Heating BTU Calculator: How Many BTUs Per Square Foot?

Question: How many BTUs do I need to heat 1500 sq ft? How many square feet will 30,000 BTU heat?

Example: To heat 1500 sq ft home, you will need anywhere between 45,000 BTU and 90,000 BTU.

These kinds of questions are very common when planning your heating needs. Adequately estimating how many BTUs you need to heat up your home is essential. The purpose of the ‘Heating BTU Calculator’ below is to pinpoint how many BTUs of heat your need as precisely as possible.

BTU or ‘British Thermal Unit’ is a unit of heat. 1 BTU is enough heat to raise the temperature of 1 pound of water by 1°F. US households require anywhere from 20,000 BTU to 300,000 BTU of heating output in the winter.

To calculate how many BTU of heating output you need, you have to know only 3 factors:

  1. Total square footage of your home, or the place you want to heat up in winter. This can be anything from a 150 sq ft room to a 3,000+ sq ft house.
  2. Your climate zone. Heating a house in Miami, Florida will obviously require less heating BTUs than heating a house in Chicago, Illinois.

To use the heating BTU calculator, you will first need to measure the place you want to heat up. You need to know if you’re heating up a 1000 sq ft, 1500 sq ft, or a 3000 sq ft home, or a 400 sq ft room, for example.

Secondly, you need to figure out what climate zone you live in. That will determine how many BTU per square foot you need for heating (more on that later on). The United States is divided into 7 main climate zones or regions. Example: Miami, Florida, is in Climate Zone 2 and requires 35 BTU of heat per sq ft. Chicago, Illinois, is in Climate Zone 5 and requires 50 BTUs of heat per square foot.

To help you figure out which climate zone you should input into the heating BTU calculator, you can use this map by Office of Energy Efficiency & Renewable Energy:

heating btu us climate zones for calculating heating btu requirements per sq ft
Climate Zone regions are based on temperature as well as humidity levels. Source: 2012 IECC – International Energy Conservation Code

With this information, you can use the Heating BTU Calculator to get a basic idea of how many BTUs you need to heat your home.

We will also explain how many square feet does a heater (furnace, space heater, etc.) with certain BTU output heat. On top of that, we will solve a few heating BTU examples below, and if you don’t find an answer, you can use the comment section and we’ll try to help you out.

Here is this handy and simple-to-use calculator for heating BTUs:

Heating BTU Calculator (Insert Sq Ft And Climate Zone)


Here is a short example of how this calculator works:

Let’s say you have a 1,200 sq ft home in Nashville, Tennessee. You’re trying to figure out how many BTU should a furnace or a central heating system produce to adequately keep your home warm during winter.

Before using the heating BTU calculator, you consult the climate zone map above and see that Nashville falls into the Climate Zone 4 region. With that, you can insert both 1,200 sq ft and ‘Climate Zone 4’ into the calculator and get the estimate of how many BTU you would need to keep your home adequately heated like this:

how many btu of heat for a 1200 sq ft home

As you can see, the best estimate is that you will require 54,000 BTU of heating during the winter season.

Now, there are two kinds of questions people ask when calculating the heating BTU. These are:

  1. How many BTUs do I need to heat X square feet? X designates the size of a home; usually between 500 and 5,000 BTUs.
  2. How many square feet will X BTU heat? X here designated the numbers of BTUs (British Thermal Units). This is a very relevant question when deciding about the size of space heaters; not furnaces or central heating systems. Normally, we speak anywhere from 1,000 BTU to 30,000 BTU here.

To help you get some answers, we have calculated two heating BTUs tables for each question:

How Many BTUs Do I Need To Heat My Home? (Table 1)

Using the BTU heating calculator, we can estimate how much heating output you require to heat a home with certain square footage.

To help you out, we’ve gathered the heating BTU requirements for 500 sq ft to 5,000 sq ft homes.

