The chart
| Room size (sq ft) | Starting BTU |
|---|---|
| 150-250 | 6,000-9,000 |
| 250-400 | 9,000-12,000 |
| 400-550 | 12,000-15,000 |
| 550-700 | 15,000-18,000 |
| 700-1,000 | 18,000-24,000 |
| 1,000-1,300 | 24,000-30,000 |
| 1,300-1,600 | 30,000-36,000 |
These are based on the ~20 BTU-per-square-foot rule for a reasonably insulated space with standard 8-foot ceilings. Find your square footage (length x width), then read across.
What moves the number up or down
The chart is a baseline. Adjust for your actual room:
- Insulation. A well-sealed, well-insulated room needs less; a drafty, poorly-insulated one (or an uninsulated garage) needs more.
- Ceiling height. Above 8 feet, you're conditioning more air volume - size up.
- Sun exposure. A room with big west- or south-facing windows and heavy afternoon sun needs more capacity.
- Climate. Hot-summer or cold-winter climates push the number up, especially for heating in the cold - see cold-weather performance.
- Use and occupancy. A kitchen (heat-producing appliances) or a room that's regularly full of people needs more than a quiet bedroom.
Why you shouldn't just round up
It's tempting to buy "a little extra to be safe." Don't. An oversized mini split cools the room too fast, shuts down, and cycles on and off - which wastes energy, wears the unit, and (worst of all) doesn't run long enough to pull humidity out, leaving the room cold and clammy. Right-sizing matters more with a mini split than with old on/off equipment. The full reasoning is in the sizing guide.
From chart to confident answer
Use the chart to narrow the range, then:
- Adjust for the factors above.
- Confirm with a real calculation - a good installer does a load calculation, which beats any rule of thumb.
- Shortlist units at your target BTU in the product finder, or see the size-by-size picks in best mini split by size.
FAQ
What size mini split do I need for a room? Start at ~20 BTU per square foot - so about 12,000 BTU for a 400-550 sq ft room - then adjust up for poor insulation, high ceilings, heavy sun, or extreme climate. Confirm with a load calculation.
How many square feet will a 12,000 BTU mini split cool? Roughly 400-550 sq ft in a well-insulated space with standard ceilings, less in a drafty or sunny room.
Is it better to oversize or undersize a mini split? Neither - right-size it. Oversizing causes short-cycling and poor dehumidification; undersizing leaves the room uncomfortable in extremes. Aim for the correct BTU, not a cushion.
Does the chart work for heating too? It's a cooling-first baseline. Heating load in a cold climate can be higher, so in cold regions size to the heating need and choose a cold-climate model.