AC Tonnage Calculator: Insert Sq Ft, Get Tons (+ Chart)

Before buying an air conditioner, you have to make the AC tonnage calculation. Not sizing an AC unit properly can cause $100s in wasted unit costs or future electricity costs.

According to the DOE,

“…an air conditioner generally needs 20 BTU for each square foot of living space.”

We need to convert BTU to tonnage. 1 ton equals 12,000 BTU. That means that, on average, we will need 0.0016 tons per square foot.

To properly get the tonnage of the AC you need, you can use the AC tonnage calculator here:

Calculated Tonnage:

0.00 Tons

 

For example, a standard 1,500 sq ft house would need a 2.5-ton air conditioner. To help us out, here is a quick table showing how many tons (and BTU) air conditioners we need for certain square footage:

Tonnage Table

Area (Square Feet): BTU Tonnage
600 sq ft 12,000 BTU 1 Ton
900 sq ft 18,000 BTU 1.5 Tons
1,200 sq ft 24,000 BTU 2 Tons
1,500 sq ft 30,000 BTU 2.5 Tons
1,800 sq ft 36,000 BTU 3 Tons
2,100 sq ft 42,000 BTU 3.5 Tons
2,400 sq ft 48,000 BTU 4 Tons
2,700 sq ft 54,000 BTU 4.5 Tons
3,000 sq ft 60,000 BTU 5 Tons
3,300 sq ft 66,000 BTU 5.5 Tons
3,600 sq ft 72,000 BTU 6 Tons

Example 1: 1.5 ton AC covers how many square feet?

Now you can answer this question. A 1.5 ton AC unit covers 900 square feet of living space.

Example 2: How many square feet does a 3-ton air conditioner cool?

How many square feet does a 3-ton air conditioner cover is quite easy to answer as well. 3-ton is equal to 36,000 BTU. If you apply the 20 BTU per sq ft rule of thumb, you can see that a 3-ton air conditioner cools about 1,800 square feet spaces.

Here is the most frequently asked question about tonnage, BTU, and area (square footage):

How Many Tons Of AC Per Square Foot? (Tonnage Per Square Foot)

Simple answer: You need 0.0016 tons of AC per square foot.

In short, if you’re looking to cool down a 600 sq ft area, you would need:

AC Capacity = 600 sq ft * 0.0016 tons/sq ft = 1 Ton

In general, for every 600 sq ft, you need 1 ton of AC. This is a general estimate.

You can use the ‘tonnage per square foot’ formula to calculate how many tons of AC you need. Here is the formula:

AC Capacity (Tonnage) = (INSERT AREA IN SQ FT) * 0.0016 tons/sq ft

You can insert the area and calculate how many tons of AC you need per certain square footage.

When you calculate the tonnage, you can check out several 1-4 ton mini-split air conditioners here:

Hope this helps. If you have any questions, you can pose them in the comments below.

73 thoughts on “AC Tonnage Calculator: Insert Sq Ft, Get Tons (+ Chart)”

  1. 12/19/2020
    I live in the Dallas, TX area
    My A/C unit is 3.5 tons
    We have 18″ and 9″ return ducts to the upstairs unit.
    My house was built by Ryland, square footage is 2661 and insulated well.
    Although there are hot & cold spots in the house, typically in rooms with large windows, our unit does not run all the time in winter or summer and has lasted 11 years so far.

    Based on the tables & calculations I can find online, my unit should be somewhere between 4.5 tons and almost 6 tons.

    I would expect an under-spec’d system to run all the time just trying to keep up. But ours does not. We had an A/C service come out recently and he suggested that we add more return ducts.

    I am trying to grasp what the symptoms are when the unit is under-sized. I would have expected the unit to burn out long before now if it was working to too hard.

    What am I missing?

    Reply
    • Hello Greg, you have sound reasoning. There are two potential symptoms of under-sized units: not sufficient cooling, and a shorter lifespan of the AC unit. Your cooling is sufficient and the modern ACs are made to last 20 years even if you run them on 100% all the time (regular maintenance is required). Given your situation, you will probably have very little symptoms of these sorts. In short, you’re not missing anything. Hope this helps.

