What size air conditioner for 400 sq ft? BTU answer and adjustments
400 sq ft of floor area needs roughly 8,000 BTU/h of cooling at the standard 20 BTU/sq ft — 0.67 tons, 2.3 kW. Sunny exposure pushes that to 8,800, heavy shade trims it to 7,200, a kitchen adds 4,000 (→ 12,000), and a crowd adds 600 per extra person.
| Scenario | Rule | BTU/h |
|---|---|---|
| Baseline room | 400 × 20 | 8,000 |
| Very sunny | +10% | 8,800 |
| Heavily shaded | −10% | 7,200 |
| Kitchen | +4,000 | 12,000 |
| Four regular occupants | +600 × 2 | 9,200 |
Conversions: 12,000 BTU/h = 1 ton = 3.517 kW. 8,000 BTU/h ≈ 0.67 tons ≈ 2.3 kW. Figures are cooling capacity to look for on the spec sheet, not electrical input.
From BTU to a product
For 400 sq ft, shop for an 8,000–10,000 BTU window unit or a 9k mini-split. If you land between sizes in a humid climate, choose the smaller inverter model — longer cycles remove more moisture, and moisture is most of the discomfort.
Method and the Manual J threshold in our BTU sizing guide; equipment options on window units, mini-splits and portables (read SACC, not marketing BTU).
Frequently asked questions
How many BTU for 400 square feet?
8,000 BTU/h as a baseline (20 BTU/sq ft), 8,800 for a very sunny room, 7,200 for a shaded one, plus 4,000 for a kitchen and 600 per occupant beyond two. That is about 0.67 tons or 2.3 kW.
What size AC unit is that?
An 8,000–10,000 BTU window unit or a 9k mini-split. For whole-house systems serving several enclosed rooms, a Manual J load calculation replaces this room-level rule.
Is 20 BTU per square foot always right?
It is the room-unit baseline ENERGY STAR and the DOE use; ceiling height above 8 ft, poor insulation and big west glazing push it higher, while a well-insulated shaded room can sit lower. The adjustments above capture the common cases.