Mini Split Calculator: Size Each Zone, Head and Outdoor Unit

Every zone is sized on its own: area x zone rate x insulation x sun x max(1, ceiling / 8) plus 600 BTU/h for each occupant above two. The head is the smallest of 6,000, 9,000, 12,000, 18,000 or 24,000 BTU/h that lands between 100% and 125% of that cooling load. When the smallest head above the load is more than 125%, the zone is flagged oversized. When the load is above 24,000, it is split across identical heads.

With the default three zones, the 400 sq ft living room (10,000 BTU/h) gets a 12,000 head, the 200 sq ft bedroom (5,000) gets a 6,000 head, and the 150 sq ft office with one occupant (3,750) gets a 6,000 head flagged at 160%. Cooling totals 18,750 BTU/h, the heads connect 24,000, and the smallest ladder size at or above 18,750 is a 24,000 BTU/h outdoor unit. Heating totals 26,250 BTU/h, which is why the winter table matters more than the cooling label.

Each band starts at the room's cooling load and extends to 125% of it. A head inside the band is a fit. In zone 4 a 400 sq ft room needs 10,000 BTU/h, so the 12,000 head sits inside; a 300 sq ft room at 7,500 gets the 9,000. Switch the zone to see rooms whose band falls between two sizes.

Zone 4, average insulation and sun, 8 ft ceilings and two occupants.

• Adding every head's nameplate to match the outdoor unit; connected capacity and delivered capacity are different numbers in the combination table.
• Sizing the outdoor unit from the sum of peaks when the zones never peak together, or from the smallest zone when they do.
• Forgetting the smallest active zone: one small room calling alone can sit below the outdoor unit's minimum output.
• Choosing from cooling alone in a cold climate and then finding the heating table short on the coldest night.
• Expecting one head to condition rooms behind closed doors or down a hallway.
• Ignoring line-set length and lift, which reduce capacity and have published limits.

A ductless system pairs an outdoor condenser with one or more indoor heads joined by insulated refrigerant lines, a control wire and a condensate drain. High-wall heads are the most common, while recessed cassettes, low-static ducted air handlers and floor consoles can share the same outdoor unit when a wall head will not suit the room. Plan the line-set route before fixing head locations, because length and elevation limits apply.

An inverter compressor modulates its output, but only within a range. Maximum capacity matters at design conditions and minimum capacity matters in spring and fall, when a 10,000 BTU/h room may need a third of that. Ask for the minimum cooling capacity of the exact indoor and outdoor pairing and for its heating capacity at your winter design temperature. Dry mode behavior varies by product and is not dedicated dehumidification.

A single-zone outdoor unit gives all of its capacity to one head. A multi-zone unit shares capacity across heads through a branch box or direct lines, which can reduce the number of outdoor units but makes the combination table essential. For the layout trade-offs against ducted equipment, read mini-split vs central air, and for a room-level walk-through see what size mini-split you need.

Write down the smallest room that can call while every other head is satisfied. In the default plan that is the 150 sq ft office at 3,750 BTU/h, well under the 6,000 BTU/h head and far under the 24,000 BTU/h outdoor unit. Ask what the outdoor unit delivers in that case and how the controls share refrigerant. A wide advertised compressor range does not tell you what reaches that one room.

Bring the zone totals to the equipment capacity checks with data for the exact indoor and outdoor match, including heating output against the 26,250 BTU/h default winter total. For a single room before choosing a head, the room-by-room BTU calculator gives the load and window-unit comparison, and the room schedule adds walls and windows.