What size mini split do I need?
Load in BTU for your room and climate, matched to the mini split sizes actually sold.
A mini split is sized to the room it serves, not the house. Get it right and it runs long, quiet cycles that hold both temperature and humidity. Get it too big, which is the common error, and it blasts, stops, and leaves the room cold and damp.
Before you buy anything. This is a square-foot estimate, not an ACCA Manual J load calculation. A Manual J uses your building’s real construction, orientation and air leakage, and on a well-sealed house it usually comes out smaller than this. Use this to sanity-check a quote, not to replace one.
Worked example: a 400 sq ft room in climate zone 4
Zone 4 covers Washington DC, Seattle, St Louis and Nashville. With an 8 ft ceiling, average insulation and two occupants:
That last row is the one to look at. Mini splits come in fixed steps, so the honest answer is often "the smallest one that covers it, and it is still somewhat oversized". A good installer will tell you the same thing. An installer who jumps two sizes "to be safe" is making the room worse.
Standard mini split sizes
Ductless units are sold in these nominal capacities. There is nothing between them, which is why the calculated load rarely matches a unit exactly.
| BTU | Tons | Typical area, zone 4 |
|---|---|---|
| 6,000 | 0.5 | up to 260 sq ft |
| 9,000 | 0.8 | up to 390 sq ft |
| 12,000 | 1.0 | up to 520 sq ft |
| 15,000 | 1.3 | up to 650 sq ft |
| 18,000 | 1.5 | up to 780 sq ft |
| 24,000 | 2.0 | up to 1,040 sq ft |
| 30,000 | 2.5 | up to 1,300 sq ft |
| 36,000 | 3.0 | up to 1,560 sq ft |
| 42,000 | 3.5 | up to 1,820 sq ft |
| 48,000 | 4.0 | up to 2,080 sq ft |
Area figures assume an 8 ft ceiling and average insulation in zone 4. Change the climate zone and they move substantially, which is exactly why a single national chart misleads.
What moves the number
| Input | Effect | Why |
|---|---|---|
| Floor area | Linear | Double the area, double the load. |
| Ceiling height | Linear on volume | A 9 ft ceiling is 12.5% more air to condition than an 8 ft one. Most charts ignore this. |
| Climate zone | Up to 2x | Cooling runs 30 to 35 BTU per sq ft in zone 1 and 15 to 18 in zone 8. Heating runs the other way. |
| Insulation and glazing | Selects within the band | A leaky older house sits at the top of its zone's range, a sealed modern one at the bottom. |
| Sun exposure | Up to 15% | Large west and south glazing is a real cooling load and a small heating credit. |
| Occupants | 600 BTU each | Counted past the first two, who are already in the baseline. |
| Kitchen | 4,000 BTU | Appliance and cooking gain inside the conditioned space. |