What Size Heater for a 30 × 40 ft Garage?

A 30 × 40 ft garage with insulated walls and ceiling needs about 23,880 BTU (6,999 W) to hold 50°F when it is 20°F outside, so a 7,500 W electric heater drawing 31.3 amps at 240 V. With no insulation it needs 48,870 BTU.

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Example job shown. Change the numbers to match yours.

Heater size to buy

23,880 BTU per hour

Heat loss plus a 25% margin

In watts (electric): 6,999 W
Next standard electric size: 7,500 W
Current at 240 VThe circuit breaker needs to be rated at 125% of this: 31.3 A
Running cost, if it runs non-stop: 1.19 dollars per hour
Heat the garage loses to hold the temperature: 19,104 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 30 × 40 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	48,870 BTU	14,323 W	15,000 W	62.5 A
Walls and ceiling insulated	23,880 BTU	6,999 W	7,500 W	31.3 A
Well insulated, door too	12,588 BTU	3,689 W	4,000 W	16.7 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	39,800 BTU	15,000 W
10°F	31,840 BTU	10,000 W
20°F	23,880 BTU	7,500 W
30°F	15,920 BTU	5,000 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	15,920 BTU	5,000 W	$0.79
50°F	23,880 BTU	7,500 W	$1.19
60°F	31,840 BTU	10,000 W	$1.59
70°F	39,800 BTU	15,000 W	$1.98

A 30 × 40 ft garage or pole barn shop

A 30 × 40 ft building is a big shop. Insulated, with an 8 ft ceiling, it needs 23,880 BTU (6,999 W). Shops are often taller, and a 12 ft ceiling raises that to 31,320 BTU.

At this size gas, propane or a ducted system is usually cheaper to run than electric heat. If you choose a fuel-burning heater, it must be vented or rated for the space, with a carbon monoxide alarm.

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 30 × 40 ft garage?

About 23,880 BTU, or 6,999 W, to hold 50°F when it is 20°F outside, with insulated walls and ceiling and an 8 ft ceiling. That is a 7,500 W electric heater. With no insulation it needs 48,870 BTU.

How many amps does a heater for a 30 × 40 garage draw?

A 7,500 W heater draws 31.3 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 30 × 40 ft garage?

About $1.19 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 30 × 40 garage?

A 1,500 W heater gives 5,118 BTU an hour. That is not enough even when the garage is well insulated (12,588 BTU). It will take the chill off a small area near you.

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.