What size mini-split do I need for each room?

This calculator’s heads are 9,000 through 36,000 BTU/hr. It picks the smallest head at or above the room load. This page does not assign a head to a room type.

The head sizes in this calculator

The mini-split tab uses those seven sizes. The room load is the same function as the room-BTU tab. Use the calculator. Window-unit chart sizing is a different table: 300 sq ft is one ENERGY STAR page. Certified cooling capacity, SEER2, EER2, and HSPF2 for single-zone models are on the ENERGY STAR mini-split chart.

Those seven sizes are this calculator’s head list, printed above.

Multi-zone vs several single-zone units

For two or three rooms, compare a single-zone unit with a multi-zone unit before you default to multi-zone. This page does not print prices. Multi-zone wins when outdoor space is limited or rooms need simultaneous different setpoints. Multi-zone condensers divide their capacity across active heads, and if all zones call at once each head gets less than its nameplate — one reason pros size multi-zone systems to the sum of room loads with attention to diversity, not just head count.

The oversizing trap, quantified

An oversized fixed-speed unit can satisfy the thermostat and shut off before the room has dried out, which leaves the air cold and clammy and makes the compressor cycle. Inverter mini-splits throttle down instead of shutting off. A head far above the load is capacity you can't use. This page does not print a percent of oversize that is safe.

DIY vs pro install

Pre-charged line-set units have made some single-zone installs a homeowner job: hang the head, drill the line-set opening, connect the line set, and have an electrician do the circuit. Multi-zone refrigerant handling, the vacuum pull, and charge weighing are not that job.

ENERGY STAR mini-splits at the capacities this page sizes for

This page lists calculator heads of 9,000, 12,000, 15,000, 18,000, 24,000, 30,000, and 36,000 BTU/hr. The rows below are copied from the ENERGY STAR mini-split chart where cooling capacity is exactly 9,000, 12,000, or 18,000 Btu/h. Each figure is the published value, and each row keeps its ENERGY STAR record link. The duct tab takes an equipment rating in BTU/hr: open it at 9,000 BTU/hr.

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Run your own numbers in the HVAC load calculator, then check airflow needs in the duct CFM guide.

Why a short cycle feels clammy: how an air conditioner works. What to change before you buy a larger head: air conditioner efficiency.

A head that is already old is a different question from a head that is the wrong size: how long air conditioners and mini-splits last. A ductless heat pump still needs a heating load beside the cooling load: heat pump sizing.