What size mini split for a basement?
A basement of about 800 sq ft needs a 24,000 BTU mini split. Basements stay cool in summer, so heating is usually what sets the size.
Method: ENERGY STAR cooling chart (as in our AC calculator), with heating scaled up for cold winters since heat pump output falls with the outdoor temperature.
Mini split capacity
24,000 BTU
2 tons · 800 sq ft
| Cooling need | 18,000 BTU |
| Heating need | 20,700 BTU |
| Size that covers both | 24,000 BTU |
- • Heating is scaled up 15% because heat pump output drops in the cold. A cold-climate model keeps its rated heat down to about −13°F.
- • Mini splits work best slightly matched, not oversized: a unit that's too big cycles on and off and loses efficiency.
Mini split size chart
Common single-zone capacities and the room size each covers in a mild climate (ENERGY STAR cooling method, 8 ft ceilings).
| Mini split | Typical room |
|---|---|
| 6,000 BTU | up to 250 sq ft |
| 9,000 BTU | 250–400 sq ft |
| 12,000 BTU | 400–550 sq ft |
| 18,000 BTU | 550–1,000 sq ft |
| 24,000 BTU | 1,000–1,500 sq ft |
| 30,000 BTU and up | 1,500+ sq ft or multi-zone |
How the calculation works
Cooling comes from the same ENERGY STAR chart used in the AC size calculator, with the usual adjustments for sun, people, kitchens and tall ceilings. Heat pumps put out less heat as the outdoor air gets colder, so this calculator adds 15% for cold winters and 30% for very cold ones, then picks the standard size that covers both heating and cooling.
More mini split guides
Frequently asked questions
What size mini split for 500 sq ft?
About 12,000 BTU in a mild climate. In a cold climate, ask for a cold-climate model or go up one size.
Can I use a mini split to heat in winter?
Yes. A mini split is a heat pump, and standard models heat efficiently down to around 5°F. Cold-climate models work to about −13°F.
Is bigger better for a mini split?
No. Oversized units cycle on and off, which wastes energy and leaves rooms humid in summer.
When do I need a multi-zone system?
When you want to condition several separate rooms. One outdoor unit then feeds an indoor head in each room, which is usually better than one big head heating the whole floor.