How Many BTU Do I Need?
Find your room's floor area in the table below for a base BTU/hr figure, published by Energy Star for sizing a room air conditioner. That base figure assumes ordinary sun exposure, no kitchen use, and up to two occupants — for a room that's sunny, shaded, a kitchen, or more crowded than that, the BTU calculator applies the published adjustments for you automatically. This page is the lookup table itself, plus a worked example for sizing several separate room units across one home.
Energy Star room air conditioner sizing table
This is the published base table room-AC capacity is sized from — find the first row whose area covers your room.
| Room area | Base capacity needed |
|---|---|
| Up to 150 sq ft | 5,000 BTU/hr |
| Up to 250 sq ft | 6,000 BTU/hr |
| Up to 300 sq ft | 7,000 BTU/hr |
| Up to 350 sq ft | 8,000 BTU/hr |
| Up to 400 sq ft | 9,000 BTU/hr |
| Up to 450 sq ft | 10,000 BTU/hr |
| Up to 550 sq ft | 12,000 BTU/hr |
| Up to 700 sq ft | 14,000 BTU/hr |
| Up to 1,000 sq ft | 18,000 BTU/hr |
| Up to 1,200 sq ft | 21,000 BTU/hr |
| Up to 1,400 sq ft | 23,000 BTU/hr |
| Up to 1,500 sq ft | 24,000 BTU/hr |
| Up to 2,000 sq ft | 30,000 BTU/hr |
| Up to 2,500 sq ft | 34,000 BTU/hr |
US EPA / DOE Energy Star, Room Air Conditioner Buying Guidance (sizing table) publishes this table directly; the calculator implements it verbatim. From there, adjust the base figure: add 10% for a very sunny room, subtract 10% for a heavily shaded one, add 4,000 BTU flat for kitchen use, and add 600 BTU for each occupant beyond two. The calculator's FAQ covers why each adjustment works the way it does, and what to do when your result lands between two retail unit sizes.
A full example: sizing three rooms in one apartment
Central air isn't always an option, and a common real question is how to size several separate window or through-wall units for one home rather than a single room. Take a small apartment with three rooms, each getting its own unit: a 200 sq ft living room with ordinary sun, a 150 sq ft bedroom with ordinary sun, and a 120 sq ft sunny kitchen. The living room falls in the "up to 250 sq ft" band, 6,000 BTU/hr, with no adjustments. The bedroom falls in the "up to 150 sq ft" band, 5,000 BTU/hr, also with no adjustments. The kitchen also falls in the "up to 150 sq ft" band — base 5,000 BTU/hr — but it's sunny, adding 10% (5,500), and it's a kitchen, adding a flat 4,000 BTU (9,500 BTU/hr total). Each room needs its own correctly sized unit rather than one average figure — buying three 7,000 BTU units because that's the rough average across the three rooms would undersize the kitchen and oversize the bedroom. Summed, the apartment needs about 20,500 BTU/hr of total cooling capacity across three separate units, which is also a useful figure for estimating the combined electrical load and running cost, even though each unit is bought and installed separately.