The short answer
A 300 sq ft room with two exterior walls, a roof overhead and average insulation needs a 6,000 BTU mini split head in Houston (summer design temperature 97°F, cooling load 5,200 BTU/h) and the same 6,000 BTU head in Seattle (85°F, 4,200 BTU/h). In Minneapolis, where it falls to -6°F, heating needs 4,500 BTU/h, still a 9,000 BTU head. Rooms with more exterior wall, glass or sun need larger heads.
The same room in every state
A 300 sq ft room with two exterior walls, a top-floor ceiling and average insulation, sun and 2 people, sized for the climate of each state's largest city. Loads are in BTU/h.
| Alabama Huntsville | 3A | 91 | 4,800 | 2,800 | 6,000 |
|---|---|---|---|---|---|
| Alaska Anchorage | 7 | 72 | 3,600 | 4,400 | 9,000 |
| Arizona Phoenix | 2B | 110 | 5,600 | 1,600 | 6,000 |
| Arkansas Little Rock | 3A | 95 | 5,000 | 2,700 | 6,000 |
| California Los Angeles | 3B | 86 | 4,200 | 1,400 | 6,000 |
| Colorado Denver | 5B | 92 | 4,600 | 3,800 | 6,000 |
| Connecticut Bridgeport | 5A | 86 | 4,400 | 3,200 | 6,000 |
| Delaware Wilmington | 4A | 90 | 4,600 | 3,100 | 6,000 |
| District of Columbia Washington | 4A | 92 | 4,800 | 2,800 | 6,000 |
| Florida Jacksonville | 2A | 94 | 5,000 | 1,700 | 6,000 |
| Georgia Atlanta | 3A | 92 | 4,800 | 2,600 | 6,000 |
| Hawaii Honolulu | 1A | 89 | 4,700 | 200 | 6,000 |
| Idaho Boise | 5B | 97 | 4,900 | 3,200 | 6,000 |
| Illinois Chicago | 5A | 89 | 4,500 | 3,800 | 6,000 |
| Indiana Indianapolis | 4A | 89 | 4,600 | 3,600 | 6,000 |
| Iowa Des Moines | 5A | 90 | 4,600 | 4,200 | 9,000 |
| Kansas Wichita | 4A | 97 | 5,100 | 3,400 | 6,000 |
| Kentucky Louisville | 4A | 91 | 4,700 | 3,200 | 6,000 |
| Louisiana New Orleans | 2A | 93 | 4,900 | 1,800 | 6,000 |
| Maine Portland | 6A | 84 | 4,200 | 3,800 | 6,000 |
| Maryland Baltimore | 4A | 92 | 4,800 | 3,000 | 6,000 |
| Massachusetts Boston | 5A | 88 | 4,500 | 3,300 | 6,000 |
| Michigan Detroit | 5A | 88 | 4,500 | 3,600 | 6,000 |
| Minnesota Minneapolis | 6A | 88 | 4,500 | 4,500 | 9,000 |
| Mississippi Jackson | 3A | 94 | 4,900 | 2,400 | 6,000 |
| Missouri Kansas City | 4A | 92 | 4,800 | 3,600 | 6,000 |
| Montana Billings | 6B | 92 | 4,600 | 4,400 | 9,000 |
| Nebraska Omaha | 5A | 92 | 4,700 | 3,900 | 6,000 |
| Nevada Las Vegas | 3B | 108 | 5,500 | 2,000 | 6,000 |
| New Hampshire Manchester | 5A | 90 | 4,600 | 3,600 | 6,000 |
| New Jersey Newark | 4A | 92 | 4,800 | 3,200 | 6,000 |
| New Mexico Albuquerque | 4B | 94 | 4,700 | 2,800 | 6,000 |
| New York New York | 4A | 88 | 4,500 | 3,100 | 6,000 |
| North Carolina Charlotte | 3A | 93 | 4,900 | 2,600 | 6,000 |
| North Dakota Fargo | 6A | 88 | 4,500 | 4,900 | 12,000 |
| Ohio Columbus | 4A | 89 | 4,600 | 3,500 | 6,000 |
| Oklahoma Oklahoma City | 3A | 97 | 5,100 | 3,000 | 6,000 |
| Oregon Portland | 4C | 90 | 4,500 | 2,400 | 6,000 |
| Pennsylvania Philadelphia | 4A | 92 | 4,800 | 3,100 | 6,000 |
| Rhode Island Providence | 5A | 86 | 4,400 | 3,500 | 6,000 |
| South Carolina Charleston | 3A | 92 | 4,800 | 2,300 | 6,000 |
| South Dakota Sioux Falls | 6A | 90 | 4,600 | 4,400 | 9,000 |
| Tennessee Nashville | 3A | 93 | 4,900 | 2,900 | 6,000 |
| Texas Houston | 2A | 97 | 5,200 | 2,000 | 6,000 |
