Mini Split Size Calculator
Size a ductless mini split room by room: each indoor head's BTU, the total load and the multi-zone outdoor unit, for cooling or year-round heat pump heating.
Size each indoor head for its room (about 20 BTU per sq ft for cooling, more where heating is the bigger load), using standard 6k, 9k, 12k, 18k and 24k BTU heads. Then pick an outdoor unit at least as big as the total load: a living room (12k) plus two bedrooms (9k each) usually needs a 24,000–30,000 BTU multi-zone unit.
Mini split head size by room area (cooling, average conditions)
| Room | Load | Head size |
|---|---|---|
| Small bedroom (150 sq ft) | 5,000 BTU | 6,000 BTU |
| Bedroom (250 sq ft) | 6,000 BTU | 6,000 BTU |
| Large bedroom (350 sq ft) | 8,000 BTU | 9,000 BTU |
| Living room (450 sq ft) | 10,000 BTU | 12,000 BTU |
| Open living area (550 sq ft) | 12,000 BTU | 12,000 BTU |
| Great room (700 sq ft) | 14,000 BTU | 15,000 BTU |
Multi-zone tips
- Each room gets its own head; open-plan spaces can share one.
- Most multi-zone outdoor units let connected heads add up to about 130% of the outdoor capacity, because rooms rarely peak at once.
- In cold climates, choose a cold-climate (hyper-heat) model and check its heating capacity at your winter design temperature, not just its rating.
Frequently asked questions
What size mini split do I need?
Match each indoor head to its room's load: roughly 6,000–9,000 BTU for bedrooms, 12,000 for a living room and 18,000–24,000 for large open areas, adjusted for sun, ceilings and climate.
How many square feet does a 12,000 BTU mini split cool?
About 450–550 sq ft in average conditions, less with lots of sun or high ceilings.
Can I put a 12k head on a 24k outdoor unit with two 9k heads?
Yes. 12k + 9k + 9k is 30k connected on a 24k outdoor unit, a 125% ratio, which most multi-zone systems allow. Check the manufacturer's combination table.
Should I size a mini split for heating or cooling?
For a heat pump that heats the house, size for whichever load is larger. In cold climates that is usually heating.
Is one big mini split better than several small heads?
One head can only condition the space it can blow into. Closed rooms need their own heads, or they will stay hot or cold.
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Room cooling loads use the ENERGY STAR sizing chart and adjustments; heating uses BTU-per-square-foot rules by climate zone. Head and outdoor sizes are common nominal capacities. Combination limits vary by manufacturer; this is a planning estimate, not a Manual J or Manual S selection.