BTU Calculator
Estimate room heating and cooling BTU from size, insulation, and climate
Estimate the heating and cooling BTU for one room from floor area, ceiling height, climate, and insulation. This is a sizing rule of thumb, not an ACCA Manual J load calculation. To turn BTU into watts, use the power converter. To price the electricity, use the electricity cost calculator.
Calculator Inputs
The base factors assume an 8 ft ceiling
Results
Enter values above to see your results
Room-size estimate only. It is not an ACCA Manual J load calculation or an equipment specification.
How to use this calculator
- Enter length, width, and ceiling height.
- Choose the climate and how well the room is insulated.
- Add the number of people usually in the room, and whether the windows get strong afternoon sun.
How the calculation works
Cooling starts at 20, 25, or 30 BTU per square foot for a cool, moderate, or hot climate. Heating starts at 60, 45, or 30 BTU per square foot for those same climates, because a cool climate needs more heat and a hot climate needs more cooling. Both are scaled by ceiling height ÷ 8.
Poor insulation multiplies the result by 1.15. Good insulation multiplies it by 0.85. Afternoon sun adds 10% to cooling only. Each person beyond two adds 400 BTU to cooling only, because people add heat to the room.
A 15 by 12 foot room
At an 8 foot ceiling in a moderate climate with average insulation and two people, cooling is 180 × 25 = 4,500 BTU per hour. Heating is 180 × 45 = 8,100 BTU per hour. A window unit is often sold at the next size up from the cooling number.
Worked example
15 ft × 12 ft, 8 ft ceiling, moderate climate
15 × 12= 180 sq ft180 × 25= 4,500 BTU/h180 × 45= 8,100 BTU/hBTU per square foot at an 8 ft ceiling
Before insulation, sun, people, and ceiling-height adjustments.
| Climate | Cooling | Heating |
|---|---|---|
| Cool | 20 | 60 |
| Moderate | 25 | 45 |
| Hot | 30 | 30 |
Tips & Best Practices
- •Measure the room you are conditioning, not the whole house, if you are sizing a window unit.
- •A kitchen or a room full of electronics needs more cooling than this estimate.
Frequently Asked Questions
How many BTU do I need to cool a room?
A moderate climate starts near 25 BTU per square foot at an 8 foot ceiling. A 180 square foot room is about 4,500 BTU before sun and extra people.
Is this the same as Manual J?
No. Manual J uses insulation levels, windows, orientation, and duct losses. This page is a room-size estimate for a first look at equipment size.
Why is heating different from cooling?
A cool climate needs more heat per square foot and less air conditioning. A hot climate is the reverse. The two factors are paired that way on purpose.
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