These BTU requirements have, depending on where in the US you live, quite a large interval. Example: How many BTU do I need to heat 1,500 square feet?

Answer: 45,000 BTU – 90,000 BTU. The exact number depends on where you live. If you live in Climate Zone 1 (very hot climate), you will require 45,000 BTU. If you live near the Canadian border – Climate Zone 7 (very cold climate), you’ll need 90,000 BTU. Most people live somewhere in between and will require around 67,500 BTUs. We’ll designate that as the “standard climate” in the BTU heating table below:

Heating BTUs Table (Rough Estimates)

Home Size (Heating): Standard Climate Very Cold Climate Very Hot Climate
500 square feet 22,500 BTU 30,000 BTU 15,000 BTU
1000 square feet 45,000 BTU 60,000 BTU 30,000 BTU
1500 square feet 67,500 BTU 90,000 BTU 45,000 BTU
2000 square feet 90,000 BTU 120,000 BTU 60,000 BTU
2500 square feet 112,500 BTU 150,000 BTU 75,000 BTU
3000 square feet 135,000 BTU 180,000 BTU 90,000 BTU
3500 square feet 157,500 BTU 210,000 BTU 105,000 BTU
4000 square feet 180,000 BTU 240,000 BTU 120,000 BTU
4500 square feet 202,500 BTU 270,000 BTU 135,000 BTU
5000 square feet 225,000 BTU 300,000 BTU 150,000 BTU

How Many Square Feet Will 1,000 – 30,000 BTU Heat?

In much the same way, we can answer how many square feet will a heater with certain heating output (expressed in BTU) heat.

Heating Output: Square Footage (Standard Climate) Square Footage (Very Cold Climate) Square Footage (Very Hot Climate)
1,000 BTU 22,2 sq ft 16,6 sq ft 33,3 sq ft
3,000 BTU 66,6 sq ft 33,3 sq ft 100 sq ft
5,000 BTU 111,1 sq ft 83,3 sq ft 166,6 sq ft
10,000 BTU 222,2 sq ft 166,6 sq ft 333,3 sq ft
15,000 BTU 333,3 sq ft 250 sq ft 500 sq ft
20,000 BTU 444,4 sq ft 333,3 sq ft 666,6 sq ft
25,000 BTU 555,5 sq ft 416,6 sq ft 833,3 sq ft
30,000 BTU 666,6 sq ft 500 sq ft 1000 sq ft

Now you can answer how many square feet will a 5,000 BTU heat. On average, it can heat about a 110 sq ft room. In the cold north, 5,000 BTU will be enough to heat 80 sq ft, and in the hot south, you will be able to heat a 170 sq ft room with such a space heater.

Let’s look at one example:

How Many Square Feet Will 40,000 BTU Heat? (Example)

Let’s say we have a 40,000 BTU heater (it could be a house heater or a 40,000 BTU patio heater).

Let’s also presume we live in a standard climate (Climate Zone 3). In this climate zone, you will need about 40 BTU to heat 1 sq ft of space.

Here’s how many square feet can a 40,000 BTU heat:

Area = 40,000 BTU / 40 BTU per sq ft = 1,000 sq ft

In a standard climate, 40,000 BTU is enough to heat a 1,000 sq ft area. Obviously, if you live in colder climate, a 40,000 BTU heater will heat an area below 1,000 sq ft. If you live in a warmer climate, 40,000 BTU will heat more than 1,000 sq ft area.

The key question, as you can see, is in which Climate Zone you live. Based on Climate Zone, you know how many BTU of heat you need per square foot.

Let’s have a look at how many BTU of heat you need in a specific Climate Zone:

How Many BTU Of Heat Do You Need Per Square Foot? (Depends On Climate Zone)

To create a heating BTU calculator, you need to know how many BTU of heat per square feet you need in a certain climate zone. Obviously, in the cold north, you will need more BTUs per square foot than in the warm south. How many BTUs exactly?

