Room BTU Calculator

The ENERGY STAR sizing table

Room air conditioners, window units, through-the-wall units and portable units, are sized in British thermal units per hour of cooling. The most widely used method is the ENERGY STAR table, which starts from floor area: 5,000 BTU/h for up to 150 ft², 6,000 for 150 to 250 ft², 8,000 for 300 to 350 ft², 10,000 for 400 to 450 ft², 12,000 for 450 to 550 ft² and so on up to 34,000 BTU/h for 2,500 ft². These figures assume an 8 ft ceiling, two occupants and average sun. A 200 ft² bedroom needs a 6,000 BTU/h unit; a 320 ft² living room starts at 8,000.

Adjustments that matter

The base figure is then adjusted. A heavily shaded room needs 10% less; a very sunny room 10% more. Each regular occupant beyond two adds 600 BTU/h, about the heat a person gives off while active. A kitchen adds 4,000 BTU/h for cooking appliances. Ceilings higher than 8 ft increase the volume of air to cool, so a common rule scales the load in proportion. A 320 ft² living room with big west-facing windows and three regular occupants needs 8,000 × 1.1 + 600 = 9,400 BTU/h, so a 10,000 BTU/h unit is the right purchase. A 500 ft² kitchen-diner needs 12,000 + 4,000 = 16,000 BTU/h.

Bigger is not better

The most common mistake is buying a unit that is too large on the assumption that it will cool faster. An oversized unit does cool the air quickly, but it then switches off before it has run long enough to remove moisture. Air conditioners dehumidify by condensing water on a cold coil, and that takes time. The result is a room that feels cold and clammy, with more on-off cycling, more wear and higher bills. Within the ENERGY STAR band, choose the nearest size up, not two sizes up.

Tons, kilowatts and whole houses

One ton of cooling is 12,000 BTU/h, the heat needed to melt a ton of ice in a day. A 9,400 BTU/h requirement is 0.78 tons or 2.75 kW of cooling. Whole-house central systems should not be sized from floor area at all. In the United States the standard is an ACCA Manual J calculation, which accounts for insulation, windows, orientation, air leakage and local design temperatures; rules of thumb such as 500 ft² per ton routinely oversize systems in well-insulated modern homes.

How to use it

1. Enter the floor area of the room in square feet or square metres and the ceiling height.
2. Choose the sun exposure: heavily shaded, average or very sunny.
3. Enter the number of people who regularly use the room and whether it is a kitchen.
4. Read the cooling load and the nearest common unit size, and avoid sizing up further.