How BTU sizing works
A BTU (British thermal unit) is the amount of heat needed to raise one pound of water by one degree Fahrenheit. Air conditioner and heater capacity is given in BTU per hour: how much heat the unit can remove from, or add to, a room every hour.
The starting point for cooling is floor area. ENERGY STAR publishes a chart that matches room size to capacity for a standard 8 ft ceiling. Small rooms need more BTU per square foot than large ones, which is why a simple "20 BTU per square foot" rule under-sizes bedrooms and over-sizes big open spaces.
The four adjustments that matter
- Sun: add 10% if the room is very sunny, take off 10% if it is heavily shaded.
- People: each person gives off heat. Add 600 BTU for every person above two who regularly uses the room.
- Kitchen: stoves and ovens add a lot of heat, so add 4,000 BTU.
- Ceiling height: taller rooms hold more air. We scale the result in proportion to the height above 8 ft.
Why bigger is not better
An oversized air conditioner reaches the thermostat setting fast and shuts off before it has pulled enough moisture out of the air. The room ends up cold and clammy, and the constant on-off cycling wastes energy and wears the compressor. An undersized unit runs nonstop and never quite catches up on hot afternoons. Buy the size the calculator gives you, rounded up to the next unit on the market.
Buying a portable air conditioner? Look for the SACC rating (Seasonally Adjusted Cooling Capacity). Since 2017 the U.S. Department of Energy has required it, and it is usually several thousand BTU lower than the older ASHRAE number some boxes still print in big type. Compare the SACC figure to the number above.
Heating: BTU by climate zone
For heating, contractors use a rule of thumb in BTU per square foot that depends on how cold your winters are: about 30 to 35 in hot climates and 50 to 60 in cold ones. Poor insulation pushes the number up, a tight, well-insulated home brings it down. Switch the calculator to Heating to apply it, or use the heating BTU calculator for furnaces and space heaters.
To compare with electric heaters: one watt equals 3.412 BTU per hour, so a standard 1,500 W space heater puts out about 5,118 BTU per hour.
BTU, tons and kilowatts
Central air conditioners and heat pumps are sold in tons. One ton of cooling is 12,000 BTU per hour, so a 2-ton system delivers 24,000 BTU per hour. Outside the U.S., capacity is usually given in kilowatts: 1 kW is 3,412 BTU per hour. The calculator shows all three.
This tool is built for one room or one open space. For a whole-house central system, ask for a Manual J load calculation: it accounts for windows, insulation, air leaks and ducts, and it is what HVAC professionals use to size equipment.