How Do I Size a Ductless Mini Split System?

Start With BTUs Per Square Foot

The simplest way to size a ductless mini split is to match capacity to the floor area you plan to cool or heat. Measure the room in square feet, then multiply by a BTU factor based on how well the space is insulated. For cooling only, use a factor of 20 BTUs per square foot for a well insulated home, 25 BTUs per square foot for a home with average insulation, and 30 to 35 BTUs per square foot for a poorly insulated home.

For example, a well insulated 500 square foot room needs roughly 10,000 BTUs, while the same room with poor insulation could need 15,000 to 17,500 BTUs.

Adjust for Real Conditions

The BTU factor is a starting point. Add capacity for rooms with tall ceilings, large or sun facing windows, heavy appliance heat such as a kitchen, or frequent occupancy. Trim capacity for shaded, low traffic spaces. Avoid oversizing, since a unit that is too large cools quickly then shuts off, which leaves humidity behind and wears on the system.

Single Zone or Multi Zone

A single zone mini split pairs one outdoor condenser with one indoor head for a single room. A multi zone system connects several indoor heads to one outdoor unit so you can size each room on its own and run only the zones you use. Total the BTUs for every room you want to serve to confirm the outdoor unit can support the load.

Confirm Your Number

Use our ductless mini split sizing calculator to check your estimate against your square footage and insulation. Once you know the BTU target, browse units at or just above that figure for the best fit.