Here is a neat table with heating BTUs per square foot for all 7 Climate Zones (check the heating map above with Climate Zones for reference):

Climate Zone BTUs Per Sq Ft
Climate Zone 1 30 BTU per sq ft
Climate Zone 2 35 BTU per sq ft
Climate Zone 3 40 BTU per sq ft
Climate Zone 4 45 BTU per sq ft
Climate Zone 5 50 BTU per sq ft
Climate Zone 6 55 BTU per sq ft
Climate Zone 7 60 BTU per sq ft

As you can see, in the north, you need about double the heating output to heat 1 square foot compared to the extreme south.

Example: With 35,000 BTU, you can heat a 1,000 sq ft home in Florida. If you’re from Chicago, a 35,000 BTU heater will adequately heat a 600 sq ft home.

You can also check a similar BTU calculator for air conditioning here.

If you need additional advice, you can give us some insight into how big a home you need to heat and where, and we’ll do our best to help you out with the heating BTU calculation.

36 thoughts on “Heating BTU Calculator: How Many BTUs Per Square Foot?”

    • Hello Adam, you’re right. We presuppose the standard ceiling height of 8 ft. For every additional ft above 8 ft, you need to add 12.5%. So, for 9 ft ceiling, you need to multiply the BTUs by 1.125 to get the most adequate estimation.

  1. This is 100% the wrong way to determine heating loads for a space. The btu/sq ft method does not compensate for any other factors and assumes the worst in terms of energy efficiency of the home. Windows, doors and insulation play a huge role in the final number. It also does not consider the exterior wall size compared to the interior sq ft size either which is critical in getting an accurate calculation. For example, an L shaped home will have the same heating load as a square shaped home but the square shaped home has 33% more internal sq footage.
    Oversizing furnaces to accomodate unrealistic heating loads leads to premature failure due to air starvation and short cycling as well as poor comfort due to insufficient runtime.
    The only accurate way to determine what the load actually is is to have a Manual J Energy Audit performed on the space.
    Ask a contractor or your local gas company for someone qualified to perform an energy audit. Doing things correctly the first time saves money and frustration in the future.
    Good luck.
    State of WI Master HVAC Technician

    • Hello Alex, you’re 100% correct, the only proper way to account for all the insulation, shapes, exterior wall sizes, and so on, is to consult the Manual J. The goal here is to get a rough estimate; a guestimate at best. For everybody who is not an HVAC expert, it’s important to get a glimpse of how many BTUs per square foot should be used for heating. The actual BTU calculation, in accordance with Manual J, is, of course, performed on the space by an HVAC expert.

    • You are correct, it’s not a one size fits all. We live I. Alaska with a well insulated home with double stud exterior walls. We are hearing about 2000 sqft along with 1000 swft daylight basement on less than 800 gallons of heating oil.
      I know there are many old homes that use a lot of energy. I think all home should be brought up to some sort of energy standards. People
      Talk about cars using energy but no one seems to care much about older homes.

