Mini Split Sizing Calculator
Pick the right mini split capacity for a room, sun exposure included.
How this calculator works
BTU = area × 20 × (height ÷ 8) × exposure + (occupants − 2) × 600
Standard mini split heads come in 6,000, 9,000, 12,000, 18,000 and 24,000 BTU. Round to the nearest, and prefer the smaller size when between — inverter units modulate down efficiently but struggle when grossly oversized.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ANSI/ACCA 2 Manual J (Residential Load Calculation) 2016, 8th edition — the room-by-room load calculation ductless equipment should be selected against
Standards are revised on a cycle. The edition above is the one this figure was checked against — confirm it against the edition your jurisdiction has adopted.
Worked example
With room area 400 sq ft, ceiling height 8 ft, exposure adjustment 1 , regular occupants 2 , this calculator returns 8000 .
Head size for rooms of common type
Click through any row to load those measurements. The address bar keeps them, which makes the result something you can paste into a message.
| Job | Size | Capacity needed |
|---|---|---|
| Bedroom | 200 sq ft × 8 ft × 1 × 2 | 4,000 |
| Living room | 400 sq ft × 8 ft × 1 × 4 | 9,200 |
| Sunroom, heavy glazing | 250 sq ft × 8 ft × 1.3 × 2 | 6,500 |
| Converted garage | 440 sq ft × 9 ft × 1.1 × 2 | 10,890 |
| Small office | 150 sq ft × 8 ft × 1 × 2 | 3,000 |
Heads come in 6, 9, 12, 18 and 24k BTU. Between two sizes, take the smaller — inverters modulate down better than up.
Frequently asked questions
What size mini split do I need for a 400 square foot room?
About 8,000 BTU under average conditions, so a 9,000 BTU head — the nearest standard size. A very sunny room of the same size would justify the 12,000.
What are the standard mini split sizes?
Six, 9, 12, 18 and 24 thousand BTU for indoor heads. Multi-zone outdoor units combine several heads, though total head capacity may exceed the outdoor unit rating since zones rarely peak together.
Should I oversize a mini split?
No. Inverter compressors modulate down well but have a minimum output, and a grossly oversized unit short cycles, fails to dehumidify and wastes energy. When between sizes, take the smaller.
Can one mini split heat a whole house?
Only in an open-plan, well-insulated home. Mini splits move very little air between rooms, so closed doors mean uneven temperatures. Most homes need a head per zone.