Mini-splits (ductless heat pumps with one outdoor unit and one or more indoor heads) are sized per zone, not per house. Each indoor head matches its own room or open area, and standard head sizes are 9k, 12k, 18k, 24k, and 36k BTU/hr. Get the sizing wrong and you either short-cycle (oversized) or run continuously without reaching setpoint (undersized). Here's the per-zone math and the standard sizes to round to.

The per-zone formula

Mini-splits need to handle the larger of the room's cooling or heating load:

cooling = ft² × 25 × sun_adj heating = ft² × climate_factor design = max(cooling, heating)

Example: 350 ft² master bedroom in Boston, normal sun. Cooling = 350 × 25 = 8,750 BTU/hr. Heating = 350 × 45 = 15,750 BTU/hr. Design load = 15,750 (heating drives). Round up to 18,000 BTU/hr (18k) head.

The mini-split sizing calculator runs both loads and snaps to the nearest standard head size.

Standard mini-split head sizes

Indoor heads come in fixed sizes from every major manufacturer (Mitsubishi, Daikin, Fujitsu, LG):

  • 9k BTU (9,000 BTU/hr): Small bedrooms, small offices, 250-400 ft² in mild climate
  • 12k BTU: Medium bedrooms, small living rooms, 400-550 ft²
  • 18k BTU: Large bedrooms, medium open spaces, 550-800 ft²
  • 24k BTU: Open-concept living, large rooms, 800-1,100 ft²
  • 36k BTU: Large open spaces, small whole-home single-zone, 1,100-1,500 ft²

Above 36k for a single zone is rare in residential — open spaces over 1,500 ft² heat and cool more evenly with two smaller heads (e.g., two 18k) than one big one. Two heads also let one run while the other is off.

Why per-zone sizing beats whole-house sizing

Central AC and forced-air furnaces share one air handler. They mix air across rooms, so a 3-ton system handles a 1,800 ft² house at the average load.

Mini-split zones don't share air. A 9k BTU bedroom head handles only that bedroom — the kitchen across the hall is on its own zone. So per-zone load is what matters, not house total. The per-zone factor (25 BTU/ft² cooling) is slightly higher than central AC (20 BTU/ft²) because:

  • No air mixing means the zone has to fully condition its own load
  • Doors are often open between zones, leaking conditioned air
  • Each zone has its own envelope (windows, walls) with no “averaging”

Multi-zone systems — sizing the outdoor unit

Multi-zone systems (one outdoor unit, 2-8 indoor heads) need the outdoor unit sized for the simultaneous load — usually 60-80% of the sum of all heads, because not every zone hits peak load at the same time.

Example: a 4-zone Mitsubishi system with two 9k heads and two 12k heads. Sum = 42k BTU. Outdoor unit sized at ~30k (about 72% of sum) handles the simultaneous load. Manufacturer configurators (Mitsubishi's Selection Tool, Daikin's VRV, etc.) match outdoor units to head combinations.

Cold-climate mini-splits — what changes

Standard mini-splits lose capacity rapidly below 20°F outdoor temperature. Hyper-heat or cold-climate models (Mitsubishi H2i, Daikin LV-Series) maintain 100% rated capacity down to 5°F and produce useful heat to -15°F. In climates where January nights drop below 0°F:

  • Spec hyper-heat models, not standard models — capacity at design temp matters more than rated capacity
  • Add 20% to the heating load for the design temperature, since the unit derates as outdoor temp drops
  • Consider supplemental electric resistance for the coldest hours, or pair with a backup furnace

What pros do differently

Don't oversize.A 12k head in a 250 ft² bedroom short-cycles, doesn't dehumidify, and runs less efficient than a properly-sized 9k. The 9k modulates down smoothly; the 12k either cycles on/off or runs at 50% all day.

Match the line set length to the manufacturer spec. Mini-split refrigerant lines have charge limits — over 50 ft typically requires extra refrigerant added at install. Pre-charged units only handle 25-30 ft of line set without adjustment.

Verify the head height clearance. Wall-mounted heads need 6 in clearance above and 4 in to either side. Ceiling cassette heads need 9-12 in plenum depth. Confirm before ordering.

Quick FAQ

What size mini-split for a 400 ft² room? Cooling-driven: 12k BTU. Heating-driven in cold climate (50 BTU/ft²): 24k. Run the calculator with your climate to see which controls.

Can one mini-split heat a whole house? Up to about 1,500 ft² with a 36k single-zone, if the house is relatively open. For multi-room layouts, multi-zone (multiple heads on one outdoor unit) is the right setup.

Do mini-splits work below freezing? Standard models lose capacity rapidly below 20°F. Hyper-heat models hold 100% capacity to 5°F and produce useful heat to -15°F.

Whole-house central system instead? Run the AC tonnage calculator for central AC sizing or the furnace size calculator for ducted heating.