What Size Mini Split Do I Need?

Plan on about 25 BTU per square foot for a mini split, a little more than a whole-house rule because a ductless head conditions one room to its design temperature. So a 12,000 BTU (1 ton) unit suits about 500 sq ft, an 18,000 BTU about 750 sq ft, and a 24,000 BTU about 1,000 sq ft. Use 27 BTU per sq ft in a warm climate, or up to 30 if you are relying on it for heat in a cold climate. Add capacity for vaulted ceilings, a sunny room, or a kitchen.

About 480 to 500 square feet. A 12,000 BTU mini split is one ton of cooling and is the most common single-room size, good for a large bedroom, a living room, or a small open area. A 9,000 BTU unit covers about 360 sq ft (a standard bedroom or office), and an 18,000 BTU covers about 720 to 750 sq ft (a large great room).

A single-zone mini split is one outdoor unit paired with one indoor head, sized to that room, and ranges from about 6,000 to 36,000 BTU. A multi-zone system is one outdoor unit feeding several indoor heads, each sized independently to its own room; the outdoor condenser is sized to support the combined heads (up to about 55,000 BTU). Size each room's head to that room first, then pick an outdoor unit that covers the total.

Yes. Almost all mini splits are heat pumps, so the same unit heats and cools. Its cooling BTU and heating BTU are usually close, but heating output drops as the outdoor temperature falls, so in a cold climate you size to the heating load at your design temperature and choose a cold-climate (hyper-heat) model rated to hold capacity well below freezing. If you only need cooling, size to the cooling BTU; if heating is the harder job, size to that.

Less bad than oversizing a conventional system, but still not ideal. Inverter mini splits modulate their output, often down to 15 to 20 percent of capacity, so a slightly oversized unit just runs at reduced output. But a badly oversized unit still short-cycles, controls humidity poorly, and costs more than needed. Size to the load and only round up one step if you are between sizes.