Calculating Mini Split Size

Work out what size ductless mini split you need. Add each room, enter its size and conditions, and the calculator estimates cooling and heating BTU, suggests an indoor unit size for each zone, and totals the outdoor unit capacity for a multi-zone system.

How mini split sizing works

A ductless mini split has an outdoor compressor connected to one or more indoor heads, each serving a room or zone. Sizing means matching each indoor head to its room’s heating and cooling load, and matching the outdoor unit to the combined load. Capacity is measured in BTU per hour (BTU/h); 12,000 BTU/h is one “ton”. Indoor heads commonly come in 6,000, 9,000, 12,000, 18,000 and 24,000 BTU/h sizes, and outdoor units from about 9,000 to 48,000 BTU/h.

The rule of thumb behind the calculator

The base figure and the sun, occupant and kitchen adjustments follow widely published room air conditioner sizing guidance. The climate factor raises capacity where heating needs dominate, because a heat pump must meet the heating load on the coldest days. These rules give a starting point; the professional standard in North America is an ACCA Manual J calculation, which accounts for windows, walls, air leakage, orientation and local design temperatures.

Worked example: two-zone system

A 20 × 16 ft living room (320 sq ft) with 8 ft ceilings, normal sun and average insulation in a moderate climate needs about 6,400 BTU/h; a 9,000 BTU/h head suits it with headroom. A 14 × 12 ft bedroom (168 sq ft) needs about 3,400 BTU/h, which a 6,000 BTU/h head covers. The combined load is about 9,800 BTU/h, so an 18,000 BTU/h two-zone outdoor unit is a common pairing — outdoor units are often sized to the sum of the heads, and manufacturers publish which head combinations each outdoor unit supports.

Avoid oversizing

Bigger is not better with mini splits. An oversized unit cools a room quickly and then cycles off, which leaves humidity high and reduces comfort and efficiency. Inverter-driven mini splits can modulate down to a fraction of their rated capacity, but every model has a minimum output: a 12,000 BTU/h head in a small, well-insulated bedroom may still run in short cycles. Choose the smallest head that meets the load, and consider a single head for adjoining open-plan spaces instead of one in every room.

Heating with a mini split

• Heat pump capacity falls as outdoor temperature drops; check the manufacturer’s capacity at your winter design temperature, not just the rated capacity at 47 °F (8 °C).
• Cold-climate models maintain much more heating output at low temperatures; look for published capacity at 5 °F (−15 °C).
• In very cold areas, size for the heating load and keep a backup heat source for extreme cold.
• Place heads where warm air can reach the rooms that need it; bedrooms with closed doors may need their own heads.

Single-zone or multi-zone?

Where to place indoor heads

Wall-mounted heads are usually installed high on an exterior wall, which keeps the line set and condensate drain short. Place them where the airflow can sweep the room without blowing directly onto a bed, sofa or desk, and keep them clear of curtains and furniture above. In open-plan spaces, one well-placed head often serves more area than two badly placed ones. Ceiling cassettes, floor consoles and slim ducted units are alternatives for rooms where a wall head is not suitable, such as rooms with large windows or where appearance is a priority.

Before you buy

1. Measure each room and note windows, sun direction and insulation.
2. Get a Manual J or equivalent load calculation from an HVAC contractor, especially for whole-home systems.
3. Check the unit’s efficiency ratings (SEER2 and HSPF2 in the US) and cold-weather capacity.
4. Plan head locations, line-set routes and condensate drainage.
5. Check electrical requirements and permits.
6. Check rebates and incentives for heat pumps in your area.

Common sizes at a glance