      • I agree with the concept of bringing all older homes up to the standards of newer homes but in my case I would have to take our city to court to do it. I purchased a house built in the late 1800’s and I had my cousin, who had a blown cellulose insulation company, insulate the house and I replaced all the windows and doors with dual pane replacement windows and fully insulated doors without pulling a permit for reasons I will explain. My Father and I did it on a long weekend when we knew inspectors would be out of the city just in case. It is a 3 unit apartment house and I live in one unit and rent the other 2, the total living space is just under 3K square feet. I replaced the heating units in each apartment with Rinnai gas heaters that at that time were the most energy efficient. The one thing about these old houses that you need to keep in mind is that the raw wood started out life as a 4×4 and after over 100 plus years is now about a 3×3 or less so that limits heat gain. The worst part is that my house was placed on the list of historic homes without my knowledge and if I were to try building up the interior walls now that I can afford to do it to make it more energy efficient I would be forced to replace the windows with energy efficient replicas of the original windows and doors. My neighbor priced it out and the replicas cost about $1,000 per window. My house has 22 windows most of which are 60″ tall, and they are mostly facing south so there is that heat gain in the winter on sunny days. What is even more ridicules is that the original windows are operated using weights on either side of the window so I assume that the “replacements” to match the historic windows would require that so that leaves an open cavity that blows in cold air. After we replaced the windows my Father and I removed the decorative molding, removed the old weights and used blown foam insulation to stop the drafts of cold air. All of that would have to be scraped out. Try reasoning with a city council that has no knowledge of construction and believes that all landlords are wealthy. They aren’t. There’s an old joke about landlords that goes something like this: how do you become a millionaire as a landlord? Start with 2 million. I have done the best I could with the budget I had and my tenants are content with the low cost of heating their apartments. Due to the size of the windows they also use less electricity. That is why if you drive around and you see old houses and multi-family houses with stone foundations you will often see that they are slowly being allowed to go to hell. That is because of all of the ordinances that mandate that we use appropriately sized (3″ was the old standard) wooden cladding. Of course over time it breaks down and rots, it needs to be painted regularly and yet no one manufactures any other sized cladding other than 4″ that would make the home more energy efficient and we are not allowed to use 4″ cladding like the cement cellulose products that are available and last a long, long time. That is just the tip of the iceberg when it comes to trying to deal with governments that want it all. Energy efficient homes and rental units but do not want us to use anything modern. Oddly, not one inspector has ever commented negatively on the changes we made and in the last 25 years all I have received is complements on how well I maintain my property. They ask about the vinyl siding and I tell them the truth, it was sided before I bought it.

    • Not arguing your point here, but the web page is only giving consumers a rough estimate, which is the best anyone can do on their own before taking your advice to get a more accurate calculation by a service which unfortunately are not always honest. My advice is to do the general estimate first, THEN get an estimate from a service and if the numbers are off significantly, get another estimate from another service and if they are both off, chose one and INTERROGATE that service until all discrepancies make sense to you.

      • Agreed, these are rules of thumb…Im in KC…notice that in Missoui its green, and in Kansas its Yellow…so I teach my guys that the total heat required has many more factors that shoudl be consulted with by a professional engineer…

  2. Love goe your map lists places in a Zone 8, most of interior Alaska, but then the calculator, nor the rest of the page, doesn’t have data for zone 8, only 7.

    • Hello Gary, you’re right, the is a very limited number of people that live in Zone 8. We will include it right away, thank you for the heads up.

  3. Used the calculator to determine that i need 101,250 btu’s “required heating output in winter”. Got that, but is that per hour, day, week, once I get the temp there and just to maintain? If I use the formula and plug in kerosene cost per hour I,m spending over $2000 per month. What am I not getting?

    • Hello Thomas, thank you for a very good question. It’s the maximum BTUs per hour. Now, if you use kerosene and would like to estimate your heating costs, here’s a way how you can do that:

      1 gallon of kerosene contains 131,890 BTUs of energy. Kerosene cost per gallon can vary quite a bit; let’s say $2.00/gallon is the average. You would need 101,250 BTU or 101,250 BTU / 131,890 BTU = 0.77 gallon of kerosene or $1.44 per hour maximum. Realistically, however, when the set temperature is met, the heating output of your furnace will fall to 20-60% and you will see a reduction of this cost.

    • The issue is that the post is using BTU and BTUh interchangeably which they are not.
      Heating equipment is rated in BTUh (not BTU) as it is an output of heat over a period of time, whereas units of fuel are rated in BTU (total heat available within that unit of fuel).
      The calculator above is calculating a rough BTUh (although it is listed as BTU). what you would be missing in the calculation to convert to fuel use, would be the heating hours to multiply their listed BTU by to get BTUh. Once you have that BTUh, you would divide that by the BTU of the fuel (133,500 for Kerosene)

      • Hi Joe, yes, BTU and BTU/h are often used interchangeably in HVAC, but as you have correctly pointed out, they are definitely not the same. BTU itself is heat content while BTU/h is heat flow. We hope that our readers will be able to distinguish between the two.

