Mini Split Sizing Calculator
Size a mini-split for a room: enter the area and a climate factor to get the recommended BTU unit and cooling load in tons.
How this calculator works
A mini-split has to remove enough heat from a room to keep it comfortable. That heat load scales with the room’s floor area and how demanding its climate and insulation are.
This calculator multiplies the room area by a BTU-per-square-foot factor, around 30 for a moderate, average-insulation room, to get the cooling load in BTU per hour. Because mini-splits only come in fixed sizes, it then rounds up to the nearest nominal unit. Enter the area and the factor to get the recommended unit and the load in tons.
Formula: Cooling load (BTU/hr) = room area × BTU-per-sq-ft factor (≈30 for a moderate, average-insulation room). Round up to the nearest nominal unit (9,000 / 12,000 / 18,000 / 24,000 / 36,000 BTU). 12,000 BTU = 1 ton.
Worked example
A 500 sq ft room at a factor of 30 BTU per square foot:
- Room area: 500 sq ft
- Cooling load = 500 × 30 = 15,000 BTU/hr
- Nearest nominal unit: 18,000 BTU (≈1.25 tons)
Room area at 30 BTU/sq ft
| Area | Load | Recommended unit |
|---|---|---|
| 300 sq ft | 9,000 BTU | 9,000 BTU |
| 500 sq ft | 15,000 BTU | 18,000 BTU |
| 750 sq ft | 22,500 BTU | 24,000 BTU |
| 1000 sq ft | 30,000 BTU | 36,000 BTU |
How to use
Enter the room’s floor area in square feet and a BTU-per-square-foot factor that suits its climate and insulation. The calculator returns the cooling load in BTU per hour, rounds up to the nearest nominal unit, and shows the equivalent in tons so you can match it to available equipment.
Sizing here is a planning estimate. A professional Manual J load calculation accounts for windows, insulation, and climate — confirm equipment size with an HVAC contractor.
Frequently asked questions
How do I size a mini-split for a room?
Multiply the room's floor area by a BTU-per-square-foot factor, around 30 for a moderate room with average insulation. That gives the cooling load in BTU per hour. Then round up to the nearest nominal unit, since mini-splits come in fixed sizes. In the worked example, a 500 sq ft room needs 15,000 BTU/hr, which rounds up to an 18,000 BTU unit.
What is a BTU and how does it relate to tons?
A BTU, or British thermal unit, measures heat, and a mini-split's rating in BTU per hour is how much heat it can move out of a room each hour. Cooling capacity is also quoted in tons, where 12,000 BTU equals 1 ton. So an 18,000 BTU unit is about 1.25 tons of cooling capacity.
Why round up to a nominal unit size?
Mini-splits are not made in every size. They come in fixed steps such as 9,000, 12,000, 18,000, 24,000, and 36,000 BTU. Your calculated load rarely matches one of these exactly, so you round up to the next available size. Rounding up rather than down ensures the unit can actually cover the room's full cooling load on a hot day.
Does the BTU-per-square-foot factor change?
Yes. The factor of about 30 suits a moderate room with average insulation, but a sun-baked room, a poorly insulated space, or a hot climate needs a higher figure, while a shaded or well-sealed room needs less. This calculator uses the factor you enter, so adjust it for the room, and confirm with a professional load calculation for anything precise.
Estimates only. Verify quantities with your supplier before purchasing.