BTU calculator for heating and cooling
Use this BTU calculator to find how much heating and cooling a house needs, in BTU per hour and in tons.
Where the heating comes from
Where the cooling comes from
What a BTU is
A BTU is a small bit of heat. BTU/h is how much heat moves in one hour. A heater or an AC unit is sized in BTU/h. One ton of cooling is 12,000 BTU/h.
The math in plain words
Heat moves through walls, the roof, the floor and the windows. It also rides in on air that leaks in. For each part we multiply its size by how well it holds heat back, then by how far apart the inside and outside temperatures are.
For cooling we also add the sun that comes through the windows and the heat from people and appliances. We check each hour of a hot day and keep the worst one.
What your three choices do
You pick insulation and sun. We turn each pick into the numbers below. The glass is always 12 percent of the floor area. Sun decides which sides it sits on.
| Insulation | Walls | Ceiling | Windows | Air leaks |
|---|---|---|---|---|
| Poor | R-7 | R-11 | Single pane | Loose |
| Average | R-13 (2x4) | R-30 | Double, clear | Average |
| Good | R-19 (2x6) | R-49 | Double, low-E | Tight |
Shaded puts less glass on the south and west and treats it as partly shaded. Sunny puts more glass on the south and west with no shade. Normal splits it evenly. The ducts run through the attic. The floor sits over a crawl space.
Why one number per square foot fails
A rule like "this many BTU per square foot" ignores your weather, your windows and your ducts. A small house with a lot of west glass can need more cooling than a bigger shaded one. Use the result here as a starting point, then check it with the app or a full design.
Get the full version in the app
The web page gives a fast estimate. The iPhone app does the whole job.
- Every window, wall and duct, not just six choices
- Load split by room with CFM and duct sizes
- A PDF report with your logo
- Works with no signal. Your jobs stay on your phone.
Questions people ask
How many BTU do I need per square foot?
It depends on your weather, your insulation and the sun on your windows. Two houses with the same floor area can need very different BTU/h. Enter your ZIP code above instead of using one number for every home.
How do I change BTU/h into tons?
One ton of cooling is 12,000 BTU/h. Divide the cooling BTU/h by 12,000. A house that needs 24,000 BTU/h needs 2 tons.
Why are the heating and cooling numbers different?
Heating fights the cold air outside on the coldest night. Cooling fights the hot air, the sun through the glass and the moisture in the air on the hottest afternoon. They use different weather, so the two numbers rarely match.
How close is this estimate?
It is a good first number from a real heat flow method. It uses six choices, not a full survey of your house. For a final size, use a full room by room design.
More free tools
An estimate in the style of ACCA Manual J. It does not replace a certified room by room design. Manual J is a trademark of ACCA. HVAC Load is not affiliated with ACCA.