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Mini Split Sizing Calculator

A mini split sizing calculator finds the right size ductless heat pump for a room or zone - in both BTU and tons. Enter the area and a couple of details and it recommends a standard unit size, so you neither freeze in an oversized unit nor undersize and run it flat out.

Your measurements

Result

Sizing matters more for mini-splits than for window units: an oversized mini-split short-cycles and never settles into the efficient, quiet, dehumidifying run it is designed for. This aims for the right size, not the biggest.

How it is calculated

We start near 24 BTU per square foot, then adjust for sun (plus or minus 10%) and insulation (up to plus 15% for drafty, minus 10% for well-sealed), and round to the nearest standard size (9k, 12k, 18k, 24k, 36k, 48k BTU). Tons are BTU divided by 12,000.

Steps

Enter the room or zone area in square feet.
Pick the sun exposure - a sun-facing room needs more.
Pick the insulation level - older, draftier rooms need more.
Read the recommended size in BTU and tons.
For several rooms, size each zone separately or add a head per room.

Worked example

A 500 sq ft average room with average insulation comes to about 12,000 BTU - a 1-ton mini-split.

Common questions

What size mini-split do I need?

Roughly 20-25 BTU per square foot, adjusted for sun and insulation. A 500 sq ft room is usually a 12,000 BTU (1-ton) unit.

Is bigger better for a mini-split?

No. Oversizing makes it short-cycle, which wastes energy and leaves the room humid. Size it to the load.

What does tons mean?

One ton of cooling equals 12,000 BTU per hour, so a 1.5-ton unit is 18,000 BTU.

Can one mini-split do a whole house?

A single head suits one open zone. For multiple rooms, use a multi-zone system with a head in each room.