BTU Calculator: Air Conditioner Size for a Room
Find the cooling BTU per hour, tons and kilowatts a room needs from its size, insulation, sun, occupants and kitchen. Add a temperature rise to get a heating load too.
The first two are free; each extra person adds 600 BTU/h.
How many degrees to warm the room. Blank hides the heating load.
Enter the room length, width and ceiling height.
About the BTU calculator
The BTU Calculator estimates how many BTU per hour a room needs for cooling, and optionally for heating, from its size and a few conditions. Enter the length, width and ceiling height, each in its own unit (feet, inches, yards, metres, centimetres or millimetres), then choose the insulation quality, how much sun the room gets, how many people use it and whether it includes a kitchen. The result appears in BTU per hour, in tons of cooling and in kilowatts.
If you also enter a temperature rise, the calculator estimates a heating load as well. The rise is the difference between the outdoor design temperature and the indoor temperature you want, in degrees Fahrenheit or Celsius.
This is an estimate from rules of thumb. It is a good starting point for comparing window units, portable units and mini splits, but it is not a substitute for a professional load calculation.
How to calculate BTU
Steps
- Enter the room length, width and ceiling height, and pick a unit for each. Units can differ from one field to the next.
- Choose the insulation: poor, average or good.
- Choose the sun exposure: shaded, average or sunny.
- Enter the number of occupants. The default is 2.
- Tick room includes a kitchen if appliances add heat to the space.
- Optionally enter a heating temperature rise and pick Fahrenheit or Celsius.
- Read the cooling load, and the heating load if you gave a temperature rise.
Formulas
Cooling load
The base load is 20 BTU/h for every square foot of floor area. It is then scaled for ceiling height, insulation and sun, and increased for extra people and a kitchen:
Here is length times width in square feet, is the number of occupants, and is 1 with a kitchen and 0 without. The height factor is the ceiling height divided by 8 ft when the ceiling is higher than 8 ft, and 1 otherwise. Lower ceilings are not reduced. The insulation factor is 1.1 for poor, 1.0 for average and 0.9 for good. The sun factor is 0.9 for shaded, 1.0 for average and 1.1 for sunny, so shade lowers the load and strong sun raises it.
Tons are BTU/h divided by 12,000, and kilowatts are BTU/h multiplied by 0.00029307107.
Heating load
is the room volume in cubic feet and is the temperature rise in degrees Fahrenheit. A rise given in Celsius is multiplied by 1.8 first, so 10 °C counts as 18 °F.
Worked examples
Bedroom. A room 15 ft by 12 ft with an 8 ft ceiling, average insulation, average sun, 2 occupants and no kitchen has 180 square feet. The load is 180 x 20 = 3,600 BTU/h, which is 0.30 ton or 1.06 kW.
Sunny kitchen-diner. A poorly insulated room 20 ft by 15 ft with a 9 ft ceiling, 4 occupants and a kitchen faces strong sun. The area is 300 square feet, so the base is 6,000 BTU/h. The height factor is 9 / 8 = 1.125, giving 6,750. Poor insulation (1.1) gives 7,425, and sunny (1.1) gives 8,167.5. Two extra occupants add 1,200 and the kitchen adds 4,000, for a total of 13,368 BTU/h, or 1.11 tons and 3.92 kW.
Heating. The 15 x 12 x 8 ft room holds 1,440 cubic feet. With a 30 °F rise the load is 1,440 x 30 x 0.133 = 5,746 BTU/h, or 1.68 kW.
Reading the result
Window air conditioners commonly come in sizes from about 5,000 to 12,000 BTU/h, and one ton of cooling equals 12,000 BTU/h. Compare your result with the rating on the unit's label. Rounding up to the next available size is normal, but a unit that is 10 to 20 percent oversize can cool the room too quickly, switch off before it removes much moisture, and short-cycle. For a room-by-room view of unit sizes, see the Air Conditioner Size Calculator. The Temperature Converter helps if your design temperatures are in a different scale.
Accuracy and limits
The calculator does not model climate zone, window area and glazing, air leakage, duct losses or the heat from lighting and equipment. A Manual J load calculation by a qualified contractor is the right method for sizing HVAC equipment. Use this tool to get a sensible range, not a final specification.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
To measure the floor first, use the Area Calculator. For plain unit changes, the Length Converter converts room dimensions between feet, metres and inches.
Frequently asked questions
How is the cooling BTU calculated?
The base is 20 BTU/h per square foot of floor area. That is multiplied by a ceiling height factor (height divided by 8 ft when above 8 ft), an insulation factor (1.1 poor, 1.0 average, 0.9 good) and a sun factor (0.9 shaded, 1.0 average, 1.1 sunny). Then 600 BTU/h is added for each occupant beyond two, and 4,000 BTU/h if the room includes a kitchen.
Is this the same as a Manual J load calculation?
No. This is an estimate from rules of thumb. A Manual J load calculation by a contractor accounts for climate zone, window area and glazing, air leakage and duct losses, which this tool does not model. Use a Manual J calculation when sizing heating or cooling equipment.
Does a lower ceiling reduce the result?
No. The height factor only applies when the ceiling is above 8 ft, where it is the height divided by 8. A 7 ft ceiling uses a factor of 1, so the load is never reduced below the floor-area figure.
How is the heating load worked out?
Enter a temperature rise. The calculator multiplies the room volume in cubic feet by the rise in degrees Fahrenheit, by 0.133 and by the insulation factor. A rise in Celsius is multiplied by 1.8 first, so 10 °C counts as 18 °F. A 15 x 12 x 8 ft room with a 30 °F rise needs about 5,746 BTU/h.
How do BTU/h, tons and kilowatts relate?
One ton of cooling is 12,000 BTU/h, and one BTU/h is 0.00029307107 kW. A load of 3,600 BTU/h is 0.30 ton or 1.06 kW. The calculator shows all three so you can match a spec sheet in any of these units.
Why can an oversized unit be a problem?
A unit that is 10 to 20 percent too large can reach the set temperature quickly and switch off before it has removed much humidity. This is called short-cycling. It can leave the room cool but clammy and wear the compressor faster.
Is my data sent anywhere?
No. The BTU sums run in your browser, so your room dimensions and settings are never uploaded or stored on a server. They stay on this device and are not shared with anyone.