Furnace Size Calculator: What Size Furnace Do I Need? (BTU Calculator)

How many BTU furnace do I need?

Most homeowners have difficulties figuring out what size of furnace do they need. Fortunately, calculating furnace size (be it natural gas, propane, electric or oil) is not rocket science.

To help future furnace owners out, we have developed a simple furnace sizing calculator.

You only need to know two things; your home square footage and the climate zone you’re in.

After the furnace sizing calculation, you’ll find how the size of the furnace is calculated. We also made 3 examples for:

  1. What size furnace do I need for a 1000 sq ft house? (Example 1)
  2. What size furnace do I need for a 2000 square foot home? (Example 2)
  3. What size furnace do I need for a 3000 sq ft home? (Example 3)

Below you will also find a chart with calculated BTUs and furnace tonnages for houses from 500 sq ft to 3,000 sq ft.

Note about furnace input vs output: Furnace BTUs you will find on the specs sheet are always the output BTUs (regardless of efficiency or AFUE rating). An 80,000 BTU 94 AFUE furnace will have a heat output of 80,000 BTU. Due to 90% efficiency (90 AFUE), the input will be 85,106 BTU.

You can freely use the calculator to roughly estimate the capacity of a furnace you need. Use this map to determine the climate zone (or climate region) you’re living in:

climate zones to calculate how big a furnace you need
5 US climate zones: Size of the furnace heavily depends on where you live.

Furnace BTU Calculator (Input Square Footage And Climate Zone)


The basic principles the furnace sizing calculator is based on are:

  • Larger homes need more BTU than smaller homes (directly proportional to square footage).
  • Homes in colder climates (Region 5) need to generate more heat than holmes in warmer climates (Region 1).
  • Note: Furnace efficiency doesn’t matter here. An 80 AFUE furnace (with 100,000 BTU rating) and an efficient furnace (94 AFUE 100,000 BTU) will both produce 100,000 BTU of heating output. It’s just that an efficient furnace will need less fuel (propane, gas, electricity) to create this output.

To properly size a furnace, we need to use BTU heating needed per square foot for each climate zone:

Region 1 Region 2 Region 3 Region 4 Region 5
35 BTU per sq ft 40 BTU per sq ft 45 BTU per sq ft 50 BTU per sq ft 60 BTU per sq ft

Essentially, homes in the far North (Minessota, for example) need almost twice as much heating load as homes in the far South (Texas, for example). We have elaborated on winter heating requirements (in BTU) and heating factor here.

Basically, BTU stands for British Thermal Unit. 1 BTU is enough energy to heat 1 pound of water by 1 degree.

For an exact estimation of furnace size, you should ask your local furnace installers or technicians. That’s because the required BTU output doesn’t only depend on the square footage of your home, but also on the number of windows, the home’s insulation R-values (higher number R-value home require less heating), and so on.

Here are the 3 examples that illustrate how the calculator works.

What Size Furnace Do I Need For A 1000 Sq Ft House? (Example 1)

Let’s say we have a nice 1,000 square foot home in Los Angeles, California. We need two data points:

  1. Home square footage: 1,000 sq ft.
  2. Climate zone: Los Angeles, California, is in Region 2.

We input both of these data points into the calculator. Here is what we get:

for 1000 sq ft home we need 40000 btu furnace

In short, we need a 40,000 BTU furnace.

What about if we have a 1,000 sq ft home in the heart of Minnesota? That’s Region 5. Here is what we get with the furnace sizing calculator:

in cold climate 1000 sq ft home needs 60000 btu furnace

We see that for a 1,000 sq ft home we need:

  • 40,000 BTU furnace in California.
  • 60,000 BTU furnace in Minnesota.

Here are the results for 1000 square foot houses in all climate zones:

Region 1 Region 2 Region 3 Region 4 Region 5
35,000 BTU 40,000 BTU 45,000 BTU 50,000 BTU 60,000 BTU

What Size Furnace Do I Need For A 2000 Square Foot Home? (Example 2)

2000 sq ft houses need double the furnace capacity compared to 1000 sq ft house.

Here are calculated 2000 sq ft home estimates for furnace size:

Region 1 Region 2 Region 3 Region 4 Region 5
70,000 BTU 80,000 BTU 90,000 BTU 100,000 BTU 120,000 BTU

In many cases, there is a dilemma if a heat pump would be better than a furnace. There are many advantages that heat pumps have over furnaces (you can read about them in our furnace vs heat pump comparison article here).

