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, 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:
- What size furnace do I need for a 1000 sq ft house? (Example 1)
- What size furnace do I need for a 2000 square foot home? (Example 2)
- 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.
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:
Table of Contents
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).
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 as homes in the far South (Texas, for example). We have elaborated on winter heating requirements (in BTU) here.
For an exact estimation of furnace size, you should ask your local furnace installers.
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:
- Home square footage: 1,000 sq ft.
- Climate zone: Los Angeles, California, is in Region 2.
We input both of these data points into the calculator. Here is what we get:
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:
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:
- Gas furnaces (low running costs, powerful furnaces).
- Oil furnaces (powered by heating oil).
- Electric furnaces (low unit cost, high running costs, most convenient 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).
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.
Completely agree
Do you count the basement in the square foot if it is heated too?
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.
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.
Hello Dan, you’re right, HVAC people prefer to speak about tons. The ton to BTU conversion is fairly easy; 1 ton equals 12,000 BTU. So, when an HVAC guy speaks about 3-ton furnace, that’s 3*12,000 BTU = 36,000 BTU heating output.
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.
Don’t you have to factor in the efficiency of the furnace?
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.
does the hieght of the ceilings make a difference I have 10 feet ceilings in a 1000 sq ft home in region !
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.
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.
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.