Sizing guide
How big a mini split does the room need?
Start at 20 BTU per square foot, adjust for the room, then pick the head size that covers the result. One size up is fine; two sizes up wastes money and short-cycles. An undersized unit never catches up on the hottest day.
Size a room
Size one room here.
The same rules as the tables below and the builder: 20 BTU per square foot, adjusted for ceiling height, insulation and sun.
Windows beyond two, more than two people, a kitchen and your climate add to the load. The builder asks for each of them.
Recommended 12K head, estimated load 9,900 BTU.
Estimated load
9,900Btu/h
- 450 sq ft× 20 BTU9,000
- 8 ft ceiling× 1.009,000
- Average insulation× 1.009,000
- Sunny or west-facing+10%9,900
A 12,000 BTU headCovers about 400 to 600 sq ft
It takes a few minutes. Nothing is ordered until you check out.
Head sizes
Head sizes and the rooms they cover.
Room sizes assume 8 ft ceilings, average insulation and average sun (20 to 30 BTU per sq ft). Each size links to the units in it.
| Size | Room size |
|---|---|
| 6,000 BTU | 200 to 300 sq ft |
| 9,000 BTU | 300 to 450 sq ft |
| 12,000 BTU | 400 to 600 sq ft |
| 15,000 BTU | 500 to 750 sq ft |
| 18,000 BTU | 600 to 900 sq ft |
| 24,000 BTU | 800 to 1,200 sq ft |
| 30,000 BTU | 1,000 to 1,500 sq ft |
| 36,000 BTU | 1,200 to 1,800 sq ft |
| 48,000 BTU | 1,600 to 2,400 sq ft |
| 54,000 BTU | 1,800 to 2,700 sq ft |
| 60,000 BTU | 2,000 to 3,000 sq ft |
Adjustments
What moves the number.
Apply each one that fits the room to the 20 BTU per square foot starting point. The builder applies the same rules.
| Condition | Adjustment |
|---|---|
| Ceiling above 8 ft | × height ÷ 8 |
| Poor insulation | +15% |
| Good insulation | −10% |
| Sunny or west-facing | +10% |
| Shaded | −10% |
| Each window beyond two | +500 BTU |
| Each person beyond two | +600 BTU |
| Kitchen | +4,000 BTU |
| Hot climate (FL, TX, AZ, Gulf states) | +10% |
| Cold climate (northern states) | +15% |
Worked example
A 400 sq ft room needs an 18K head.
A living room with 9 ft ceilings, average insulation, a west-facing wall with four windows and an open kitchen. The square footage alone suggests 9,000 BTU. The adjustments almost double it.
Several rooms: one condenser or several?
A multi-zone condenser drives two to six heads and keeps the outside of the house tidy, but every head runs off one compressor and the manufacturer limits the total connected capacity. Separate single-zone systems keep running if one fails and are simpler to install. The builder shows both kinds side by side when they fit.
The math
- 400 sq ft× 20 BTU8,000
- 9 ft ceiling× 9 ÷ 89,000
- West-facing glass+10%9,900
- Two windows beyond two+1,00010,900
- Open kitchen+4,00014,900
Pick the 18,000 BTU head. 12,000 would run flat out on the hottest day and still fall behind.
Vaulted ceilings, sunrooms or very old houses: call and we size it by hand.