Too small and it runs flat out without keeping up. Too big and it cycles on and off, wastes energy and leaves the room clammy. Here is a simple starting point.
A Starting Point by Room Size
ENERGY STAR’s room air conditioner chart is a useful guide for cooling:
- About 350 to 400 sq ft: around 9,000 BTU
- About 450 to 550 sq ft: around 12,000 BTU
- About 700 to 1,000 sq ft: around 18,000 BTU
- About 1,400 to 1,500 sq ft: around 24,000 BTU
When to Size Up or Down
- Very sunny room: add about 10%.
- Heavily shaded room: reduce by about 10%.
- Kitchen: add more for the heat from cooking.
- Poor insulation, high ceilings or cold winters: size for heating, not just cooling.
Heating Needs Can Be Bigger
If the mini split will be your main heat, heating usually decides the size. Heat pumps lose output as it gets colder, so check each model’s heating capacity and low-temperature limit. See How Cold Can Mini Splits Heat?
Get a Proper Load Calculation
For whole-floor or main-heat installs, ask your installer for a load calculation (often called a Manual J). It accounts for windows, insulation and your local climate.
Common Sizes in Our Reviews
Most of the units we cover are 12,000 BTU, like the DELLA Serena. For large open areas, the Senville AURA 24,000 BTU is the bigger option.
Want to see how a mini split fits with the rest of your home? Our Whole-Home Energy Calculator adds up what a thermostat, an EV charger, a backup battery and more would mean for your monthly bill.