      Reply
    • If you have one 3.5 ton unit for a two-story production built home of 2661 sqft I’d have a HVAC contractor or mechanical engineer run it through the manuals for size and air flow. Well worth the money to be comfortable and efficient.

      Reply
  2. I bought a 15 year old two story home in Houston. My appraisal shows the house is 4300 sqft (2400 on first floor and 1900 on second floor). There’s high open ceiling from entrance all the way through the living room. Property faces west, so it gets a lot of evening sun in the front. The house has tall high ceiling glass windows in the back, but the morning sun in the back is partly shaded by high a ceiling patio. The house seems to have good insulation (but it is 15 years old). The house has the original system installed by the builder with two 3 ton units (Evaporator coils were changed in 2013). I am about to replace this system. Is this tonnage adequate for this house? Should I go to a 4 ton/3.5 ton combo or perhaps a 5 ton/4 ton combo? At what point do we run into the problems of an oversized AC?

    Reply
    • Practically, the tonnage is sufficient if your home was sufficiently cool in the summer. Theoretically, 1 sq ft requires, on average, 20 BTU. 4300 sq ft translates to 86,000 BTU, or about 7.2 tons. Add Houston climate, high ceilings, and subtract good isolation and you’re looking at about 8 tons.

      Oversized ACs are more expensive to buy and install; that’s the only major drawback. Hope this helps.

      Reply
    • Hello Joseph, according to EPA’s recommendation, you would need 20 BTU per 1 sq ft. That means about 240,000 BTU in your case. 12,000 BTU equals 1 ton. The safe bet would be to buy a 4-ton unit (48,000 BTU in total).

      Reply
  3. I have a commercial space rented as a barbershop with aprox 6-7 workers at a time and about 6-10 customers in the shop at a time during peak hours. The space currently has a 6ton ac unit. The space is only 1200 sq ft and a floor to ceiling about 12ft. Will a 3.5-4 ton unit be enough to properly cool my shop?

    Reply
    • Hello Isa, 4 ton (48,000 BTU) unit should be theoretically enough to cool down spaces up to 2,400 sq ft. In practice, the Energy Star guide recommends that you add 600 BTU per person. Let’s say you have 17 people in the shop, that would be 17*600 BTU = 10,200 BTU. So, you still have about 38,000 BTU left; that’s more than enough to adequately cool down your shop.

      Reply
  4. Must a/c condensers be placed w/I 2 feet of house, or can they be more remote?
    I would like them placed away from the house, but do not know if that is possible without problems.

    Reply
  5. Could you help me with Line-set or AC refrigerant line sizes ??
    3 ton AC unit waiting for electric run of 30 amp 220volt , but some knuckle head ran a 20amp yellow romex and what seems to be about a 3/4 line set, could be smaller maybe a 5/8 line set
    on 2nd deck patio , 2 floor to be AC’d is about 1800 sq ft
    I think the line set is to small
    The electric is definitely to small
    Nameplate rating is 25.7 amps

    Reply
    • Hello Micheal, sure thing. Well, if you need a 30 amp, 25.7 amps won’t cut it. It’s hard to talk about the specific situation but a 3 ton AC usually has a 3/4 line set. Hope this helps.

      Reply
  6. I LIVE IN THE CHICAGO AREA AND AM GOING TO REPLACE MY AC I NOW HAVE A 3 1/2 TON COMPRESSOR THE SALESMAN SAYS THAT ALL WE NEED IS 2 1/2 TON I FEEL THAT IT MIGHT BE TO SMALL FOR OUR 1800 SQUARE FOOT HOUSE WE ON A SLAB WHAT IS THE CORRECT SIZE TO INSALL LOOKING AT 16 SEER

    Reply
    • Hello Ray, for the Chicago area, 2 1/2 ton unit (30,000 BTU) just might be enough. 3 1/2 ton unit (42,000 BTU) might be overkill for 1,800 sq ft in Chicago. In Florida, for example, that would be adequate.

