Mini split size by room
Indoor head size for heating and cooling in a room with 8 ft ceilings and average insulation. In hot climates, cooling sets the size; in cold climates, heating does.
| Room area | Hot IECC 1–2 | Warm IECC 3 | Mixed IECC 4 | Cold IECC 5 | Very cold IECC 6–7 |
|---|---|---|---|---|---|
| 150 sq ft | 6k | 6k | 6k | 9k | 9k |
| 250 sq ft | 9k | 6k | 9k | 12k | 15k |
| 350 sq ft | 9k | 9k | 12k | 15k | 24k |
| 500 sq ft | 15k | 12k | 15k | 24k | 2 heads |
| 750 sq ft | 24k | 18k | 24k | 2 heads | 2 heads |
k = thousand BTU/h. Common indoor head sizes are 6,000, 9,000, 12,000, 15,000, 18,000 and 24,000 BTU/h. A space too large for one 24,000 BTU head is better served by two smaller heads at opposite ends.
Indoor heads and outdoor units
A single-zone system has one indoor head and one matched outdoor unit. A multi-zone system connects two to five heads to one outdoor unit. Each head is sized for its own room, and the outdoor unit is sized for the combined load.
- Multi-zone outdoor units come in 18,000, 24,000, 30,000, 36,000, 42,000 and 48,000 BTU/h sizes.
- Manufacturers allow the connected heads to add up to more than the outdoor unit, typically up to about 130%, because rooms rarely need full output at the same time.
- The calculator picks the smallest outdoor unit that covers the combined load and keeps the connected heads within 130% of its size.
Sizing a mini split for heating
Mini split heads are rated at 47 °F outdoors. On the coldest night, output drops. The calculator divides each room's heat loss by the share of capacity a cold-climate mini split keeps at your design temperature: 95% in hot zones, 90% in warm, 85% in mixed, 75% in cold and 65% in very cold zones.
That is why a 250 sq ft bedroom needs a 6,000 BTU head in Georgia but a 15,000 BTU head in Minnesota. If you only want cooling, or have another heat source for the coldest days, choose Cooling only.
Common sizing mistakes
- One big head for a whole floor. Ductless heads move air well within their own room but not around corners or through doors. Closed bedrooms need their own heads.
- Oversizing small rooms. A 12,000 BTU head in a 120 sq ft bedroom will short-cycle and leave the room clammy. Many brands make 6,000 BTU heads for small rooms.
- Ignoring the kitchen. Cooking adds about 4,000 BTU/h of heat. Add the kitchen as its own room type.
- Matching heads to a too-small outdoor unit. If all rooms call for full output at once, an undersized outdoor unit cannot keep up.
Frequently asked questions
What size mini split do I need for a 12 × 12 room?
A 144 sq ft room usually needs a 6,000 BTU head, or a 9,000 BTU head in very cold climates if you also heat with it.
What size mini split for 500 sq ft?
About 12,000 to 15,000 BTU in hot, warm and mixed climates, and 24,000 BTU in cold climates for heating and cooling.
Can one mini split cool a whole house?
Only an open-plan home. Most houses need a head in each closed room, connected to one multi-zone outdoor unit or several single-zone units.
Is a 12,000 BTU mini split too big for a bedroom?
For a typical 150 sq ft bedroom, yes, unless you live in a very cold climate and heat with it. Oversized heads short-cycle and dehumidify poorly.
How many zones can one outdoor unit handle?
Most multi-zone outdoor units support two to five indoor heads. Larger homes often use two outdoor units.
These results are estimates for planning. Confirm head and outdoor unit sizes with the manufacturer's combination tables and a room-by-room load calculation.