What size air conditioner for 900 sq ft? BTU answer and adjustments
For 900 square feet, the baseline is 18,000 BTU/h (20 BTU per sq ft) — about 1.50 tons or 5.3 kW of cooling. Adjust to 19,800 BTU for a sun-baked room, 16,200 for a shaded one, 22,000 if it is a kitchen, and add 600 BTU per regular occupant beyond two.
| Scenario | Rule | BTU/h |
|---|---|---|
| Baseline room | 900 × 20 | 18,000 |
| Very sunny | +10% | 19,800 |
| Heavily shaded | −10% | 16,200 |
| Kitchen | +4,000 | 22,000 |
| Four regular occupants | +600 × 2 | 19,200 |
Conversions: 12,000 BTU/h = 1 ton = 3.517 kW. 18,000 BTU/h ≈ 1.50 tons ≈ 5.3 kW. Figures are cooling capacity to look for on the spec sheet, not electrical input.
Which unit, in practice
For 900 sq ft, shop for an 18,000–24,000 BTU mini-split, or central/ducted equipment for enclosed layouts. 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.
Related: BTU sizing explained, window vs portable, and for whole-house equipment the central AC guide. Metric version: puissance par m² (French).
Frequently asked questions
How many BTU for 900 square feet?
18,000 BTU/h as a baseline (20 BTU/sq ft), 19,800 for a very sunny room, 16,200 for a shaded one, plus 4,000 for a kitchen and 600 per occupant beyond two. That is about 1.50 tons or 5.3 kW.
What size AC unit is that?
An 18,000–24,000 BTU mini-split, or central/ducted equipment for enclosed layouts. 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.