Mini-Split Sizing Calculator — BTU/hr & Head Size
Right-size a ductless mini-split head: the BTU/hr cooling load a room needs and the nearest standard head size to buy.
How it works
A ductless mini-split serves one zone, so sizing a head starts with the same area rule the whole industry uses: roughly 20 BTU/hr per square foot of floor in a mild climate, rising toward 25 in a moderate one and 30 where the head also carries a cold heating season. Multiply your room area by that factor to get the cooling load the head must cover.
Manufacturers stock heads in fixed capacities — 6,000, 9,000, 12,000, 15,000, 18,000, 24,000, 30,000, and 36,000 BTU/hr — so the calculator rounds your load up to the smallest head that meets it. A 400 square-foot room in a moderate climate needs about 10,000 BTU/hr, which a 12,000 BTU head covers comfortably.
Bigger is not better with mini-splits. An oversized head short-cycles: it blasts to temperature, shuts off, and never runs long enough to wring humidity out of the air, leaving the room cold and clammy. Modern inverter heads modulate well, but you still want the smallest head that carries the load. For a whole house, or a room with heavy sun or cathedral ceilings, a Manual J load calculation is the definitive sizing before you buy.
load = 400 × 25 = 10,000 BTU/hr (2.93 kW)
nearest head ≥ load = 12,000 BTU/hr head