The key cut-off homeowners can use is the capacity. That’s because even the best heat pumps can at most produce only about 60,000 BTU. Anything above that, you will need a big furnace that can produce a larger heating output; it’s almost impossible to adequately heat a 2,000 sq ft home with a heat pump alone. If interested in heat pumps, can also check how many BTU heat pump you need here.

What Size Furnace Do I Need For A 3000 Square Foot Home? (Example 3)

Furnace size for a 3,000 square foot home range from 105,000 BTU to 180,000 BTU, depending on the climate zone you’re in. Here is a table with calculated 3,000 sq ft furnace sizes for all 5 regions:

Region 1 Region 2 Region 3 Region 4 Region 5
105,000 BTU 120,000 BTU 135,000 BTU 150,000 BTU 180,000 BTU

We hope everybody will be able to use the furnace capacity calculator and get a good estimate of the size of the furnace they need for winter heating.

Chart For Furnace Tonnages And BTUs For 500 – 3,000 Sq Ft Houses

To help you out, we have calculated how many ton furnaces you need for small, middle-sized, and big houses. We choose Region 3 (with 45 BTU per sq ft) for these furnace sizing estimates.

This will answer the question of how many ton furnace do I need for my home for some homeowners:

House Size (Sq Ft): Furnace Size (in BTUs): Furnace Size (in Tons):
500 sq ft home 22,500 BTU furnace 1.9 tons furnace
600 sq ft home 27,000 BTU furnace 2.3 tons furnace
700 sq ft home 31,500 BTU furnace 2.6 tons furnace
800 sq ft home 36,000 BTU furnace 3.0 tons furnace
900 sq ft home 40,500 BTU furnace 3.4 tons furnace
1000 sq ft home 45,000 BTU furnace 3.8 tons furnace
1100 sq ft home 49,500 BTU furnace 4.1 tons furnace
1200 sq ft home 54,000 BTU furnace 4.5 tons furnace
1300 sq ft home 58,500 BTU furnace 4.9 tons furnace
1400 sq ft home 63,000 BTU furnace 5.3 tons furnace
1500 sq ft home 67,500 BTU furnace 5.6 tons furnace
1600 sq ft home 72,000 BTU furnace 6.0 tons furnace
1700 sq ft home 76,500 BTU furnace 6.4 tons furnace
1800 sq ft home 81,000 BTU furnace 6.8 tons furnace
1900 sq ft home 85,500 BTU furnace 7.1 tons furnace
2000 sq ft home 90,000 BTU furnace 7.5 tons furnace
2100 sq ft home 94,500 BTU furnace 7.9 tons furnace
2200 sq ft home 99,000 BTU furnace 8.3 tons furnace
2300 sq ft home 103,500 BTU furnace 8.6 tons furnace
2400 sq ft home 108,000 BTU furnace 9.0 tons furnace
2500 sq ft home 112,500 BTU furnace 9.4 tons furnace
2600 sq ft home 117,000 BTU furnace 9.8 tons furnace
2700 sq ft home 121,500 BTU furnace 10.1 tons furnace
2800 sq ft home 126,000 BTU furnace 10.5 tons furnace
2900 sq ft home 130,500 BTU furnace 10.9 tons furnace
3000 sq ft home 135,000 BTU furnace 11.3 tons furnace

For example, the 6-ton furnace is sufficient for heating a 1600 sq ft home in Region 3.

You can check our articles about different types of heating furnaces:

Another useful resource is connected with falling efficiency and mold infestation in furnaces. To prevent the loss of efficiency, HVAC UV lights for furnaces are used (here are the details).

35 thoughts on “Furnace Size Calculator: What Size Furnace Do I Need? (BTU Calculator)”

  1. Many factors other than building square footage need to be considered before choosing a furnace but it gives you a ballpark estimate to start with.
    The size and type of duct work also needs considered.
    The ceiling height and the amount and type of insulation in the attic and walls.
    A 3000 sqft home with 11 foot ceilings will need a much larger furnace than one with 8ft ceilings.
    And those with 20ft high cathedral ceilings and “open” concepts may result in all the money you spend for heat merely keeping the ceiling warm instead of you.
    The type of windows and doors also need to be factored in.
    Are you on a hill top or in a valley or in town where winds are buffered and blocked.
    What is the coldest temperature you have had in the past.
    HVAC companies often tell homeowners their old furnace was oversized and recommend a much smaller furnace.
    Then when those nights hit 25 below, their new furnace is running constantly and can’t keep up.
    It really takes a professional with decades of experience to make these determinations so it’s not a DIY project.