| Utah Salt Lake City | 5B | 98 | 4,900 | 3,000 | 6,000 |
| Vermont Burlington | 6A | 87 | 4,400 | 4,200 | 9,000 |
| Virginia Virginia Beach | 3A | 92 | 4,800 | 2,600 | 6,000 |
| Washington Seattle | 4C | 85 | 4,200 | 2,400 | 6,000 |
| West Virginia Charleston | 4A | 89 | 4,600 | 3,200 | 6,000 |
| Wisconsin Milwaukee | 5A | 87 | 4,400 | 3,900 | 6,000 |
| Wyoming Cheyenne | 5B | 88 | 4,300 | 4,000 | 6,000 |
How it works
The formula
We add up the heat that crosses the room's outside surfaces on a design-temperature day, with the same numbers as our central AC calculator but for one room. Then we pick the head size. Only the exterior walls and, on a top floor, the ceiling count. Walls shared with other rooms, the floor and ducts do not.
walls = U × net wall area × (ΔT + 10°F sun)
ceiling = U × floor area × (ΔT + 40°F hot attic), top floor only
glass = U × window area × ΔT
sun = window area × 70 × SHGC
air leaks = 1.08 × CFM × ΔT
people and lights = 230 × people + 400
humidity = 200 × people + 0.68 × CFM × grains
heating = walls + ceiling + glass + air leaks, with the heating ΔT, no sun and no people
Assumptions
- Indoors is kept at 75°F for cooling and 70°F for heating, as in Manual J.
- The room is a square box. Each exterior wall is as long as the room is wide, and every exterior wall has windows of 8% of the floor area (two walls: 15%).
- Insulation levels set the U-factors, the windows' solar heat gain coefficient and air leakage. Average is R-13 walls (U 0.075), an R-30 attic (U 0.033), double-pane windows (U 0.5, SHGC 0.55) and 0.45 air changes an hour. Leakage is scaled to the number of exterior walls (one quarter per wall).
- "Sun" scales the sun through windows and on walls: 0.7 for shaded, 1.3 for full sun.
- There are no ducts, so no duct loss, which is why a head is smaller than the same room on central air.
- Head size rule. The cooling target is 95% to 115% of the cooling load, as in ACCA Manual S. The head must also cover the heating load, because the same head heats the room, and it delivers less heat as it gets colder: we assume a variable-speed head gives 90% of its rating at 17°F and 75% at 5°F, as in our heat pump calculator. We take the smallest head that does both. If the heating load needs a bigger head than cooling allows, heating wins and we say so. If even the smallest head is above the range, it's still the answer.
- Head sizes: 6,000, 9,000, 12,000, 15,000, 18,000, 24,000, 30,000, 36,000 BTU. Rooms above 36,000 BTU need two heads.
- Multi-zone: up to 5 rooms on one outdoor unit of 18,000, 24,000, 30,000, 36,000, 42,000, 48,000 BTU. We size the outdoor unit to the sum of the room loads by the same rule, and list the heads separately. Makers allow an outdoor unit smaller than the sum of the heads only under their own diversity rules, so check their combination chart.
- Heating uses the nameplate size. Cold-climate heat pump models hold most of their capacity down to 5°F; standard models lose much more. In a cold climate, check the maker's capacity at your winter design temperature.
Worked example: a 300 sq ft room in Houston
8 ft ceiling, two exterior walls, top floor, average insulation and sun, 2 people. Summer design 97°F, zone 2A, so ΔT = 22°F.
