Enter the size of the room, how sunny it is and how many people use it. The calculator applies the ENERGY STAR sizing chart to give the cooling capacity you need in BTU per hour and the next common mini split or ductless heat pump size.
How to use the mini split size calculator
- Enter the length and width of the room or zone the indoor unit will serve.
- Choose the sun exposure. A room with big west or south windows is very sunny; a room under trees is heavily shaded.
- Enter how many people usually use the room, and tick the box if it is a kitchen.
- Buy the size shown, or the next one up if you are between sizes. For a whole house, a Manual J load calculation by an HVAC contractor is the reliable way to size.
Mini split sizing formula
Area (ft²) = length × width
Base BTU = ENERGY STAR chart value for that area
BTU needed = base × sun factor (0.9 shaded, 1.1 sunny)
+ 600 × (people − 2) + 4,000 if a kitchen
Tons = BTU ÷ 12,000
The adjustments are the ones ENERGY STAR gives with its room air conditioner chart. A ton of cooling is 12,000 BTU per hour (NIST), so a 9,000 BTU mini split is a ¾ ton unit.
Worked example: a 20 × 16 ft bedroom
- Area: 20 × 16 = 320 ft², which falls in the 300–350 ft² row: 8,000 BTU.
- Normal sun and two people: no change. Buy the next common size, a 9,000 BTU (¾ ton) mini split.
- If the room is very sunny and three people use it: 8,000 × 1.1 + 600 = 9,400 BTU, so step up to 12,000 BTU (1 ton).
ENERGY STAR sizing chart
| Room area | BTU per hour | Common mini split size |
|---|---|---|
| 100–150 ft² | 5,000 | 6,000 BTU |
| 150–250 ft² | 6,000 | 6,000 BTU |
| 250–300 ft² | 7,000 | 9,000 BTU |
| 300–350 ft² | 8,000 | 9,000 BTU |
| 350–400 ft² | 9,000 | 9,000 BTU |
| 400–450 ft² | 10,000 | 12,000 BTU |
| 450–550 ft² | 12,000 | 12,000 BTU |
| 550–700 ft² | 14,000 | 15,000 BTU |
| 700–1,000 ft² | 18,000 | 18,000 BTU |
| 1,000–1,200 ft² | 21,000 | 24,000 BTU |
| 1,200–1,400 ft² | 23,000 | 24,000 BTU |
| 1,400–1,500 ft² | 24,000 | 24,000 BTU |
| 1,500–2,000 ft² | 30,000 | 30,000 BTU |
| 2,000–2,500 ft² | 34,000 | 36,000 BTU |
Mini split or heat pump for heating
The ENERGY STAR chart sizes for cooling. A ductless heat pump that also heats the room may need more capacity in a cold climate, because heat loss on the coldest night can be larger than the cooling load. Check the heating capacity the maker lists at low outdoor temperatures, and for a whole house ask for a Manual J load calculation, which also accounts for insulation, windows and air leaks.
Tips for choosing a mini split
- Do not oversize. A unit that is too big cools in short bursts and removes less humidity.
- One head per room. An indoor unit cools the space it can blow into; walls and doors block it.
- Ceiling height matters. The chart assumes ordinary 8 ft ceilings; vaulted rooms need more.
- Insulate first. Better insulation and air sealing can drop the size you need; see the insulation calculator.
Questions
What size mini split do I need for 500 square feet?
About 12,000 BTU (1 ton). The ENERGY STAR chart gives 12,000 BTU for 450 to 550 ft² with normal sun and two people.
How many square feet does a 12,000 BTU mini split cool?
About 450 to 550 ft² in a room with normal sun. In a very sunny room it covers less, roughly up to 450 ft².
What size mini split for a 12 × 12 room?
A 144 ft² room needs about 5,000 BTU, so the smallest common unit, 6,000 BTU, is enough. Many brands start at 9,000 BTU, which is also fine with an inverter unit.
How many tons is a 24,000 BTU mini split?
Two tons. Divide the BTU by 12,000.
Sources
- Room Air Conditioner: sizing chart, ENERGY STAR (U.S. EPA), archived by the University of North Texas Libraries.
Used for: BTU per hour by room area; −10% shaded, +10% sunny, +600 BTU per person over two, +4,000 BTU for a kitchen. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Ton of refrigeration = 12,000 Btu/h; foot definitions.