What size air conditioner do I need?
The quick rule: about 20 BTU per square foot of room area. So a 300 sq ft room needs roughly 6,000 BTU, a 500 sq ft room about 10,000 BTU, and a 1,000 sq ft space about 20,000 BTU. To express it in tons, divide BTU by 12,000 (1 ton = 12,000 BTU/hr). Here's the chart by room size, plus the adjustments that matter — and why an oversized unit is the wrong move.
The math
Start from the room's floor area and the per-square-foot baseline:
- BTU per hour = room area (sq ft) × 20 BTU/sq ft
- Tons of cooling = BTU ÷ 12,000
For a 15 × 20 ft room: 300 sq ft × 20 = 6,000 BTU (about half a ton).
AC size by room size
| Room size | Cooling needed (≈20 BTU/sq ft) | Tons |
|---|---|---|
| 150 sq ft (small bedroom) | ~3,000 BTU | 0.25 |
| 250 sq ft (bedroom) | ~5,000 BTU | 0.4 |
| 400 sq ft (living room) | ~8,000 BTU | 0.7 |
| 550 sq ft (studio) | ~11,000 BTU | 0.9 |
| 700 sq ft | ~14,000 BTU | 1.2 |
| 1,000 sq ft | ~20,000 BTU | 1.7 |
| 1,500 sq ft | ~30,000 BTU | 2.5 |
Window and portable units are usually labeled in BTU; mini-split and central systems are usually labeled in tons.
Get the exact BTU and tonnage for your room
Enter your room size (and tweak the BTU-per-sq-ft rate) and the calculator gives the BTU per hour and the equivalent tons of cooling.
Open the BTU CalculatorAdjust for your room
The 20 BTU rule assumes a typical room with ~8 ft ceilings. Tune it for the conditions:
| Condition | Adjustment |
|---|---|
| Very sunny room | add ~10% |
| Heavily shaded room | subtract ~10% |
| Kitchen | add ~4,000 BTU |
| Each person over two (regular) | add ~600 BTU |
| High or vaulted ceilings | add for the extra volume |
Why bigger isn't better
It's tempting to oversize "to be safe," but that backfires. An oversized AC cools the air fast and shuts off before it pulls out much humidity — then short-cycles on and off. The result is a room that feels cold and clammy, higher energy bills, and more wear on the unit. A right-sized system runs in longer, steadier cycles that hold the temperature and dehumidify properly. For a whole-house or permanent system, this room estimate is a starting point — get a professional Manual J load calculation before buying, since insulation, windows, and air-sealing all change the real number.