      Reply
    • I live in Florida and have a 3.5 ton a/c for 1925 sq ft. Can I add two runs to cool lanai that is 232 sq ft or do I need a new small unit for lanai?

      Reply
  7. Hi,

    I am sizing units for a commercial space of approximately 2500 sq feet with 14 foot ceilings. It will be a store so I anticipate no more than 20 people present at a time. But, the previous system was a 10 ton unit connected to three ducts. A new A/C contractor suggested I instead install three 4 ton units, each running off one of the ducts (They do cover the space in zones). He claims that will be more cost-efficient and avoid needing something called an “economize.” Does this make sense? Thanks

    Reply
    • Hello Lydia, the combined cooling output of three 4 ton units will be 12 tons; that’s sufficient cooling output for your situation. 4-ton units can be more energy-efficient than 10-ton units; you can get a lot of 4-ton units with 20+ SEER rating (here are some examples of 4-ton central air conditioners with SEER and costs).

      “Economize” is not an expression we in HVAC are familiar with. The best thing would be to ask what is the SEER rating on 4-ton and 10-ton units. If the difference in SEER rating is about 2 or greater, go with that unit. Example: If a 10-ton unit has 18 SEER, and 4-ton units have 20 SEER, go with 4-ton units. Hope this helps.

      Reply
      • An economizer uses outside air for cooling when the outside temp is below a certain point, typically about 60F, instead of refrigerated cooling. That way you’re only using the blower, not the compressor. A lot of techs don’t like them and they can be cranky, but they’re also a huge energy saver.

        Reply
  8. Hi,

    I am looking to add an A/C unit to a warehouse space, 4700 sq ft with 20 ft ceilings, 1 over head door to be opened 4-5 times a day for a minute or so, 3 employees, in a cold climate (Minnesota) so it will only be running 3-4 months out of the year. What size tonnage unit shoud I be looking at? Thank You.

    Reply
    • Hello Jeff, that’s a huge warehouse with 20 ft ceilings. To calculate the AC tonnage required, you can follow the ’20 BTU per 1 sq ft (8 ft ceiling)’ rule of thumb. So 4700 sq ft * 20 BTU = 235.000 BTU or about 20 tons; but that’s if you would have 8 ft ceilings. You have a 20 ft ceiling; you need to add an additional 150% to that calculation. This would yield a total tonnage of about 50 tons; which is quite a lot. We’re talking about 10 5-ton units for example.

      These calculations, however, might not be as precise when we’re talking about the tonnage required to cool down a large warehouse. We would recommend talking to an HVAC expert on-site. You can call your local HVAC companies or fill in this form and you’ll get some information and free quotes for the warehouse. Sorry, that’s the best we can do; estimating the total tonnage requirements in warehouses is not something we have a lot of experience with.

      Reply
  9. I am in Ohio and have a 1,300 sq. ft. commercial space that is divided up into 6 rooms off a hallway, 1 small bathroom and an entrance area about 8’x15′. There are usually 5-7 people in the space at a time. Currently it has a 3.5 ton roof top ac unit located on the far rear corner. It has about a 25′ run of 12″ diameter flex duct connecting it to the main duct. When temps are above 80 degrees & 50% humidity the unit can not keep the office below 75 degrees and constantly runs. I was quoted to replace it with a 4 ton ac unit centrally mounted on the roof connecting just above the main duct line. Based on the square footage of the space I am concerned that 4 tons is oversized and won’t efficiently remove the humidity. Is this an accurate concern or is more tonnage better?

    Reply
    • Hello John, you have quite a complicated situation here. Calculating the exact tonnage is quite difficult; the best thing to do is to use the 3.5-ton rooftop unit as the benchmark. If that unit doesn’t lower the temperature sufficiently, you need a bigger one; and a 4-ton unit has about 17% higher cooling output than a 3.5-ton unit. There is no concern about overkill here or the consequential short cycling due to 50% humidity. As far as we can see, the 4-ton unit is adequate for this situation.

