Mini-split sizing calculator
A ductless mini-split is sized one room at a time. Enter the floor area of each room that gets an indoor unit. The calculator gives the cooling capacity for each one and the combined capacity the outdoor unit has to support.
- Zone 1, 400 sq ft12,000 BTU/h
One zone needs a single-zone system: one indoor unit and a matching outdoor unit of the same capacity.
These are cooling capacities for rooms with 8-foot ceilings and average insulation. In a cold climate, size for heating instead and check the capacity the unit keeps at your winter design temperature.
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Mini-split size by room size
A ductless mini-split is sized one room at a time. This table gives the ENERGY STAR chart figure for each room size and the indoor unit size the calculator picks. It assumes average sun, two people and an 8 foot ceiling.
| Room floor area (sq ft) | Chart capacity (BTU/h) | Mini-split size (BTU/h) |
|---|---|---|
| 100 up to 150 | 5,000 | 6,000 |
| 150 up to 250 | 6,000 | 6,000 |
| 250 up to 300 | 7,000 | 9,000 |
| 300 up to 350 | 8,000 | 9,000 |
| 350 up to 400 | 9,000 | 9,000 |
| 400 up to 450 | 10,000 | 12,000 |
| 450 up to 550 | 12,000 | 12,000 |
| 550 up to 700 | 14,000 | 15,000 |
| 700 up to 1,000 | 18,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 | 24,000 |
| 1,200 up to 1,400 | 23,000 | 24,000 |
| 1,400 up to 1,500 | 24,000 | 24,000 |
| 1,500 up to 2,000 | 30,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 | 36,000 |
“Up to” means just under. The calculator rounds up to the next size on its list: 6,000, 9,000, 12,000, 15,000, 18,000, 24,000, 30,000 and 36,000 BTU/h. The list is the calculator’s own, not ENERGY STAR’s.
How the calculator works
ENERGY STAR publishes a sizing chart for room air conditioners. We found no government chart for mini-splits, so the calculator applies the room chart to each zone. The result is a rule of thumb.
For each zone, the calculator takes the chart figure for the floor area. It subtracts 10% for a heavily shaded room, adds 10% for a very sunny one and adds 600 BTU/h for each person beyond two. It then rounds up to the next indoor unit size.
The total is the sum of the indoor unit sizes. A zone above 2,500 square feet, or one that needs more than 36,000 BTU/h, must be split into two zones.
Limits of this calculator
- It sizes for cooling only. In a cold climate, size for heating instead. Use the heating capacity the unit keeps at your winter design temperature, the cold outdoor temperature the heating is sized for. It comes from the manufacturer’s data. ACCA Manual S selects equipment from that kind of performance data.
- Multi-zone outdoor units accept only listed combinations. The manufacturer’s tables show which indoor units fit.
- Open floor plans and high ceilings change the load. The chart assumes an 8 foot ceiling, and ENERGY STAR says to choose a higher rating for taller ceilings.
- The kitchen allowance is left out. ENERGY STAR adds 4,000 BTU/h for a kitchen.
- Rounding up adds capacity. ENERGY STAR warns that an oversized room air conditioner cools the room before it removes humidity.
Sources
- ENERGY STAR, Room Air Conditioners (chart and adjustments): https://www.energystar.gov/products/room_air_conditioners
- ACCA, Manual J, residential load calculation: https://www.acca.org/standards/technical-manuals/manual-j
- ACCA, Manual S, equipment selection: https://www.acca.org/standards/technical-manuals/manual-s
Related
For one room or a whole house, use the BTU calculator. For a ducted central system, use the AC tonnage calculator. Read how long heat pumps last on the heat pump lifespan page.
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Calculator last reviewed on October 5, 2026. Method and sources.