How to Use the Air Conditioner Size Calculator
US air conditioners are rated in BTU/h rather than "room sizes," and the right BTU rating depends on more than just square footage. This calculator starts from a common baseline of about 20 BTU per square foot, then adjusts for sun exposure (north, south, east, or west-facing) and floor level, since these change how much heat actually builds up in the room.
West-facing rooms take on intense direct sun in the afternoon, and top-floor rooms absorb extra radiant heat from the roof, so both need a bigger unit than the base estimate for the same square footage. Buying strictly by square footage without factoring these in can leave a room under-cooled on hot afternoons — it's safer to round up to the next common AC size shown in the result.
Frequently Asked Questions
West-facing rooms get intense direct sun in the afternoon, and top-floor rooms absorb extra heat from the roof, so both need more cooling capacity than the base estimate for the same square footage.
Square footage is just the room's size, while BTU/h (cooling capacity) measures how much heat the unit can actually remove per hour. This calculator computes the required BTU/h first, then you match it to a common AC size.