Home › Calculators › BTU Calculator
BTU Calculator: Air Conditioner Size by Room
Enter the room size and a few facts about it. The calculator estimates the BTU rating the air conditioner needs, the number printed on every window unit and mini split, so you can buy the size that actually cools the room instead of the one that was on sale.
How This Calculator Works
The base rule is about 20 BTU per square foot of floor area for a normally insulated room with 8 foot ceilings and two occupants. The calculator then adjusts: higher ceilings add volume to cool, a sunny room adds heat load, and each person beyond two adds about 600 BTU of body heat. It rounds to the nearest 100 BTU; you then pick the closest standard unit size, rounding up only slightly.
Worked Example
A 12 by 15 foot bedroom is 180 square feet, giving a base of 3,600 BTU, which in practice means the standard 5,000 to 6,000 BTU unit. The same room at 250 square feet, sunny, with 10 foot ceilings works out near 6,200 BTU, so an 8,000 BTU unit is the honest pick once adjustments stack up.
Standard AC sizes by room area (base conditions)
| Item | Figure |
|---|---|
| Up to 150 sq ft | 5,000 to 6,000 BTU |
| 150 to 250 sq ft | 6,000 to 8,000 BTU |
| 250 to 350 sq ft | 8,000 to 10,000 BTU |
| 350 to 450 sq ft | 10,000 to 12,000 BTU |
| 450 to 550 sq ft | 12,000 to 14,000 BTU |
| 550 to 700 sq ft | 14,000 to 18,000 BTU |
Frequently Asked Questions
What happens if I buy an oversized air conditioner?
It cools the air fast and shuts off before removing humidity, leaving the room cold and clammy, and it cycles on and off, which wastes energy and wears the unit. Bigger is not better in AC sizing.
What about kitchens and home gyms?
Add roughly 4,000 BTU for a kitchen's cooking heat, and treat exercise rooms like sunny rooms. Server or equipment loads need their heat output added directly.
How many BTU per square foot is the rule?
20 BTU per square foot is the standard planning figure for typical US homes. Very old, poorly insulated houses often need more often need professional load calculation (a Manual J) for whole systems.
Does this calculator size a whole house system?
No. It sizes a single room unit. Whole house central systems need a proper load calculation covering windows, insulation, ductwork and climate zone.