      Reply
  10. I bought a manufactured home that has 2293 sq ft the company I bought it from installed a 3 ton central unit. Is this unit enough to cool my home? Haven’t moved into the home yet. Please advise me what to do.

    Reply
    • Hello Teresa, it’s hard to say without the additional information, but let’s say we use a rule of thumb ’20 BTU per sq ft’. For a 2293 sq ft home, you would need roughly 2293*20 BTU = 45,860 BTU; this is about 3.8 tons. If you leave in a colder climate (New York, for example), the 3-ton central unit might suffice. If you live in a hot climate (Florida, for example), a 4-ton unit would be a safer option.

      Here is a suggestion: move into the new home and see how well the 3-ton central unit performs. If it doesn’t adequately lower the temperature and humidity, you can always buy an additional window or portable unit as an auxiliary cooling unit.

      Reply
  11. Our house is 1232 sq. feet on a slab with high ceilings in every room going from 8 feet to 12 in some rooms and 16 in others (perhaps the average would be 13 feet?). Our previous AC is 2 ton. It cooled the house ok except on those rare super hot periods. We are in Southeast Iowa. The company for the new AC has ordered a 2.5 ton. Will this be overkill? Do we need 2.5? I do not want the house to become more humid.

    Reply
    • Hello Irene, air conditioners both cool down air and dehumidify it. With a bigger unit, it will become less humid, no worries about that. 2 ton AC unit is, in theory, undersized. For 13 ft ceilings, you would require at least 25 BTU per sq ft. At 1232 sq ft, that would be 30,800 BTU or about 2.5 tons. Even in practice, 2.5-ton seems like a very adequate sizing. Hope this helps.

      Reply
  12. I live in Houston, Texas. The house is 2346 sf, the wall is 9 ft, the sun is everage, the weather is is hot. Based on the two calculations above, the AC should between 4 and 5 tons. The builder installed 5 tons. During the day time, it take 20 minutes to cool the house. At night time, it takes 5 minutes which is a short cycle.
    I think the AC is oversized, do you think so ? It should be better undersized than oversized

    Reply
    • Hello David, for the summer heatwave, the 5-ton unit is a good idea. Short cycling is an indication of an oversized AC unit (with rotary compressors); you can run it at the lowest speed to prevent that from happening. Right now, it might feel like you have an oversized AC but when the heatwave kicks in, you are ready for it. It’s always a problem with the bigger AC systems with a high tonnage cooling capacity.

      Reply
  13. We live in Austin, TX. The first floor is 1,800 square feet with the living room and kitchen making up 1,000 feet of that space. Both those rooms are the farthest from the handler/return and have 12’ ceilings. The unit is a new 3 ton 18 seer smart system (compressor talks to the handler). My assumption is it is undersized. The kitchen and living room are ~5° warmer than the stat and do not reach desired temp. My thought is to supplement with two mini ducts in the living room. Thoughts?

    Reply
    • Hello Michael, the basic rule of thumb by DOE is ’20 BTU per sq ft of living space with 8 ft ceiling’. Your 12 ft ceilings are 50% higher; so let’s say 30 BTU per sq ft. You get 1,800 sq ft * 30 BTU per sq ft = 54,000 BTU or 4.5 tons (this is the minimal figure; you have to add 4,000 BTU for kitchen and about 20% because Austin, TX is hot). In short, the 3 ton AC system is undersized and not by a small margin. Practically you see this: 5°F warmer than thermostat temperature with, presumably, 3 ton running on 100% cooling output all the time.

      Your solution with two mini ducts is great. You’re looking at two 12,000 BTU or even 18,000 BTU mini splits. If you need some help, you can check some of the best 12,000 BTU ductless mini splits here. Hope this helps.

      Reply
  14. I need to replace my 27 year old HVAC system. My current air conditioning system is a 3.5 ton SEER 12. My question is if I go with a 21 SEER can I go down to a 3 ton system? The HVAC tech told me to go with a 4 ton system. Our house is about 900 sq ft per floor with 2 floors above ground plus the basement for a total of 2,700 sq ft. We have not had any problems heating or cooling our house with the older system.