    • Sadly when they give an estimate it’s in and out. I’ve never had one ask about or determine the insulation or what the coldest temp is ( we’re in Boston). It’ helpful for the homeowner to have a starting point as they are running around giving estimates all day and are not going to spend a lot of time given that there many others competing for the job. at least that’s my experience in a busy and cold ( in the winter) city.

      • Try out coolcalc.com mj8 . Its low cost and you can do your heating and cooling loads room by room or by the whole house. It accommodates the insulation value by the year the house was built . Once you get the hang of it its awesome

    • Hello Jane, basements, on average, need a bit more heating per square root. The calculations above are rough estimates; every home is different and that’s why the BTU output should be adjusted accordingly to the requirements of your home.

        • Hi David, well, the 1000 sq ft house calculation is quite straightforward; depending on which climate region you live in, you can see how many BTU per sq ft you should include in the calculation. For a 1000 sq ft basement, things are a bit complicated. Since the basement usually doesn’t need to be heated up to the same temperature as the house (living space), you can take 50% of the BTU per sq ft for the house. This is a very rough estimation; the point here is to keep the basement warm and, most importantly, reduce the humidity levels (basements usually have higher humidity levels and you will often see mold and mildew there because of it). Hope this helps.

  2. How do I convert the BTU requiem to to tonnage. My HVAC guy always speaks in tonnage. I have a 1650 sq ft home with a 3 ton furnace.

  3. Does the required size change for baseboard heat vs. forced air heating? Also, are there systems that will switch automatically between heating water for baseboard heat and heating water for household usage (e.g., shower, washing machine, etc.)? If so, can you comment on them?

    • Hello Alex, an intriguing question. Currently, we don’t really have the adequate knowledge to answer this question. We might think about creating an article about baseboard heat vs. forced air heating and the size changes that may incur. Thank you for the question; sorry we can’t be more helpful.

    • Hello Shelly, that’s a very good question. In the furnace BTU rating, the efficiency is already figured in. So you don’t have to additionally factor in the efficiency.

    • Not if you’re looking at the “Output BTU” value of the furnace. If you’re looking at the input BTU rating, then yes you need to factor in the BTU rating. An 80% efficent furnace rated at 100,000 BTU input will produce 80%, or 80,000 BTU’s on the output side. The output is the important value here.

      • Hello KB, that’s exactly the right assessment. Some producers list input BTUs and others output BTUs; as KB pointed out, it’s important to note the difference when it comes to furnace sizing.

      • So is “Furnace Capacity” = “Output BTU”? Using the calculator above, it says I need 113,550 BTU “Furnace Capacity”. Does that mean I need to get either a 142,000 BTU Furnace @ 80% or 120,000 BTU Furnace @ 95%?

        • Hello Walter, this is a very smart question. The furnace capacity always equals output BTU. The inefficiency losses are already factored in. Example: If you need a 113,550 BTU furnace, you buy a 113,550 BTU furnace. If such a furnace is 80% efficient, you’ve correctly calculated that it needs to actually produce 142,000 BTU. The specifications of 142,000 BTU 80 AFUE will read ‘113,550 BTU’ output. In short, the inefficiency is already factored in, you don’t have to do extra calculations. Thank you for such an insightful question, hope it helps others as well.

    • Hello Billie, it does make a considerable difference. Usually, all the calculations are made for standard 8 ft ceiling height. If you have a 10 ft ceiling, you need to add +25% to the calculation. So, if the calculator tells outputs 50,000 BTU, you actually have to add 25%; so your total is 62,500 BTU.

  4. Have a high efficiency furnace, 13 years old, that does not function when temps fall below 32 degrees. It has been serviced and most parts replaced. Is it dead or r there other factors such as dirty ducts? The square footage is 2700 feet.

    • Hello Carmen, what is the tonnage of the furnace? It seems like it might be a bit undersized given it’s not capable to function properly when the temp drops below 32.

      • So what is consider a good high efficiency size furnace for a 2700 sq. home with high ceilings? I seem to have the same problem as Carmen.

        • Hello GC, well, it depends on where you live, how cold the winter is, and your insulation. The rough estimate would be a 10-ton unit, but it’s best to call an HVAC expert for on-site estimation in this case.

  5. I currently have a fan forced gas furnace with an output BTU = 112,000. It does a pretty decent job of heating the house which is a 4800 sq ft walkout ranch in Denver, CO.
    When I use various electric furnace sizing calculators I get very large BTU estimates e.g., above 200K BTU which corresponds to greater than a 10 ton furnace size.

