This gets you to the right capacity in about a minute. It is a starting point, not a load calculation.
Mini splits are sized in BTU. For a typical insulated room in a moderate climate, budget 20 BTU per square foot.
| Room size | Capacity | Typical use |
|---|---|---|
| 150 to 350 sq ft | 9,000 BTU | Bedroom, home office, nursery |
| 350 to 550 sq ft | 12,000 BTU | Large bedroom, small living room |
| 550 to 800 sq ft | 18,000 BTU | Living room, open kitchen and dining |
| 800 to 1,200 sq ft | 24,000 BTU | Great room, small open-plan floor |
| 1,200 to 1,800 sq ft | 36,000 BTU | Large open plan, finished basement |
Worked example
A 14 by 16 ft bedroom is 224 sq ft. At 20 BTU per square foot that is 4,480 BTU, which lands in the 9,000 BTU row. Nothing in the room is unusual, so 9,000 BTU is the answer.
Three things move the number, and they are cumulative.
Do not oversize
This is the most common mistake. An oversized unit cools the air fast, satisfies the thermostat, and shuts off before it has pulled the humidity out. The room ends up cold and clammy, and the compressor short-cycles, which shortens its life. Slightly under is better than substantially over.
One outdoor unit can run multiple indoor heads. Count the rooms you want to control separately, not the rooms you want to condition. Two bedrooms that are always the same temperature can share a head; a bedroom and a home office on opposite schedules should not.
When you add zones, size each head to its own room from the table above, then pick a condenser rated at or above the total. A 9K bedroom plus a 12K living room needs a 2-zone condenser of 18,000 BTU or more.
In a cold climate the heating load, not the cooling load, decides the size. Below about 20°F a heat pump’s output drops off, and the published BTU rating is measured at 47°F. If you are heating through a real winter, size on the heating requirement and check the unit’s rated capacity at your design temperature, which is on the AHRI certificate linked on every one of our equipment pages.
Still unsure?
A room that is unusual in any way (very old windows, a sunroom, a garage conversion, a space over an unconditioned crawlspace) deserves a real Manual J load calculation from a contractor. The table above will not get those right.
Every figure here is either arithmetic or standard sizing practice. Nothing on this page comes from a manufacturer’s marketing copy.