How to size a mini split
Each indoor head cools one room or open area, so mini splits are sized room by room. The calculator works out each room's cooling load from its area, your climate zone, insulation and sun, then adds extra for kitchens, sunrooms and garages. It picks the nearest standard head: 6,000, 9,000, 12,000, 18,000, 24,000, 30,000 or 36,000 BTU.
For a multi-zone system, the outdoor unit should roughly match the total of all the heads. Check the manufacturer's allowed combinations before you buy.
Mini split size chart
For a room with average insulation and 8-foot ceilings in a mild climate.
| Room size | Cooling load | Head size |
|---|
| 150 sq ft | 3,750 BTU | 6,000 BTU |
| 250 sq ft | 6,250 BTU | 6,000 BTU |
| 350 sq ft | 8,750 BTU | 9,000 BTU |
| 450 sq ft | 11,250 BTU | 12,000 BTU |
| 550 sq ft | 13,750 BTU | 18,000 BTU |
| 700 sq ft | 17,500 BTU | 18,000 BTU |
| 900 sq ft | 22,500 BTU | 24,000 BTU |
| 1,200 sq ft | 30,000 BTU | 30,000 BTU |
Common questions
What size mini split do I need for a 500 sq ft room?
About 12,000 BTU in a mild climate. In a hot climate, or a sunny room, step up to 18,000 BTU; in a cold climate with good insulation, 12,000 BTU is usually enough for cooling.
What size mini split for a 1-car or 2-car garage?
A typical 1-car garage (about 250 sq ft) needs 9,000–12,000 BTU, and a 2-car garage (about 450 sq ft) needs 12,000–18,000 BTU. Garages are usually poorly insulated, so the calculator adds 25%.
How do I size a multi-zone mini split?
Size each indoor head for its own room, then choose an outdoor unit whose capacity covers the total of the heads. Most manufacturers allow the heads to add up to 100–130% of the outdoor unit size.
Is it OK to oversize a mini split?
Slightly, yes. Inverter mini splits slow down at low load, so they handle oversizing better than standard ACs. Going more than one size up still costs more and can reduce dehumidification.
Can a mini split heat in winter?
Yes — most mini splits are heat pumps. In cold climates (zones 4–5), choose a cold-climate or hyper-heat model rated to keep heating well below freezing.