Key takeaways
- Start with about 20 BTU per square foot of floor area for cooling.
- 12,000 BTU per hour equals one ton of cooling — size the unit in tons.
- Add 600 BTU per occupant beyond two, ±10% for sun, and 4,000 BTU for a kitchen.
- This is a rule of thumb; a true sizing uses an ACCA Manual J load calculation.
How to size cooling
Cooling capacity is measured in BTU per hour, and 12,000 BTU per hour equals one ton of cooling. A quick estimate scales with floor area at roughly 20 BTU per square foot, then adjusts for ceiling height, sun exposure, occupants, and whether the room is a kitchen.
The per-square-foot rate starts at 20 BTU for an average room, rising to about 22 for a very sunny space or dropping to 18 when heavily shaded — roughly a 10% swing either way. Ceiling height is divided by 8 because the baseline assumes a standard 8 ft ceiling. A kitchen adds a flat 4,000 BTU for the heat thrown off by the stove, oven, and refrigerator.
Worked example: a 15 × 15 room
Take the default 15 ft × 15 ft room: that is 225 square feet. With an 8 ft ceiling and average sun, the baseline is 225 × 20 × (8 ÷ 8) = 4,500 BTU/hr. With two occupants there is no occupant add-on, and with the Kitchen option set to No nothing is added there either. Dividing 4,500 by 12,000 gives 0.38 tons, which rounds up to a ½-ton (6,000 BTU) unit. Switch Kitchen to Yes and the load jumps to 8,500 BTU.
Room AC sizing by area (Energy Star)
| Room size | Cooling capacity |
|---|---|
| 150–250 sq ft | 6,000 BTU |
| 250–300 sq ft | 7,000 BTU |
| 300–350 sq ft | 8,000 BTU |
| 350–400 sq ft | 9,000 BTU |
| 400–450 sq ft | 10,000 BTU |
| 450–550 sq ft | 12,000 BTU (1 ton) |
This is a rule of thumb
Square-footage estimates are fine for picking a window unit or mini-split, but they ignore insulation, window area, air leakage, and climate. A whole-house or ducted system should be sized with an ACCA Manual J load calculation, the industry gold standard. Oversizing is a real risk: an oversized unit short-cycles and never runs long enough to pull humidity out of the air.
Frequently asked questions
How many BTUs to cool a room?
≈20 BTU per sq ft; a 225 sq ft room ≈ 4,500 BTU, adjusted for sun, ceiling, occupants, and +4,000 for a kitchen.
How many BTU to cool a 300 square foot room?
About 6,000 BTU at 20 BTU/sq ft for average sun and an 8 ft ceiling — Energy Star rates a 6,000 BTU unit for 250–300 sq ft.
How many BTU is a ton?
One ton of cooling is 12,000 BTU/hr, so 24,000 BTU is a 2-ton unit.
Does a kitchen need extra capacity?
Yes — add 4,000 BTU for a kitchen. Set Kitchen to Yes and the calculator adds it automatically.
How do ceiling height and sun change it?
Load scales by ceiling ÷ 8; sun moves the rate to ≈22 BTU/sq ft (very sunny) or ≈18 (heavily shaded).
How do I size the unit in tons?
Divide BTU by 12,000 and round up to the next half ton when selecting equipment.
Room-size capacity guidance follows Energy Star; the gold standard for load sizing is an ACCA Manual J calculation. The 12,000 BTU per ton conversion is exact.
Last reviewed June 2026