Size a mini split one zone at a time: a 144 sq ft bedroom needs about 5,000 BTU/h, so a 6,000 BTU/h (6k) head; a sunny 320 sq ft living room needs 8,800 BTU/h, a 9k head. Add the heads to pick the outdoor unit: 9k + 6k + 9k for a living room, bedroom and kitchen is 24,000 BTU/h, a 24k multi-zone outdoor unit.
How the mini split size is calculated
Each zone is sized with the ENERGY STAR room air conditioner method, the same one behind the BTU calculator: the chart capacity for the floor area, scaled for ceilings over 8 ft, 10% more for a very sunny room or 10% less for a heavily shaded one, 600 BTU/h for each regular occupant beyond two and 4,000 BTU/h for a kitchen.
Zone BTU/h = chart BTU × (ceiling ÷ 8 ft) × sun factor + 600 × (people − 2) + 4,000 if kitchen
The head is the smallest standard wall unit at or above that number. Standard sizes come from Mitsubishi's MSZ-GS and MSZ-FX lines: 6,000, 9,000, 12,000, 15,000, 18,000 and 24,000 BTU/h. A zone that needs more than 24,000 BTU/h is split over two equal heads, which also spreads the air better in a long room.
The outdoor unit is the matching single-zone unit for one head. For several heads it is the smallest multi-zone size at or above the heads' total; Mitsubishi's MXZ outdoor units run from 20,000 to 60,000 BTU/h (20k, 24k, 30k, 36k, 42k, 48k and 60k models).
Worked example: three zones
| Zone | Area | Load | Head |
|---|---|---|---|
| Living room, very sunny | 320 sq ft | 8,000 × 1.10 = 8,800 BTU/h | 9k |
| Bedroom | 144 sq ft | 5,000 BTU/h | 6k |
| Kitchen | 120 sq ft | 5,000 + 4,000 = 9,000 BTU/h | 9k |
| Total | 584 sq ft | 22,800 BTU/h | 24,000 BTU/h of heads |
The heads add to 24,000 BTU/h, so the outdoor unit is a 24k multi-zone. Swap the kitchen for a second bedroom and the heads drop to 21,000 BTU/h; the next outdoor size is still 24k.
Mini split size chart
| Room area | Cooling needed | Head size |
|---|---|---|
| 100 to 250 sq ft | 5,000 to 6,000 BTU/h | 6k |
| 250 to 400 sq ft | 7,000 to 9,000 BTU/h | 9k |
| 400 to 550 sq ft | 10,000 to 12,000 BTU/h | 12k |
| 550 to 700 sq ft | 14,000 BTU/h | 15k |
| 700 to 1,000 sq ft | 18,000 BTU/h | 18k |
| 1,000 to 1,500 sq ft | 21,000 to 24,000 BTU/h | 24k |
The chart assumes 8 ft ceilings, average sun and two people. Anything else shifts a room into the next row, which is why the calculator works room by room.
BTU calculator for a mini split: nominal versus rated
The number in a model name is nominal. In the ENERGY STAR list of certified mini-split heat pumps, the MSZ-GS09 is rated 9,000 BTU/h of cooling and the GS12 12,000, but the GS15 is rated 14,000 and the GS24 22,400. If a zone's load sits just under a size, check the rated figure on the spec sheet before you commit.
Heating capacity changes with the outdoor temperature. The same list gives the GS12 14,400 BTU/h of heat at 47 °F in one combination and 10,100 BTU/h at 5 °F. In a cold climate, compare the zone's heat need from the BTU calculator's heat tab with the head's 5 °F rating, not the name.
Single zone or multi-zone?
One head on its own outdoor unit is the simplest and usually the most efficient layout, and each room keeps running if another unit fails. A multi-zone unit saves wall space outside and one line set run per head, but every head must be a model the outdoor unit's combination table accepts. The calculator lists the heads, outdoor unit and one line set per head as a material list you can add to your project.
Don't oversize the heads
A head much bigger than its room cycles on and off, dries the air less and costs more. ENERGY STAR's warning for room units applies here too: an oversized unit "does not provide better cooling." If a room falls on a boundary, the smaller head with good sealing is usually the better choice.
What comes next
- Power: mini splits need their own circuit. Add the outdoor unit's nameplate current to the electrical load calculator, then size the breaker in the breaker size calculator.
- A whole-house ducted system instead: the AC size calculator.
- A single room without a line set: the window AC answer on the BTU calculator.