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Mini Split Size Calculator
Quick answer: A common starting point for a single room is about 20 BTU per hour per square foot of cooling, adjusted for sun, people and kitchens, then raised for heating in colder climates. Round to the nearest standard head size, usually 6,000 to 24,000 BTU/h, and have a licensed installer confirm it before buying.
How it works
Ductless mini-split heads are sold in steps such as 6k, 9k, 12k, 15k, 18k and 24k BTU/h. The calculator multiplies room area by 20 for a cooling estimate, adds 10 percent for a very sunny room or removes 10 percent for heavy shade, adds 600 BTU/h for each regular occupant beyond two, and adds 4,000 BTU/h for a kitchen. It then multiplies by a climate factor to estimate heating need, because heating loads in cold places often exceed cooling loads. Oversizing a head is a common mistake: a unit that is too large cycles on and off, removes less humidity and may be noisier. Undersizing in a cold climate leaves the room cool on the worst nights. Read ducted vs ductless and the DIY mini-split guide before planning an install.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 400 sq ft bedroom-office, normal sun, 2 occupants, not a kitchen, moderate climate (1.2). Cooling ≈ 400 × 20 = 8,000 BTU/h. Heating ≈ 8,000 × 1.2 = 9,600 BTU/h. A 9k or 12k head would be the range to discuss with an installer.
FAQ
Can one head heat a whole floor?
Only if the floor is open; closed doors block airflow, so rooms behind them stay cooler.
Is a bigger head safer?
Not usually. It can short-cycle and handle humidity poorly.
Does sun matter for heating?
Less than for cooling, but north rooms in cold climates may need the larger size.
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