What size air conditioner for 300 sq ft? BTU answer and adjustments
The 300 sq ft answer: 6,000 BTU/h baseline (= 0.50 tons = 1.8 kW), 6,600 with strong sun, 5,400 in shade, 10,000 for a kitchen, 7,200 with four regular occupants. The formula is transparent — floor area × 20, then the ENERGY STAR adjustments.
| Scenario | Rule | BTU/h |
|---|---|---|
| Baseline room | 300 × 20 | 6,000 |
| Very sunny | +10% | 6,600 |
| Heavily shaded | −10% | 5,400 |
| Kitchen | +4,000 | 10,000 |
| Four regular occupants | +600 × 2 | 7,200 |
Conversions: 12,000 BTU/h = 1 ton = 3.517 kW. 6,000 BTU/h ≈ 0.50 tons ≈ 1.8 kW. Figures are cooling capacity to look for on the spec sheet, not electrical input.
Which unit, in practice
In equipment terms, 300 sq ft points to a 5,000–6,000 BTU window unit or the smallest mini-split head. Round to the next available size above your adjusted number — never two sizes up: an oversized unit cools fast, stops before dehumidifying, and leaves the room cold and clammy.
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 300 square feet?
6,000 BTU/h as a baseline (20 BTU/sq ft), 6,600 for a very sunny room, 5,400 for a shaded one, plus 4,000 for a kitchen and 600 per occupant beyond two. That is about 0.50 tons or 1.8 kW.
What size AC unit is that?
A 5,000–6,000 BTU window unit or the smallest mini-split head. 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.