BTU Calculator

Room size and conditions — the right BTU capacity for heating or cooling.

ft
ft
ft
Recommended capacity
7,500 BTU
Room area
300 sq ft

A rough sizing estimate based on common rules of thumb. For a precise figure, especially on larger or unusual spaces, an HVAC contractor's full Manual J load calculation accounts for far more variables.

A worked example

A 300 sq ft room with an 8-foot ceiling, average conditions, and 2 occupants needs about 7,500 BTU — the standard 25 BTU per square foot baseline.

Frequently asked questions

What happens if I buy an oversized unit?

An oversized air conditioner cools the room quickly but shuts off before properly removing humidity, leaving the space feeling cold and clammy rather than truly comfortable — bigger isn't automatically better.

What happens if it's undersized?

An undersized unit runs nearly constantly trying to reach the target temperature, which wastes energy, adds wear, and often never quite catches up on the hottest or coldest days.

Is this accurate enough for a major purchase?

It's a solid starting estimate for typical residential rooms, generally within about 10% of a contractor's calculation. For a whole-house system or an unusual space, a full Manual J load calculation from an HVAC professional accounts for more variables.