Mini-Split Sizing Calculator
Find the BTU and recommended head size for a ductless mini-split, room by room.
How mini-split sizing works
A ductless mini-split is sized room by room, not by the whole house. Each indoor head should match the heat load of the space it serves — multiply the room's floor area by a climate factor (roughly 18–30 BTU per sq ft) and adjust for insulation, sun and glass. Then round to the nearest standard head size: 9,000, 12,000, 18,000, 24,000 or 36,000 BTU.
Size each room separately
Sizing per room is what makes mini-splits comfortable and efficient: a sunny living room with lots of glass needs more than a shaded bedroom of the same size. Oversizing a head causes short-cycling and poor humidity control, just like with a central system.
| Room size | Typical head |
|---|---|
| 150–250 sq ft | 9,000 BTU |
| 250–400 sq ft | 12,000 BTU |
| 400–650 sq ft | 18,000 BTU |
| 650–1,000 sq ft | 24,000 BTU |
How many heads and what outdoor unit
Add up the BTU of all the heads you need, then choose a multi-zone outdoor (condenser) unit rated to cover that total. Manufacturers cap how many heads one condenser can run, so very large homes may need two outdoor units. See our cost calculator to price it out.
Frequently asked questions
What size mini-split do I need for a room?
Roughly room area × climate factor (18–30 BTU/sq ft) × an insulation/sun adjustment. A 400 sq ft room typically needs a 12,000 BTU head (9,000 for a small or well-shaded room).
How many mini-split heads do I need?
One per room or open area you want to control independently — size each separately and total them for the outdoor unit.
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⚠️ Rule-of-thumb estimate, not a substitute for a professional Manual J load calculation. Based on US DOE / ENERGY STAR guidance.