    Reply
    • Hello Mike, you will still need a 3.5 ton HVAC system. SEER is a measure of energy efficiency, not cooling/heating output. The cooling output should stay the same but if you upgrade from 12 SEER to 21 SEER, you will reduce your cooling/heating bills by more than 40%. You could go with a 4 ton but if the 3.5 ton was sufficient for the past 27 years, there is no need to increase the cooling output.

      Reply
  15. hello
    I would like to get some recommendation as to what tonnage AC unit we should get for a floor with 10 rooms, bathroom, hallway, altogether about 3300 sqf, about 9 feet height, all around very exposed to the sun, NYC climate. There can be even 30-40 people residing here at the same time. It can be either one big unit or two smaller ones at the opposite ends of the hallway. What would you suggest?
    Many thanks

    Reply
    • Hello Eszter, 3,300 sq ft with 12.5% above-average ceiling height would, applying 20 BTU per sq ft rule, be about 6.2 tons. Add the sun exposure and so many people residing in there, and you’re looking at a 7 to 8-ton unit. You should be looking at two 4 ton multi-zone mini-split units. For 10 rooms, you will need at least 5 air handlers to evenly distribute the cool air.

      Reply
  16. I live in florida and have a two story townhouse. The unit is a dual zone for upstairs and downstairs. We are roughly 1500-1600 square feet. What size unit would I need?

    Reply
    • Hello Evan, if you apply the 20 BTU per sq ft rule, you would require about 30,000 BTU or 2.5 ton units. Given this is hot Florida, 3 tons should be considered. Dual zone with each zone generating 1.5 tons or 18,000 BTU seems like adequate AC sizing.

      Reply
  17. I live in Savannah, Ga. My house is 3 bedroom 2 ba, 2 car garage with bonus room over garage and 2299 sqft. I have 3 rooms with 14 ft ceilings. The hvac is
    3.5 ton,10 seer and we have had this system since Dec 2006. Our master bedroom is on the opposite side of the house from the hvac unit. This room has always been very hot in the summer and very cold in the winter. We are now upgrading to a 4 ton unit, do you think that’s to big or an upsize.

    Reply
    • Hello Bernard, if you have a 10 SEER unit, upgrading to an 18+ SEER unit is definitely a smart financial choice. 14 ft ceiling is way above average 8 ft ceiling height. Upgrading from 3.5 ton to 4 ton might not be sufficient. As you’ve said, the master bedroom wasn’t only hot, it was very hot. 5 ton unit might be a more adequate sizing. To answer your question, the 4 ton unit is not too big.

      Reply
  18. First, let me say how gracious you are with information.
    I live in deep south Louisiana, heat indices have been 100+ on a regular basis this summer. Additionally, it’s super humid nearly 100% of the time. My home is 1720 sq.ft.
    The front is completely shaded and faces Northwest; the back is about 60% shady with the only direct sun on the back [with lots of windows] during the hottest part of the day.
    Currently there are two units installed in this house, both Carrier, totaling 4.5 tons. One of them is in the attic; the other in a hallway closet. There is no ceiling in that closet and it is open to the attic. I’ve been told it must be open because the heater uses natural gas. I deplore that!
    After 27 years of service, the Control Panel on one of the units has gone out.
    One HVAC company says it would cost $800-$1000 for a new control panel, not including installation.
    Considering the age of the units and my distaste for the no-ceiling situation, I believe the best decision is to replace both units with One Unit. The salesman informs I need a 5 ton unit. A Carrier 5 ton, 17 SEER with dual stage cooling is his recommendation. His bid for the entire job is $18,500.
    Naturally, I will get a couple more bids from other companies.
    I’m asking for your opinion, given the above information, what would you recommend for my situation?