    • Hello Charlene, that seems like quite a complex sunroom to calculate the heating BTU requirements for. In this case, calling an HVAC expert for on-site estimation seems like a good choice.

  4. A lot of homes are have rooms with undersized convection. Is there a chart that will tell us how many BTU’S are needed for a 5degree day compared to a 30 degree day in order maintain a set temp of say 70 degrees

    • Hello Dave, that would be a very useful chart. As far as we know, however, it doesn’t exist. There are too many factors to adequately quantify the BTUs here; the primary would be insulation (R-Values).

  5. Is the basement area included? We use the basement. The boiler, water heater, laundry, and supply cabinets are there. And I use my computer there. It’s always “open” to the ground floor with a stairwell. There’s no provision for heating it directly, but it doesn’t feel cold in the wintertime. Shouldn’t it be included in the calculation?

    • Hello George, we can usually tolerate lower temperatures in the basement as in the living spaces. If you want to adequately heat the basement, you would need quite a lot of heating output; 30 BTU per sq ft or more. In your situation, you should include the basement square footage in the calculation, and you will get a pretty good estimate of how much heating output you need.

  6. So lets say i have a 300 square foot cabin in zone 6. This says i need about 16,700 btu to heat that size area. My question is do i need 1 heater that will do 16,700 btu per hr. Or could i have say 3 separate units that do 5,600 btu ea. im thinking of installing 3 base board heaters that do 5,600 btu ea. Just not sure if this makes a difference.

    • Hello Jeremy, you just need a total heating output of 16,700 BTU. You can do that with 1 or many units. A standard 1,500W space heater generates about 5,100 BTU, for example. 3 of them should do the trick. 3 baseboard heaters will work as well.

  7. I live in Georgetown CA 95634, in a granny flat 1000sf / 2 story. I’m replacing my pellet stove with a freestanding gas fireplace. I’ve found a Trenton 24,000 BTU/hr, will this be adequate to heat my place. I don’t like it hot, I prefer to layer and my place is very well insulated. Any input would be greatly appreciated.
    Thank you for your time,

    • Hello Gerri, BTU-wise it would be best for the replacement freestanding gas fireplace to have about the same BTUs as the pellet stove. These calculations are theoretical in nature, with a lot of assumptions about insulations and so on. Practically, the best test for how many BTU of heating you need is just to try a heater out. Well, with pellet stove installed, you pretty much know how well does it heat the space up. If you’re happy with that heating output, you should get the same-BTU gas fireplace. Hope this helps.

  8. I have a special situation. I need to heat an open rectangular room up to 120º F. Dimensions are 42′ x 12′ x 9′(ceiling), total 504 sq. ft. or 4,536 cu. ft. I have to use electric (no gas in to the building). Approx what size BTU heater will I need?

    • Hi James, heating to such a higher temp (120°F) depends heavily on insulation. Now, the basic 1500W heaters produce about 5200 BTU of heating output. For a 504 sq ft room with a 9 ft ceiling, you would usually need at least 34 BTU per sq ft. To raise the temp to 120°F, you would need… blank. It’s really really hard to say without the insulation R-values. But let’s say you would need 60 BTU per sq ft; that would be about 30,000 BTU or 6 1500W space heaters. Hope this helps a bit.

  9. Hello,
    I’m confused by this. I live in zone 5, my house is 1100 sf. The calculator says I need 55,000 btus. Is this the output required ? Then how do I calculate which size furnace to buy if it is an 80% efficient furnace? Would a 60,000 BTU rated furnace be sufficient?

