Mini Split BTU Calculator
Size a ductless mini-split from room area, ceiling height, sun exposure, insulation, climate, occupants and kitchen load.
How it works
BTU = area × 20 × (ceiling ÷ 8) × sun × insulation × climate + occupants × 600 + kitchen · then round to nearest 1,000
The 20 BTU per square foot rule of thumb is only a starting point; a room’s real load swings with how much air it holds (ceiling height), how much heat pours in through glass (sun), how well the shell holds temperature (insulation), and the regional climate. Those are multipliers on the base load. People and appliances add fixed heat on top — roughly 600 BTU per occupant and about 4,000 BTU for an open kitchen — so those are added, not multiplied. The result is rounded to the nearest 1,000 because mini-splits only come in set capacities.
Worked example
A 400 ft² room with 8 ft ceilings (×1.0), average sun and insulation (×1.0), a moderate climate (×1.0) and 2 occupants, no kitchen: 400 × 20 = 8,000 BTU base, unchanged by the multipliers, plus 2 × 600 = 1,200, for 9,200 — rounded to 9,000 BTU, or 0.75 tons. That points squarely at a 9,000 BTU (¾-ton) head.
Frequently asked questions
Why not just use 20 BTU per square foot?
The flat rule ignores everything that actually drives load — a sun-baked, poorly insulated room in a hot climate can need 40% more than the same-size shaded, well-sealed room up north. The multipliers here nudge the base estimate toward what the room really demands so you do not badly over- or under-size the unit.
Is it bad to oversize a mini-split?
Yes. An oversized unit cools the air fast but short-cycles before it removes humidity, leaving the room cold and clammy, and it wears the compressor with frequent starts. Mini-splits modulate well at part-load, so sizing close to the real load — not padding it — gives the best comfort and efficiency.
Does this size heating too?
A mini-split heat pump both heats and cools, and this estimates the cooling load. Heating load depends on your winter design temperature and can be higher in cold climates, so in the north size to the larger of the two — or have a pro run a Manual J before buying.
Related calculators
Estimate for planning. Confirm with a Manual J load calculation before buying equipment.