What Size Heater for a 12 × 20 ft Garage?

A 12 × 20 ft garage with insulated walls and ceiling needs about 6,936 BTU (2,033 W) to hold 50°F when it is 20°F outside, so a 3,000 W electric heater drawing 12.5 amps at 240 V. With no insulation it needs 14,634 BTU.

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Example job shown. Change the numbers to match yours.

Heater size to buy

6,936 BTU per hour

Heat loss plus a 25% margin

In watts (electric): 2,033 W
Next standard electric size: 3,000 W
Current at 240 VThe circuit breaker needs to be rated at 125% of this: 12.5 A
Running cost, if it runs non-stop: 0.35 dollars per hour
Heat the garage loses to hold the temperature: 5,549 BTU per hour
Temperature rise: 30 °F

This is the heat needed to hold the temperature in steady conditions. A large amount of concrete, tools and vehicles will take hours to warm, and opening the door loses a lot of heat. For a quick warm-up, choose the next size up. A fuel-burning heater needs a vent or a heater rated for garages, and a garage with a car or fuel in it has special safety rules: follow the heater's manual and your local code.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

Heater size for a 12 × 20 ft garage, by insulation

An 8 ft ceiling, holding 50°F when it is 20°F outside, with a 25% margin. Amps are at 240 V for the standard electric size.

Insulation	BTU per hour	Watts	Electric size	Amps
None: bare studs, plain door	14,634 BTU	4,289 W	5,000 W	20.8 A
Walls and ceiling insulated	6,936 BTU	2,033 W	3,000 W	12.5 A
Well insulated, door too	3,598 BTU	1,054 W	1,500 W	6.3 A

How cold it gets where you live

The same garage with insulated walls and ceiling, held at 50°F, for different design temperatures outside:

Coldest outdoor temperature	Heat needed	Electric size
0°F	11,560 BTU	4,000 W
10°F	9,248 BTU	3,000 W
20°F	6,936 BTU	3,000 W
30°F	4,624 BTU	1,500 W

How warm you want it

At 20°F outside, insulated. The last column is the cost per hour at 17¢ a kWh if the heater runs non-stop at the heat-loss rate:

Temperature inside	Heat needed	Electric size	Cost per hour
40°F	4,624 BTU	1,500 W	$0.23
50°F	6,936 BTU	3,000 W	$0.35
60°F	9,248 BTU	3,000 W	$0.46
70°F	11,560 BTU	4,000 W	$0.58

A 12 × 20 ft garage: one car, or a workshop

A 12 × 20 ft space is a small one-car garage or a typical workshop bay. With the walls and ceiling insulated it needs about 6,936 BTU, so a 3,000 W heater. A 1,500 W plug-in space heater gives 5,118 BTU, which is enough here only if the space is well insulated, including the door.

For a bench you use for an hour at a time, a radiant (infrared) heater aimed at where you stand usually warms you faster than heating all the air.

Enter your own garage in the garage heater size calculator, or read what size heater for a garage for breakers, fuel and safety.

Want to change anything else? The calculator above is the full Garage Heater Size Calculator: edit any number and the results update instantly.

Tools & materials for this project

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240V electric garage heater (5,000 W)Infrared quartz heater Garage door insulation kit Weatherstripping for garage door Carbon monoxide detector

Frequently asked questions

What size heater for a 12 × 20 ft garage?

About 6,936 BTU, or 2,033 W, to hold 50°F when it is 20°F outside, with insulated walls and ceiling and an 8 ft ceiling. That is a 3,000 W electric heater. With no insulation it needs 14,634 BTU.

How many amps does a heater for a 12 × 20 garage draw?

A 3,000 W heater draws 12.5 amps at 240 V. The breaker must be rated for 125% of that, and the wire sized to the breaker.

How much does it cost to heat a 12 × 20 ft garage?

About $0.35 an hour at 17¢ a kWh if the heater ran non-stop at the heat-loss rate. In practice the thermostat cycles it, and a garage you heat only while you work costs far less.

Will a 1,500 W space heater heat a 12 × 20 garage?

A 1,500 W heater gives 5,118 BTU an hour. That is enough only if the garage is well insulated, including the door (3,598 BTU).

Related sizes and guides

Related sizes

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.