BTU Calculator Guide: Sizing an AC Unit or Heater
How BTU sizing actually works for a room air conditioner or heater, why bigger isn't always better, and the standard sizing table behind the numbers.
BTU (British Thermal Unit) measures how much heat a cooling or heating unit can move per hour. Getting the number right matters more than most people expect — both too small and too large cause real problems.
Where the base number comes from
Rather than a flat rate per square foot, this calculator uses the standard Energy Star / DOE room air conditioner sizing table, which reflects that required BTU per square foot drops as rooms get larger — a small room needs proportionally more cooling power than a large one.
What gets added on top
- Sun exposure: a heavily shaded room needs 10% less, a very sunny one needs 10% more.
- Occupants: each person beyond the first two adds 600 BTU, since bodies generate heat too.
- Kitchen: a flat 4,000 BTU for cooking-appliance heat.
Why oversizing backfires
A unit sized too large cools the air quickly and shuts off before it's run long enough to pull humidity out of the room — leaving it cold but clammy, and cycling on and off more often, which wears out the compressor faster than steady operation would.
The BTU Calculator runs the full table and every adjustment for you.