- Geometry: each wall is √300 = 17.3 ft long; windows 2 × 7.5% × 300 = 45 ft²; net walls 2 × 17.3 × 8 − 45 = 232 ft²; air leaks 9 CFM.
- Walls 0.075 × 232 × 32 = 600; ceiling 0.033 × 300 × 62 = 600; glass 0.5 × 45 × 22 = 500.
- Sun 45 × 70 × 0.55 = 1,700; air 1.08 × 9 × 22 = 200; people and lights 2 × 230 + 400 = 900.
- Humidity 2 × 200 + 0.68 × 9 × 50 = 700.
- Cooling total 5,200 BTU/h. 95%-115% of that is 4,900 to 6,000. No head is made in that range, so we take the smallest, 6,000 BTU (116% of the load).
- Heating in Minneapolis (-6°F, ΔT 76°F): walls 1,300 + ceiling 800 + glass 1,700 + air 700 = 4,500 BTU/h, which a 9,000 BTU head covers.
Too big or too small
- Too big: short cycling. Inverter heads slow down instead of shutting off, but every head has a lowest speed. A head far larger than the room reaches the setting, stops and starts again often, and wears faster.
- Too big: clammy air. At low load the coil is not cold long enough to take moisture out, so the room feels cool and damp.
- Too big: you pay for it. A bigger head and a bigger multi-zone outdoor unit cost more, and the outdoor unit may need a bigger breaker and wire.
- Too small: it can't keep up on the hottest afternoon or the coldest night, and in a cold climate you may need backup heat.
- Between sizes? Take the larger head. Inverter heads handle a bit of extra capacity much better than a central air conditioner does. The one exception is a very small room, where the smallest head is the answer.
Questions people ask
What size mini split do I need for a 300 sq ft room?
With average insulation, two exterior walls, a top-floor ceiling and average sun, a 300 sq ft room has a cooling load of about 5,200 BTU/h in Houston and 4,200 in Seattle. The heating load in Minneapolis is about 4,500 BTU/h. That gives a 6,000 BTU head in Houston, 6,000 in Seattle and 9,000 in Minneapolis, where the heating load on a design night sets the size. Many online charts say 9,000 everywhere; that adds a margin an inverter head doesn't need in a mild climate.
How many BTU per square foot do I need for a mini split?
The old rule is 20 BTU per square foot. Our room math gives 17.3 BTU per sq ft for the 300 sq ft example in Houston and 15.1 for heating in Minneapolis. Rooms with more exterior wall, big windows, full sun or a roof overhead need more; interior rooms need much less. That's why we add up the walls, windows, ceiling and people instead.
What size mini split for a 1,000 sq ft open space?
One head can cool a 1,000 sq ft open room, as long as it's one open space. With three exterior walls, a roof overhead and 4 people, the cooling load in Houston is about 19,200 BTU/h, so a 24,000 BTU head. One head only reaches the rooms it can blow air into: closed bedrooms down a hall each need their own head.
Is it bad to oversize a mini split?
Less bad than with a regular air conditioner, but still not good. An inverter head slows down instead of shutting off, which keeps humidity under control. But there's a floor: most heads can't run below about 30% of their rating, so a head far too big still cycles on and off in mild weather, and it costs more. We pick the smallest head within 95%-115% of the cooling load, and a bigger one only when the heating load needs it.
What size multi-zone mini split do I need?
Size each room's head, then size the outdoor unit to the sum of the room loads. For example, in Houston a 300 sq ft room, a 150 sq ft room and a 400 sq ft sunny room with three exterior walls get 6,000 + 6,000 + 12,000 BTU heads, and the outdoor unit is 18,000 BTU. Outdoor units are made in 18,000, 24,000, 30,000, 36,000, 42,000, 48,000 BTU. Manufacturers allow an outdoor unit smaller than the sum of the heads, because the rooms are rarely all at full load at once, but only within their own combination charts. We size to the sum and list the heads, so you can compare with a quote.
Do I need a Manual J load calculation for a mini split?
For one head in one room, our estimate is a good guide. For a multi-zone system, or a mini split that is your main heat in a cold climate, have a contractor run a Manual J and check the manufacturer's capacity at your winter design temperature. Heating capacity falls as it gets colder, and the sticker BTU is for mild weather.