    Reply
    • Hello Carmen, thank you for your question. The hands-on HVAC experts will know best. Just follow a few directions, such as you know that 4.5 tons is enough, that’s the capacity you’re looking for. In most cases, a higher SEER unit is a better investment. You’re already doing the smart thing – getting several bids from HVAC companies – this is the key thing to do if you don’t want to get swindled. If you need more quotes, you can get up to 4 quotes for free here. Hope this helps a bit.

      Reply
  19. I recently had a rialto home built. It’s 2 stories, 8ft ceilings with the living room open to the second floor. My master bedroom, media room.and master bath are not as cool. Setting the temp to 70 at night, the upstairs bedroom is 60 degrees and downstairs bedroom is 74 and airflow seems to be an issue. Our system is 2 zones, upstairs and downstairs and is a carrier, high efficiency 16 seer. It’s a 4 ton but this chart shows I should have a 5 ton for my 3100 sqft home. Hvac keeps coming back adjusting dampers but don’t seem to think the siE of the unit is the issue? Am I wrong? I carrier, the builder nor the hvac company will answer my question on the size of the unit being appropriate. Thoughts?

    I feel like If I let this go the ac unit will have to work harder and not last as long and I’ll be stuck with the bill once warranty runs out.

    Reply
    • Hello Adriana, this seems like an inadequate airflow issue indeed. You can rearrange the current setting by putting the bigger of the 2-zone units upstairs (the one with higher airflow). That would distribute the cooling more evenly. In such a case, it makes sense that you find somebody who can help you hands-on, it’s a bit of a tricky situation.

      Reply
  20. Hi there,
    I live in Arizona and house is 2-years old, 1,500 sq ft – A/C runs hard from for six months especially from June through October. My house gets super warm if we don’t keep it set at 74’ – what size will keep our house cooler and not stop the AC from continually running. ACs go out a lot out here.

    Thank you so much.

    Reply
    • Hello L.B., your AC unit seems to be undersized by 0.5 or 1 ton. It would be best if you would look for a new AC with up to 1 ton additional cooling capacity. Hope this helps.

      Reply
  21. I bought a 1900 sq ft bi level house in NJ outside of the NYC area. The lower level is cooled by a ductless mini split. The top level, which is where I need the central ac, is approximately 1400 -1500 sq ft with 8 ft ceilings. Two people will be living in the house. There is a sliding glass door that opens onto the back deck. House faces east. There is currently a 3.5 ton system which I think may be oversized. What size ac would be best, 2.5 or 3 tons? Or am I off base here?

    Reply
    • Hello Nancy, you’re asking exactly the right question. For the upper part, the 3.5-ton unit seems like overkill. Given its NJ, 8 ft ceiling, east facing, and so on, you would need about 20 BTU per sq ft. For a 1,500 sq ft area, that’s about 30,000 BTU or 2.5 tons. You’re not off base at all; the 2.5 ton AC would be a much better choice than the big 3.5-ton unit.

      Reply
  22. My national home builder put a 2 ton unit in my 1,680 sq ft home and during the summer, we have difficulty cooling 2 rooms of the house. I just had an 11 month inspection and the inspector said the unit is 2 tons and should be at least 2.5 – 3.0 tons for 1,680. The ceiling throughout are 8′. The warranty group tells me he is using old standards and they have engineering reports that claim 2 tons is sufficient. In light of the fact that i have 2 rooms 3-4 degrees warmer than the rest of the house, I think the home builder is trying not to upgrade the system appropriately. Can you comment of what size the system should be?

    Reply
    • Hello Robert, you’re correct; the 2-ton unit is undersized. The easy rule of thumb for AC is ’20 BTU per sq ft’; if you have 8 ft ceiling height which you do. 1,680 sq ft * 20 BTU per sq ft = 33,600 BTU = 2.8 ton. So, even this simple rule tells you you would need a 2.8-ton unit; something between 2.5 and 3.0 tons, as the inspector figured out. It’s difficult to admit a professional mistake, maybe that’s the reason why they are trying to convince you that everything is ok. Hope you resolve the issues as smoothly as possible.

