Mini split size calculator
Add each room with its floor area, sun exposure and how many people use it. You get the BTU each room needs by the ENERGY STAR chart, the mini split head size for each, and the outdoor unit for the whole system.
Inputs
Result
| Room | BTU/h needed | Head size |
|---|
Mini split size by room area
ENERGY STAR's sizing chart for the cooling capacity a room needs, and the standard mini split head that covers it, for an average room with two people.
| Room area, ft² | BTU/h needed | Mini split head |
|---|
How it is worked out
A 300 ft² bedroom needs 7,000 BTU/h by the chart: a 9,000 BTU/h head. A very sunny 200 ft² office needs 6,000 + 10% = 6,600: also 9,000. A 250 ft² kitchen needs 6,000 + 4,000 = 10,000: a 12,000 head. Together the three rooms need 23,600 BTU/h, which a 24,000 BTU/h three-zone outdoor unit covers.
Heads and the outdoor unit
Each room gets an indoor head sized to its own load. In a multi-zone system the heads are connected to one outdoor unit, sized to the combined load rather than the sum of the head sizes, because the rooms do not all peak at once. Manufacturers allow the connected heads to add up to more than the outdoor unit's rating; check the combination table for the brand you buy.
Do not oversize
Pick the smallest head that covers the room, not one size bigger "to be safe". Mini splits are inverter-driven and turn down well below their rating, which is why the step up to the next available size is fine; two steps up and the unit runs at its minimum, cycles on and off, and leaves the room clammy in summer. For whole systems, ACCA Manual S, the sizing standard codes reference, keeps heat pumps to 115% of the cooling load in cooling-dominated climates and 125% where heating dominates.
Heating with a mini split
The ENERGY STAR chart sizes for cooling. In cold climates, size for heating too: check the unit's heating capacity at your winter design temperature, which for cold-climate models is published at 5°F, against the room's heat loss. A heating estimate is in the furnace size calculator.
For a whole-house central system, use the AC tonnage calculator; for the circuit, the minimum circuit ampacity calculator.
Questions
What size mini split do I need?
Find the room's floor area in the ENERGY STAR chart, adjust for sun, shade, people and kitchens, and pick the next standard size up. A 300 ft² room needs 7,000 BTU/h: a 9,000 BTU/h mini split.
How many square feet does a 12,000 BTU mini split cool?
By the ENERGY STAR chart, 450 to 550 ft² for an average room. In a very sunny room, about 450 ft²; with a kitchen, much less.
How many square feet does a 9,000 BTU mini split cool?
About 350 to 400 ft² by the ENERGY STAR chart, and up to 450 ft² in a heavily shaded room.
How many BTU per square foot for a mini split?
The ENERGY STAR chart works out to about 33 BTU/h per ft² for a 150 ft² room, about 22 at 550 ft² and under 15 at 2,500 ft², since a big room has less wall and window for its floor area.
What size mini split for 1,000 square feet?
18,000 BTU/h by the ENERGY STAR chart for one open space; split into rooms, size each room and let the outdoor unit cover the total.
Can I oversize a mini split?
Go to the next size that covers the room, no further. Inverter mini splits turn down, so one step up is fine; bigger units sit at their minimum, cycle and dehumidify poorly. For whole systems ACCA Manual S keeps heat pumps to 115% of the cooling load, or 125% where heating dominates.
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