    • Hi Joan, here it is important to understand furnace input vs furnace output. The specified BTU heating capacity (60,000 BTU for example) is always the output. To produce that output, the furnace will need a larger than 60,000 BTU input (since no gas furnace is 100% efficient).
      Example: A 60,000 BTU 80% efficient (80 AFUE) furnace will produce 60,000 BTU output. It will require a 75,000 BTU input since it is 80% efficient.
      Hope this makes sense now.
      In your case, the 60,000 BTU furnace would be a reasonable choice.

  10. I have a warehouse 7,800 sq.ft. and 21 ft. high. Will 6-25kw Space Heaters be right?
    I had figured 6 Units but it’s confusing. Any help will be great and much appreciated.

    • Hi Morgan, warehouses are a different matter. This is primarily for living spaces; warehouses tend to have not-as-good insulation (lower insulation R-values), high ceilings (plenty of air to heat) and usually don’t need to be heated up to 72°F. To adequately determine the heat load needed (in BTU or kW), you should consult a local hands-on HVAC expert; they do special Manual J calculations when it comes to warehouses.

      We can play around with the calculations, of course. If we would presume the same premises that are used for heating living space, here is how this calculation would go: We have a 7,800 sq ft warehouse with 21 ft high ceiling (that’s 162.5% higher than standard 8 ft ceiling height). That means that, instead of 20 BTU per sq ft for 8 ft ceiling height, we need to use 52.5 BTU per sq ft. That’s 7,800 sq ft × 52.5 BTU/sq ft = 409,500 BTU or, if we convert these BTU to kW with this calculator, 120 kW. Practically, that is way more than what you actually need.

  11. This calculation does not allow for what temperature you are trying to reach. I was interested in how much energy I would use if I kept my vacant apartment set at 50 degrees versus 70 degrees.

    • Hi Jack, those calculations are rather complex (many variables, non-linear temperature-energy correlation). In general, the DOE says that lowering the temperature by 1 degree Fahrenheit should reduce the heating bill by about 3%. Hope this gives you a bit more insight into what temperature to set in your vacant apartment.

  12. Hello! I live in Minnesota and was recently sold a 40,000 BTU 2 Stage 96% eff. Furnace for a 1,560 sq. Ft home was built in 1953.

    I am being told by my family and their HVAC guy, along with EVERY google search, that a 40K BTU unit is well undersized. The company is standing its ground and insisting that the next size up (60K) would be oversized and would be flagged by the city inspector.

    I’m curious if they are feeding me a lot of hooey and my Furnace is in fact undersized for my climate and house age.

    • Hi Jonathan, a 60K furnace would be flagged by the city inspector? That is a rich one. 🙂
      Alright, let’s calculate a bit: 1,560 sq ft house, let’s presume the minimal 30 BTU per sq ft (that’s usually for South US, Minessotta is much colder, but let’s just try this out). That would yield a heat load of 46,800 BTU minimum. Now, this is Minessota; theoretically, you would need a lot more than 30 BTU per sq ft. So, the 60,000 BTU furnace is obviously a much more suitable choice than 40,000 BTU.
      In short, no one would bat an eye if you have a 60k furnace in Minessota (1,560 sq ft house, built in 1953 = subpar insulation, to top it off). What everybody should bat an eye at is 40k furnace for this house.
      As the calculations have shown, your family and the HVAC guy is in the right here. The company that installed the furnace… well, it seems they have a vested interest here. If they know a thing or two about furnace sizing – and they should know a lot more than just a bit – they are defending this 40k furnace knowing it’s quite undersized. Sadly, this kind of thing happens. Hope you’re going to resolve this before the next winter, best of luck!

  13. With a 500 sq. ft. home in Zone 2 I get 17,500 Btus to hear the home.
    Is the 17,500 the furnace rating? Or is is how may Btus are needed per hour or per day?

    • Hi David, that’s a good question; we need to clarify this. The 17,500 BTUs is the furnace capacity rating, not BTUs needed per time unit (hour, day). Hope this makes things clear.


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