      Reply
  23. So I’m building a home in South Texas ( RGV, coastal area) with living sq ft at 2000 with 9 ft ceilings around the edges and comes up to 10 and about 16ft in the living area cathedral style. Builder is consistent stating that one 5 ton unit is enough to cool the home without burning itself out and more tonnage will cause mold. Even though during the planning process they discussed needing 2 units and even built the 2nd AC pad. As a previous comment I’m afraid he’s using old statistics. Can you help? Thanks.

    Reply
    • Hello Anthony, you have a kind of vibrant ceiling height, but let’s say that the average ceiling height is about 12 ft. That means that – using the DOE standard – you should be looking at at least 30 BTU per sq ft. With a 2000 sq ft home, that’s 60,000 BTU or exactly 5 tons. Now, South Texas is hot, so you should be having higher-than-national-average cooling needs. That may be a good reason to look for 2 units with a net capacity larger than 5 tons. Nonetheless, the installers had an on-sight look and has more detailed information. If you do install the 5-ton unit and realize it’s too small, you can always add a 1-ton or a 2-ton unit to the areas that are not adequately air conditioned by the 5-ton unit. As of the old statistics, AC tonnage calculation is not an exact science and you can get different results using roughly the same standards. Older HVAC installers usually have a ‘feeling’ for sizing AC units that may not correspond to the current standards. Hope this helps in understanding the two different suggestions.

      Reply
  24. We just had a house built but feel they put the wrong condenser in. We have a dual zoned home between the upstairs and the down stairs. 9ft ceiling down and 8ft ceiling up. 3150sq ft and they have a 3.5 ton condenser. Trance Model 4TWR4042G, is this too small or is different when zoned like this? The builder says it’s fine but……. Thought?

    Reply
    • Hello Mike, the 3.5-ton unit is 42,000 BTU. Given the DOE’s ’20 BTU per sq ft’, you should be looking at about 3,150 sq ft * 20 BTU per sq ft = 63,000 BTU. That’s a 5.25-ton unit. Now, you don’t really be very alarmed about this. HVAC installers, in general, know how to calculate the AC tonnage for a specific house. The climate, insulation, windows, sunshine, and so on have an impact on cooling requirements as well, beyond the rule of thumb. Chances are that the 3.5 unit is adequately sized; however, you can’t really neglect the possibility that it’s undersized. Theoretical calculations are one thing. Practically, however, you can see the real-life cooling needs when the summer heatwave kicks in.

      Reply
  25. My house in Las Vegas is a two story with a total square footage of 1575 sq. ft. About half of the house is floor to ceiling high and the other half has the 2nd floor. Can you suggest how many tons of condenser unit do I need if I have to replace the old existing unit? I don’t know how many tons the existing. Thank you

    Reply
    • Hello Loreto, the best thing to do here is to check the tonnage of the existing AC unit. It’s written on the unit; either on the outdoor or the indoor unit. We can also try to theoretically calculate this: so let’s say that about 800 sq ft has 8 ft ceiling and the other 800 sq ft has about 17 ft ceiling height? Since it’s Las Vegas, where tends to be quite hot, you would need about a 2-ton unit for the first 800 sq ft and about a 4-ton unit for the second 800 sq ft. All in all, the net tonnage you’re looking at is close to 6.

      Reply
  26. I am looking to replace my ac. I live in Fort worth, TX (north tx), my home is 20 yrs old 2394 sq foot ranch home with and currently has a 5 ton original ac. I had 2 estimates for a new ac and one tech said based on my duct work looks like I have an oversized ac. We do have issues in the summer and seems the ac is always running. He sounds convinced but I’m afraid to downsize it and it not cool the house. The other guy estimate for exactly what I have currently and when asked him he said it would be 400 to 600.00 to do a calculation. Please help I’m afraid to spend the money for a smaller unit and it not be correct.

    Reply
    • Hello Anita, if you have a 5 AC unit and it has to run all the time in the summer, you have an adequately sized AC. If you experience short cycling (AC turning on and off frequently), that’s a sign of an oversized AC. Without those signs, and if you need the AC to run all the time, it seems that 5 ton AC is quite adequate, and getting a smaller unit would present a risk you don’t really have to take.

