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.
Worked example
With room area 400 sq ft, ceiling height 8 ft, exposure adjustment 1 , regular occupants 2 , this calculator returns 8000 .
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.