Sizing

Mini-split size for a 12x20 one-car garage shop

A 12 x 20 ft garage shop with average insulation, normal sun, one person and 1,500 W of tools running carries about 11,718 BTU per hour of cooling load, so shop a 12,000 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, 12 x 20 ft
Insulation	Shaded	Normal sun	Sunny
Poor or none	12,738 BTU, 18,000 unit	13,518 BTU, 18,000 unit	14,688 BTU, 18,000 unit
Average	11,118 BTU, 12,000 unit	11,718 BTU, 12,000 unit	12,618 BTU, 18,000 unit
Good	10,308 BTU, 12,000 unit	10,818 BTU, 12,000 unit	11,583 BTU, 12,000 unit

The load is 240 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 9 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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Frequently asked questions

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

With average insulation, normal sun and one person with 1,500 W of tools, the load here is about 11,718 BTU per hour, so a 12,000 BTU unit is enough.

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.