What Size AC for 400 Sq Ft?
A 400 sq ft space needs about 8,000 BTU/h of cooling (0.67 tons), so the nearest size is 8,000 BTU. This assumes an 8 ft ceiling and average sun. It is a rule of thumb (20 BTU per sq ft), not an engineered load.
AC size for 400 sq ft by situation
| Scenario | BTU/h | Tons | Suggested size |
|---|---|---|---|
| Heavily shaded (−10%) | 7,200 | 0.6 | 8,000 BTU |
| Average sun, 8 ft ceiling | 8,000 | 0.67 | 8,000 BTU |
| Very sunny (+10%) | 8,800 | 0.73 | 10,000 BTU |
| Kitchen (+4,000 BTU) | 12,000 | 1 | 12,000 BTU |
| 9 ft ceilings (+12.5%) | 9,000 | 0.75 | 10,000 BTU |
| 10 ft ceilings (+25%) | 10,000 | 0.83 | 10,000 BTU |
| Hot climate (+10%) | 8,800 | 0.73 | 10,000 BTU |
Rule-of-thumb figures for a two-person space; each extra person over 2 adds about 600 BTU.
How it is calculated
400 sq ft × 20 BTU × (8 ÷ 8) = 8,000 BTU/h
8,000 ÷ 12,000 = 0.67 tons
Sunny: 8,000 × 1.1 = 8,800 · Kitchen: 8,000 + 4,000 = 12,000
Which type of AC for 400 sq ft?
- Window unit – The usual choice at this size is a 8,000 BTU window unit. Units above roughly 15,000 BTU often need a 230V circuit, but this size normally stays on 115V.
- Portable unit – Portable units lose capacity to exhaust-hose heat. Expect to need about 10,400 BTU nominal (≈30% more) to match a window unit here.
- Mini-split – A 8,000 BTU single-zone mini-split is the quiet, efficient alternative and also heats, with no window blocked.
Frequently asked questions
How many BTU do I need for 400 square feet?
About 8,000 BTU/h for an 8 ft ceiling in average conditions (400 × 20). That is 0.67 tons, so the closest unit is 8,000 BTU.
Is 8,000 BTU enough for 400 sq ft if the room is sunny?
A very sunny space needs about 8,800 BTU, so 10,000 BTU is recommended – a step different from the 8,000 BTU for average sun.
What if the 400 sq ft space has 10 ft ceilings?
The load rises 25% to 10,000 BTU, so choose 10,000 BTU instead of 8,000 BTU. Volume matters, not just floor area.