      Reply
  27. I need to replace an old central air unit. This is an office space on 1250 sq ft in NYC. The waiting area is 400 sq ft. with heavy people traffic. The old unit was 5 ton. In my view 3 ton should be enough. I would like your suggestions.

    Reply
    • Hello Julio, it depends on the people traffic and insulation. If the 5 ton was massively oversized, the 3 ton should be a good compromise. You might want to call an HVAC expect to adequately size the unit; the difference in cost between 3 and 4-ton unit, for example, runs in several $1,000s as we have written in an article about the costs of central air conditioners here.

      Reply
  28. Hello Dear, how many tons Ac I need for the below room:
    Length: 14 FT
    Width: 12 FT
    Ceiling Height: 10 FT
    Door Size: 3.5 FT
    Climate: Very Hot
    Sun Exposure: Very Sunny.

    Reply
    • Hello Rahid, the basic rule of thumb is 20 BTU per sq ft for 8 ft ceilings. 14×12 room has 168 sq ft, and you have 10 ft ceilings instead of 8 ft ceilings; you have to add 25% to that; so you’re looking at 25 BTU per sq ft for 10 ft ceilings. That would be 4200 BTU or about 1/3 ton. Now, you have a very hot climate and high sun exposure. You would do much better with a 2/3 or 8,000 BTU unit. Hope this helps.

      Reply
  29. Hello! My house is 1539sq ft with cathedral ceilings from 11ft to 8ft tall. I want to replace my ac unit but I’m not sure which size my house needs. It is currently 17 years old and has 3.5tons. I’ve had two estimates- one person suggests 3tons due to a major mold issue that could have been cause by the Florida humidity and the air conditioning unit being too big. The other suggests that I replace the unit with the exact size I have now (3.5). I’m confused as to who’s side to take. What is the standard for a 1539sqft home?

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    • Hello Gloria, the smaller 3-ton recommendation is due to high humidity. The contractor probably concluded that your current AC is too big and that’s why it’s short cycling; that would result in adequate cooling but inadequate humidity reduction. One option here is just to get an additional dehumidifier to fight the humidity levels.

      Here’s the sizing for 1,539 sq ft: You use the rule of thumb (20 BTU per sq ft of living space; 8 ft ceiling), increase it due to cathedral ceilings (average ceiling high is presumably 9.5 ft; an increase of 19%) to about 24 BTU. That would result in 36,936 BTU or 3.1 tons. Now, Florida is hot; in most cases, contractors in Florida will increase the AC capacity to 3.5 tons. That seems like a reasonable AC tonnage.

      With the current 3.5 ton unit, do you get adequate cooling quite easily? If yes, the 3-ton unit might give you a sufficient cooling load and may (may!) decrease humidity if the high humidity is a result of an oversized AC short cycling. That, however, doesn’t seem likely. A combination of 3 ton AC unit and dehumidifier would make sense. If the 3.5-ton unit can barely give you sufficient cooling then it makes no sense to downsize it to 3-ton unit. I hope all of this is at least a bit helpful.

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  30. Hello,
    I am building new construction. Its a 1400Sq ft upstairs on top of full basement with concrete walls. Upstairs ceiling height is 8FT with cathedral ceiling in a small ((250sqft) living room. Ceiling height in basement is 9ft. Building in the North Georgia hills (zone 7) with average sun exposure. Wanting a split system for upstairs/downstairs with separate thermostats. What tonnage would you recommend for this house? Thanks much.

    Reply
    • Hello Charlie, here it would make sense to call a contractor for an on-site Manual J tonnage estimation. The simple formula you can use is 20 BTU per sq ft, which would yield you a 28,000 BTU. That’s about 2.5 tons. The Manual J estimation will probably be between 2 and 3 tons, but you really want to do that because the difference between 2 and 3 tons is quite large. Hope this helps at least a bit.

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