Window AC size calculator
CoolingRoom air conditioner BTU/h from the ENERGY STAR chart, with sun, shade and kitchen adjustments.
BTU/h · room
Mini split sizing calculator
A ductless head only serves the room it is mounted in. Size each zone on its own, then check that the heads added together have not quietly doubled the house load.
Which load: Cooling
One row per head. A ductless head serves the room it is in, not the house.
| Zone | Area | Exposure | Load | Head | Row actions |
|---|---|---|---|---|---|
| 11,000 | 12k | ||||
| 5,800 | 6k | ||||
| 3,000 | 6k | ||||
| Total | 780 | 19,800 | 24k |
3
Combined head capacity 24,000 BTU/h against a 19,800 BTU/h load
Why ductless sizing is different
A ducted system distributes one capacity around a house. A ductless system parks a fixed capacity in one room and hopes the doorway does the rest. That changes the sizing rule completely: you size to the room, not to the floor area of the house, and a sunroom with three glass walls needs several times the head that its square footage suggests.
The exposure column is doing that work. "Sunroom" multiplies the cooling load by 1.6 because glass on three sides is not a wall. A north-facing interior office gets 0.9.
The multi-zone trap
Heads add up faster than the load does
Head sizes come in a coarse ladder — 6k, 9k, 12k, 15k, 18k. Round every room up to the next rung and a house with a 30,000 BTU/h load ends up with 42,000 BTU/h of heads. The outdoor unit modulates, but each head has a minimum output, and below it the system short-cycles that zone: cold blasts, poor dehumidification, a compressor that never settles.
The oversize ratio in the panel is the check. Under about 1.3 is fine. Above 1.5, combine two small adjacent rooms onto one head or move to a ducted mini-split air handler for that part of the house.
How many heads a house needs
| Layout | Heads | Why |
|---|---|---|
| Studio or open one-bedroom | 1 | One space, doors open, air moves |
| Open-plan main floor + 2 bedrooms | 3 | Bedroom doors close at night |
| Two-storey, 4 bedrooms | 5–6 | Every closed room needs its own |
| Addition or garage conversion | 1 | The classic single-zone retrofit |
| Whole house, ducts already there | 0 | A ducted system is cheaper per ton |
If the answer is five or six heads, price a ducted heat pump against it before you commit — the ductless against central comparison works through where the crossover sits.
Heating mode is the harder case
Switch the control to heating and the numbers move: in zone 5 the heating load per square foot is roughly double the cooling load. A head sized for a bedroom's cooling will not hold that bedroom in January. In a cold climate, size the heads on the heating column and accept the cooling oversize — inverter equipment turns down far better than it turns up. The heat pump sizing calculator finds the balance point for the outdoor unit.
Sources: 10 CFR 430.32(c)(5)(ii) (read 2026-09-04)
Climate-zone notes
The BTU/h per square foot band a mini-split head is sized on is a climate-zone band: the same room takes a smaller head in a mild zone and a larger one where design temperatures are colder. Multi-head systems make that sharper, because the outdoor unit has to carry every head at the zone's design condition at once, not the sum of their rated capacities.
- Zone 1
- Zone 2
- Zone 3
- Zone 4
- Zone 5
- Zone 6
- Zone 7
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