BTU Calculator – What Size Air Conditioner Do I Need? | CalcLoft
Find out the right BTU rating and AC size for any room.
How to size an air conditioner
AC capacity is measured in BTUs. The basic rule: 20 BTUs per square foot for a standard 8 ft ceiling, plus adjustments for sun exposure and people. A 15 × 12 ft room (180 sq ft) needs about 5,000-6,000 BTU.
BTU guide for common rooms
| Room size | Recommended BTU | AC size |
|---|---|---|
| Up to 150 sq ft | 5,000 | 0.5 ton |
| 150-250 sq ft | 6,000 | 0.5-0.75 ton |
| 250-400 sq ft | 8,000-10,000 | 0.75-1 ton |
| 400-550 sq ft | 12,000 | 1 ton |
| 550-800 sq ft | 14,000-18,000 | 1.5 ton |
| 800-1,100 sq ft | 18,000-24,000 | 2 ton |
Bigger is NOT better
An oversized AC cools too fast, short-cycles, and leaves the room humid. Match the size to the room — this calculator gives the right ballpark.
BTU calculator FAQ
What size AC for a 12x12 room?
144 sq ft needs about 5,000 BTU — a small window unit or mini-split covers it.
How many BTU per square foot?
The standard rule is 20 BTU/sq ft, adjusted for ceiling height, sun and occupancy.
Do I need to convert BTU to tons?
1 ton = 12,000 BTU. An 18,000 BTU unit is a 1.5-ton system.
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