How to size a mini split, without the jargon
Sizing a mini split sounds technical, but it reduces to one question per room: how much heat does this space gain and lose? Square footage is the best first-pass proxy, and it's what our coverage ranges are built on. BTU is the unit of heating and cooling capacity; each room gets its own indoor unit, sized like this at 8-foot ceilings and average insulation:
| Room | Indoor unit |
|---|---|
| Small, up to 400 sq ft | 9,000 BTU (9K) |
| Medium, 400–600 sq ft | 12K |
| Large, 600–800 sq ft | 18K |
Then adjust for ceilings, sun, insulation and kitchens (below). A room bigger than 800 sq ft gets two indoor units.
Why bigger is genuinely worse
The intuition “get the bigger one to be safe” fails for air conditioning. A right-sized inverter mini split runs long, gentle cycles, and those long cycles are what wring humidity out of the air. An oversized indoor unit hits the target temperature quickly and shuts down before it has dehumidified, so the room reads 72° and feels like a locker room. It also cycles on and off more, which is the least efficient and noisiest way to run. When a room sits between sizes, that's not a coin flip; read the next section.
The adjustments that actually matter
- Ceiling height. Coverage assumes about 8 ft. Vaulted or 10 ft ceilings add air volume: step up a size, or at least stop rounding down.
- Sun and glass. A west-facing room with big windows can gain as much heat as a room half again its size. Step up.
- Insulation. A finished, insulated bedroom sits happily at the bottom of a range. A garage or a 1960s addition with original walls behaves a size larger than it measures.
- Kitchens. Ovens and refrigerators dump heat. A 350 sq ft kitchen-dining space is honest 12K territory.
Zones: one indoor unit per room you close a door on
A multi-zone mini split is one outdoor unit driving two or three indoor units, each with its own thermostat. The rule: air doesn't turn corners well, so an indoor unit conditions the room it can see. An open living-dining-kitchen area is one zone; the bedroom down the hall is its own. Our mini splits run two or three zones per outdoor unit: an 18K outdoor unit drives two smaller indoor units, and the 27K handles bigger pairs or three zones.
One planning trick: if there's a room you'll eventually want conditioned, a 27K two-zone mini split leaves you headroom; adding the third zone later is an indoor unit and a line set, not a new outdoor unit.
Do the two-minute version now
Our sizing calculator applies exactly this logic to your rooms and shows the mini splits that match. And if anything about your space is unusual (sunrooms, lofts, half-finished basements), call us. Sizing questions are free, and we'd rather talk you out of the wrong mini split than refund it later.