How the BTU estimate works
The rule of thumb starts at 20 BTU per hour for every square foot of floor, which assumes a ceiling about 8 ft high. The room area is the length multiplied by the width. A room that is very sunny gets 10% more, a heavily shaded room gets 10% less, and a normal room stays as it is. Each person beyond two adds 600 BTU per hour, and a kitchen adds 4,000. The total is the cooling load, and one ton of cooling is 12,000 BTU per hour. For a 12 × 15 ft room the area is 180 ft², so the base is 180 × 20 = 3,600 BTU/h. Very sunny raises it to 3,960, four people add 1,200 and a kitchen adds 4,000, giving 9,160 BTU/h, or about 0.76 tons.
What changes the number
The estimate assumes a typical ceiling, normal insulation and average windows. A room with a high ceiling, little insulation or a large west-facing window will need more cooling than the formula shows, because those features are not separate inputs. It is better to be a little over than under, but an oversized unit has its own problem: it cools the air quickly and then shuts off before it has removed much moisture, so the room can feel cold and clammy. Buy the nearest unit size at or above the result rather than the largest one available. For central air or a whole-house system, a contractor should do a proper room-by-room load calculation instead of relying on this figure.
Cooling only
This page sizes cooling, not heating. It does not size heaters or furnaces, and it should not be used for a heat pump or boiler, because heating needs depend on the climate and the building in ways floor area alone cannot show. Treat the result as a starting point for a single room air conditioner and check the label on the unit you are considering.
Frequently asked questions
What size air conditioner for a 15 × 20 ft room?
The area is 300 ft², so the base is 300 × 20 = 6,000 BTU/h. With normal sun, two people and no kitchen nothing is added, and 6,000 ÷ 12,000 = 0.5 tons.
How much does a kitchen add to the estimate?
A kitchen adds 4,000 BTU/h. A 12 × 15 ft room that is very sunny with four people is 3,960 + 1,200 = 5,160 BTU/h without the kitchen, and 5,160 + 4,000 = 9,160 BTU/h with it.
Does a shaded room need less cooling?
Yes. A heavily shaded room takes 90% of the base figure. A 20 × 20 ft room is 400 ft², so the base is 8,000 BTU/h and the shaded figure is 8,000 × 0.90 = 7,200 BTU/h, which is 0.6 tons.