Sizing

Mini-split size for a 20x20 two-car garage shop

A 20 x 20 ft garage shop with average insulation, normal sun, one person and 1,500 W of tools running carries about 15,718 BTU per hour of cooling load, so shop a 18,000 BTU mini-split. Poor insulation and a west wall can push that up a size or two.

By the GarageShopCalc editors · Updated 2026-10-13 · Researched from manufacturer specifications, code tables and verified owner reviews

Cooling load and nearest standard size

Load and unit size, 20 x 20 ft
Insulation	Shaded	Normal sun	Sunny
Poor or none	17,418 BTU, 18,000 unit	18,718 BTU, 24,000 unit	20,668 BTU, 24,000 unit
Average	14,718 BTU, 18,000 unit	15,718 BTU, 18,000 unit	17,218 BTU, 18,000 unit
Good	13,368 BTU, 18,000 unit	14,218 BTU, 18,000 unit	15,493 BTU, 18,000 unit

The load is 400 sq ft x 25 BTU/h per sq ft x an insulation factor and a sun factor, plus 600 BTU/h for one person and 1,500 W of tools at 3.412 BTU/h per watt (about 5,118 BTU/h). All of those factors are conventions, not code values, and a contractor load calculation is the right answer for a big purchase. Remember the open overhead door: a mini-split cannot beat an open 16 ft door.

Check each unit's voltage before you plan the circuit. Some DIY 12,000 BTU units run on 115V; others need a 208-240V circuit and a disconnect, which changes the subpanel size. Have a licensed electrician confirm.

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Mini-split by garage size: the other answers

Frequently asked questions

Is a 12,000 BTU mini-split enough for a 20x20 two-car garage?

With average insulation, normal sun and one person with 1,500 W of tools, the load here is about 15,718 BTU per hour, so a 12,000 BTU unit is undersized. Look at the 18,000 BTU size.

This page is researched guidance, not professional electrical, engineering or safety advice. Circuits, loads and clearances depend on your house, your service and your town: have a licensed electrician confirm any wiring, and check local code and each manufacturer's manual before you act.