BTU Calculator
The air conditioner capacity a room needs, in BTU, tons and kilowatts.
How the sizing works
The rule of thumb is about 20 BTU/h per square foot with 8-foot ceilings, scaled for ceiling height, plus 10% for sunny rooms (minus 10% for shaded), 600 BTU per person beyond two, and 4,000 BTU for kitchens. Results land near the EnergyStar room-AC chart: a 15 × 12 bedroom needs roughly a 5,000 to 6,000 BTU unit, a sunny 25 × 20 living room around 12,000 (one ton).
Bigger is not better
An oversized AC cools the air fast and shuts off before it dehumidifies, leaving the room cold and clammy while short-cycling itself to an early death. If you're between sizes, take the smaller one unless the room is sunny or crowded. Poor insulation, big window area and top-floor rooms can justify 10 to 20% more; a whole-house system deserves a proper Manual J calculation, not a rule of thumb.