Mini-Split Zone Sizing Calculator
Size a ductless mini-split head for a single zone from its area, insulation, sun and occupancy, and match it to a standard capacity.
How it works
Load = area × insulation factor × sun factor + 600 per extra person · Round up to a standard head (9k/12k/18k/24k…)
Ductless mini-splits are sold in fixed head sizes — 9,000, 12,000, 18,000 and 24,000 BTU/hr are the common ones — so zone sizing is about landing on the right step. A tight new room needs about 20 BTU/hr per square foot while an older, drafty or top-floor space can need 30+. Because mini-splits with inverter compressors modulate down efficiently, matching the load closely matters even more than with a single-speed unit: an oversized head cycles and dehumidifies poorly.
Worked example
A 400 ft² average-insulation, sunny bonus room with 2 occupants: 400 × 25 = 10,000 BTU/hr, ×1.15 for sun = 11,500 BTU/hr. That rounds up to a 12,000 BTU/hr (1-ton) head.
Frequently asked questions
Should I round a mini-split up or down?
Round up to the next standard head, but only just — an inverter unit modulates down to meet a lighter load, whereas a badly oversized head short-cycles and leaves the room humid. Avoid jumping two sizes “to be safe”.
One multi-zone outdoor unit or several single-zone units?
Single-zone systems are usually more efficient and keep running if one fails, but a multi-zone condenser saves outdoor space and refrigerant runs. Size each indoor head to its own room load either way.
Why is top-floor or sunny space rated higher?
Heat gain through a sun-struck roof or wall of glass adds load beyond what floor area alone implies, so those zones use a higher BTU-per-square-foot factor.
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This calculator is for educational purposes. Double-check important results before acting on them.