    Is an option to simply map the gas furnace output BTU to an electric furnace BTU sizing? E.g., 112,000 output BTU gas furnace = 9.4 ton electric furnace?

    • Hello there, the capacity of electric furnaces is not measured in BTUs, it’s measured in kW (kilowatts); a unit for electric power. To map the gas furnace, you need to simply convert BTU to kW. In fact, we have this BTU to kW calculator here for AC units but it basically works for any kind of unit. Basically, 1 kW is equal to 3412 BTU. In your example, 112,000 BTU corresponds to 35.2 kW. That means that a 35 kW electric furnace has about the same heating output as your current 112,000 BTU gas furnace. Hope this helps.

  6. This is extremely misleading and I wish the narrative surrounding equipment sizing would change to start including, amongst a proper manual j, a blower door test to account for air infiltration. I don’t know how these numbers and rules of thumb originated or if there was integrated tolerance for average air infiltration, but newer homes and anything that gets built now is built so tight there is little to no air infiltration making applying this sizing chart to those homes result in obscenely oversized equipment resulting in short cycles which translates to an uneven and uncomfortable home. Getting much smaller equipment in tighter homes allows for longer run cycles which translates to a more even and comfortable home. The problem is finding a quality company/technician.

  7. Great article thank you! Question: do you count bathrooms or closets in the square footage calculations if those spaces don’t have vents or ducts?

    • Hi Maria, thank you. Yes, basically, we are warming the indoor air. Wherever there is air, the furnace has to heat it. Bathrooms, for example, have air and we count them even if they don’t have vents/ducts.

  8. With the high cost of PGE here in northern California (zone 2 on your mapping) and being single in a 1500 sq ft house built in the late 90s and liking my house temp in the winter at 75 or above in the winter ( 90is ok in the summer inside) and only really using 2 or 3 rooms in the house that has volume ceilings in the living room ( never use) Family room (use some) and master bedroom ( sleep in it mostly) and my office ( use it most of the time) with 2 other unused bedrooms, I am using around 100 to 115 therms in the winter.
    I am considering putting a static electric wall heater (600W Eheat- Envi 120v) in the office, bedroom and family room instead of heating the whole house.. I benefit from the local Electric company with a winter rate – off peak ( 5-8 pm) of $0.112 per Kwh.
    Would this make sense to reduce my winter heating costs ?

    • Hi Ken, alright, this is going to be interesting to calculate. Let’s say you use 100 therms per winter, and the average cost per therm is about $3. That comes to about $300 for heating in the winter. Those 100 therms give you 9,643,200 BTU of heating output (presuming 100% efficiency here).

      Now, let’s say you would switch from gas furnace to completely electric heating (using static electric wall heaters). How much would that cost? Well, 1 kWh of electricity produces 3,412 BTU of heating output. For 9,643,200 BTU of heating output, you would need to ‘burn’ 2826 kWh of electricity. At $0.112/kWh, that comes to $317 vs $300 for gas. This seems to suggest that gas would be cheaper.

      However, you would use electric wall heater to heat only the office, bedroom and family room you are using. The gas furnace does heat the whole house, and you’re not using all the house. In this case, it would make sense to get the wall heaters (we are talking these Envi 120V heaters, right?) due to very low electricity rates you have in the winter. Hope this helps.

  9. I have a question on how to calculate the furnace BTU size:

    My house has varying ceiling heights with the bed rooms and kitchen at 8′ and 9′ while the living rooms vary from 10′, 11′ and 12′. I also have many sky lights that has tall sidewalls that range from 2′ to 5′ tall. The one in the kitchen with 8′ ceiling is 5’x5′ with wall height varing from 3′ to 5′. So what is the proper way to calculate the required BTU of the furnace?

    Also in calculating square footage, should I use the inner wall or the outer wall to measure the area. Also from room to room where the wall height changes, which wall should I use or just use the center line between the 2 walls?

    Would it be more accurate to calculate based on area of exterior walls, top floor ceiling walls, bottom floor area and window area? You have a formula for that?

    • Hi Sammy, you are correct, the ceiling height matters. The calculator assumes the standard 8 ft ceiling height. The key here is to understand that the furnace should warm the total volume of air, and that volume is determined by the ceiling height as well.

      The simple rule is to add 12.5% of BTU output needed for every ft over 8 ft ceiling height. For every ft under 8 ft ceiling height, you should subtract 12.5% of BTU output. Hope this makes a bit more sense.


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