AC BTU Calculator: What Size Air Conditioner Do You Need?
Sizing an air conditioner by guesswork costs you twice — an undersized unit runs forever, an oversized one short-cycles and never pulls the humidity out. This calculator uses the standard 20 BTU per square foot method with the adjustments that actually matter.
Room details
ft
ft
ft
Recommended cooling capacity
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In tons of cooling—
Room area—
Typical unit size to shop for—
Based on the standard 20 BTU/sq ft baseline with height, sun, occupancy and kitchen adjustments. For whole-house systems, a contractor’s Manual J load calculation is the correct tool.
How AC sizing actually works
The industry baseline for cooling is 20 BTU per square foot of floor area for a room with standard 8-foot ceilings. From there, real rooms push the number up or down: taller ceilings mean more air volume to cool, a west-facing sunroom gains far more heat than a shaded bedroom, every person adds body heat, and a kitchen adds a working stove and refrigerator to the load.
BTU = sq ft × 20 × (ceiling ÷ 8) × sun factor + 600 per person over two + 4,000 if kitchen
Worked example
A 20 × 15 ft living room (300 sq ft) with 9-ft ceilings, average sun, and 3 regular occupants: 300 × 20 × 1.125 = 6,750, plus 600 for the third person = about 7,400 BTU. You'd shop the 8,000 BTU class — round up to the next available size, not down.
Why oversizing is a real problem
A unit that's too big cools the air fast and shuts off before it has time to dehumidify. You get a room that's 72° and clammy, plus extra wear from constant start-stop cycling. Between two sizes? Go up one class only if the room is sunny or connected to open space; otherwise take the closer match.
Frequently asked questions
How many BTU do I need per square foot?
The standard baseline is 20 BTU per square foot for cooling with 8-foot ceilings. Adjust up for tall ceilings, strong sun, more than two occupants, or kitchens, and down slightly for shaded rooms.
What happens if my AC is too big for the room?
It short-cycles: it cools the air quickly and shuts down before removing humidity, leaving the room cold but damp. It also wears out faster from frequent starts and often costs more to run than a correctly sized unit.
Is 12,000 BTU the same as 1 ton?
Yes. One ton of cooling equals exactly 12,000 BTU per hour, so a 3-ton central unit is 36,000 BTU. Window and portable units are usually sold by BTU; central systems by tons.