BTU Calculator
Size heating or cooling BTUs for a room, adjusted for climate, insulation and sun.
The room
8 ft is standard
+600 BTU each past two
This estimates the BTU capacity needed to heat or cool a space, from its size and conditions.
It adjusts for ceiling height, sun exposure, insulation and occupancy, all of which change the answer.
How BTU requirements are worked out
A BTU, or British Thermal Unit, is the energy needed to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTU per hour, which is a rate rather than a quantity.
The baseline rule of thumb is 20 BTU per square foot for cooling. A 500 sq ft room needs about 10,000 BTU/hr; a 1,500 sq ft space needs about 30,000.
The critical thing, and the opposite of most people's instinct: oversizing is a real problem, not a safe margin. An oversized air conditioner cools the air quickly, shuts off, and never runs long enough to pull moisture out. The result is a room that is cold and clammy, with the unit short-cycling and wearing out faster.
Adjustments are additive on top of the square-footage baseline. One ton of cooling is 12,000 BTU/hr, which is where AC 'tonnage' comes from.
What to enter
- Room size in square feet
- Length × width. The starting point for the whole calculation.
- Ceiling height
- The rule assumes 8 ft. Higher ceilings mean more air volume, so add roughly 10% for each extra couple of feet.
- Sun exposure
- A very sunny room needs about 10% more; a heavily shaded one about 10% less.
- Room type and occupancy
- Kitchens need about 4,000 BTU extra for appliance heat. Add about 600 for each person beyond two.
- Insulation and climate
- Poor insulation and extreme climates both push the requirement up substantially.
Units and conversions
- 1 ton of cooling
- 12,000 BTU/hr. This is where air conditioner 'tonnage' comes from.
- 1 kW
- About 3,412 BTU/hr. Used for electric heating and outside the US.
- SEER
- Seasonal Energy Efficiency Ratio. Higher is more efficient, and it does not change the BTU you need.
- Heating vs cooling
- Heating typically needs more BTU per square foot than cooling, 30-60 depending on climate.
What this assumes
The 20 BTU per square foot rule assumes 8 ft ceilings, average insulation and a temperate climate. All three shift the answer.
This is an estimate. For whole-home HVAC, a professional Manual J load calculation accounts for windows, orientation, air sealing and local climate data.
How to calculate the BTU rating you need
Start from square footage, then apply the adjustments that actually apply to your room.
- × 20
- The cooling baseline for 8 ft ceilings
- adjustments
- Sun, ceiling height, kitchen use and occupancy
Measure the space. Length × width for square footage. For open-plan areas, include everything the unit has to condition.
Multiply by 20. 500 sq ft × 20 = 10,000 BTU/hr as a starting point.
Apply the adjustments. Add 10% for a very sunny room, subtract 10% for heavy shade, add 4,000 for a kitchen, add 600 per person beyond two.
Do not round up generously. Choose the nearest standard size, not the next size up. Oversizing causes short-cycling and poor humidity control.
See a worked example: a sunny 500 sq ft living room
- Size
- 500 sq ft, 8 ft ceilings
- Conditions
- Very sunny, usually three occupants
Baseline: 500 × 20 = 10,000 BTU/hr.
Sunny room: +10% = 11,000.
Third occupant: +600 = 11,600 BTU/hr.
So a 12,000 BTU unit, which is one ton. A 15,000 BTU unit would cool the room faster and dehumidify it worse.
About 11,600 BTU/hr, so a 12,000 BTU unit
Frequently asked questions
About 20 BTU per square foot for cooling, assuming 8 ft ceilings and average insulation. Heating usually needs more, in the range of 30-60 depending on climate.
So a 500 sq ft room needs around 10,000 BTU/hr before adjustments for sun, ceiling height and occupancy.
No, and this is the most common and most consequential mistake. An oversized unit cools the air fast, shuts off, and never runs long enough to remove humidity.
You end up with a room that is cold and clammy, a unit that short-cycles, higher energy use and shorter equipment life. Correctly sized beats oversized every time.
One ton equals 12,000 BTU/hr. A 3-ton unit is 36,000 BTU/hr.
The term is historical: it comes from the cooling that a ton of ice provides as it melts over 24 hours. It has nothing to do with the unit's weight.
1 kW is about 3,412 BTU/hr. So a 12,000 BTU unit is roughly 3.5 kW of cooling capacity.
That is the cooling output, not the electricity it consumes. A unit's power draw is much lower, and how much lower is what SEER measures.
Yes. The rule assumes 8 ft ceilings, and higher ceilings mean more air volume to condition.
Add roughly 10% for each additional couple of feet. A vaulted ceiling can add considerably more, since warm air stratifies at the top.
For a window unit or a single room, this estimate is fine. For whole-home HVAC, a professional Manual J load calculation is worth it.
Manual J accounts for window area and orientation, insulation values, air infiltration and local climate data. Getting whole-home sizing wrong is expensive and hard to undo.
Problems people actually run into
Sizing up for a safety margin
Choosing 15,000 BTU when the calculation says 11,600 feels prudent and makes the room worse. The unit reaches temperature too quickly and shuts off before dehumidifying.
Cold and clammy is the signature of an oversized system. Pick the nearest standard size, not the next one up.
Using square footage alone
A sunny top-floor kitchen with poor insulation and a 700 sq ft shaded basement can differ by 50% in requirement at the same square footage.
Apply the adjustments. Sun exposure, ceiling height, appliance heat and insulation all move the answer meaningfully.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026