What Size Air Conditioner Do I Need? (BTU Chart + Calculator)
20 BTU per square foot is the rule. BTU chart by room size, adjustments for sun and ceiling, and tons conversion.
How to pick the right AC size
The rule of thumb is 20 BTU per square foot of floor area, adjusted for ceiling height, sun exposure and occupants. Getting this right matters: too small and it never cools; too big and it short-cycles, wastes energy and leaves the room humid.
BTU by room size (standard 8 ft ceilings)
| Room size | BTU | AC type |
|---|---|---|
| Up to 150 sq ft | 5,000 | Window unit |
| 150-250 sq ft | 6,000-8,000 | Window / mini-split |
| 250-400 sq ft | 10,000-12,000 | Mini-split / 1 ton |
| 400-550 sq ft | 12,000-14,000 | 1-1.5 ton |
| 550-800 sq ft | 18,000 | 1.5 ton |
| 800-1,100 sq ft | 24,000 | 2 ton |
Adjustments that change your BTU
| Factor | Adjustment |
|---|---|
| Ceiling over 8 ft | +10% |
| Heavy west sun | +10-15% |
| Kitchen (heat from cooking) | +4,000 BTU |
| 4+ people regularly | +10% |
| Shaded room | -10% |
BTU vs. tons
1 ton = 12,000 BTU. An 18,000 BTU system is a 1.5-ton system. Mini-splits and central AC are sized in tons; window units in BTU.
Pro tips
- For bedrooms, size slightly smaller — longer run cycles dehumidify better
- Measure the actual room, not the whole house, for a single unit
- Check the unit's SEER rating — higher SEER = cheaper to run
FAQ
What size AC for a 12x12 bedroom?
144 sq ft needs about 5,000 BTU — a compact window unit works great.
Is bigger AC always better?
No — oversized units cool too fast, short-cycle, and leave rooms cold and damp. Match the size.
What is a "ton" of air conditioning?
One ton = 12,000 BTU/hour. It's the standard unit for central and mini-split systems.
Need a different size?
Our free calculators give you the exact answer in seconds.