BTUFit › Mini split sizing
Mini split sizing calculator
Mini split heads come in fixed sizes: 6,000, 9,000, 12,000, 18,000, 24,000, 30,000 and 36,000 BTU. Size to the room's cooling load and buy the next head up: a 300 sq ft bedroom lands on 9,000 BTU, a 500 sq ft living room on 12,000. Sun, ceilings and winter heating move the answer.
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Typical installed cost (US ballpark, 2026): a single-zone ductless system in the common 9,000–18,000 BTU range often lands around $2,000–$5,500 equipment plus pro labor, before major electrical work. Multi-zone and cold-climate models run higher. DIY is realistic only with pre-charged / quick-connect kits (sealed line sets that stay closed; no vacuum or charging by the homeowner). A 230V circuit still needs a licensed electrician. Ranges reflect public US cost guides (Angi, HomeGuide and similar), not a bid for your house.
How mini split sizing differs from window AC sizing
The load math is the same: square footage through the ENERGY STAR brackets, adjusted for sun and ceiling height. Two things change:
- Heating can govern. If the mini split is your primary heat, the winter load is usually larger than the summer one, and heat pump capacity shrinks as the outdoor temperature drops. The "primary use" setting adds 15% for mild-winter heating and 40% for cold climates, a stand-in for sizing to the heating load. For a real answer, run the heating BTU calculator and check the model's rated capacity at 5°F, not 47°F.
- Head sizes are coarse. There's nothing between 12,000 and 18,000. If your load falls just above a size, recheck your inputs before paying for the jump, and remember inverter heads modulate down, so landing one size high is survivable in a way it isn't with a window unit.
Quick reference by room size
Standard conditions (8 ft ceilings, average sun, cooling-first), every 100 sq ft from a bedroom to a small house. Change any of those assumptions in the calculator above; the two rows with their own page go deeper on the electrical and the head count at that step.
| Room | Typical space | Load (BTU/h) | Head |
|---|---|---|---|
| 200 sq ft | small bedroom | 6,000 | 6,000 BTU |
| 300 sq ft | bedroom, office | 7,000 | 9,000 BTU |
| 400 sq ft | large bedroom | 9,000 | 9,000 BTU |
| 500 sq ft | large room, studio | 12,000 | 12,000 BTU |
| 600 sq ft | big living room | 14,000 | 18,000 BTU |
| 700 sq ft | open living area | 14,000 | 18,000 BTU |
| 800 sq ft | open living area | 18,000 | 18,000 BTU |
| 900 sq ft | large open plan | 18,000 | 18,000 BTU |
| 1,000 sq ft | small apartment | 18,000 | 18,000 BTU |
| 1,100 sq ft | apartment / main floor | 21,000 | 24,000 BTU |
| 1,200 sq ft | apartment / main floor | 21,000 | 24,000 BTU |
| 1,300 sq ft | open loft / multi-zone | 23,000 | 24,000 BTU |
| 1,400 sq ft | open loft / multi-zone | 23,000 | 24,000 BTU |
| 1,500 sq ft | small house / multi-zone | 24,000 | 24,000 BTU |
Frequently asked questions
What size mini split do I need for one room?
Find the room's square footage, take the ENERGY STAR bracket load, adjust for sun and ceiling height, and buy the next head size up: heads come in 6,000 / 9,000 / 12,000 / 18,000 / 24,000 / 30,000 / 36,000 BTU. A typical 300 sq ft bedroom lands on a 9,000 BTU head; a 500 sq ft living room on 12,000.
Should I size a mini split for heating or cooling?
Whichever load is bigger, and in the northern half of the US that's heating. A head's rated heating capacity is measured at 47°F and falls as it gets colder outside, so if the mini split is your primary heat in a cold climate, set "Primary use" above to heating and look for cold-climate (hyper-heat) models with published capacity at 5°F.
Can one mini split head cool multiple rooms?
Only if the air can actually get there. A single head serves the open space it can "see"; it won't cool bedrooms behind closed doors. For a whole apartment or several rooms, price a multi-zone system (one outdoor unit, two to five heads) instead of one oversized head.
Is it OK to oversize a mini split?
Mini splits tolerate mild oversizing better than window units because inverter compressors throttle down, but most heads only modulate down to 30–40% of rated capacity. A 24,000 BTU head in a room that needs 8,000 will still short-cycle, dehumidify poorly, and cost more up front. Stay within one size step of the load.
Do I need 115V or 230V?
Most 6,000–12,000 BTU single-zone systems are available in 115V versions that run on a standard circuit. At 18,000 BTU and above, you're in 230V territory and may need an electrician to run a new circuit, so budget for it.
How much does a mini split cost installed?
A single-zone ductless system in the common 9,000–18,000 BTU range often lands around $2,000–$5,500 for equipment plus pro labor, before major electrical work; multi-zone and cold-climate models run higher. DIY only fits pre-charged / quick-connect kits, and a 230V circuit still needs a licensed electrician. Labeled 2026 US ranges, not a quote.