How this calculator works
A garage loses heat through its walls, ceiling, door and floor edge, and through air leaking in. The heat needed to hold the temperature is the rate of loss per degree times the temperature difference. We estimate the loss per degree from the surface area of the garage and a heat-loss factor for the insulation, plus the air leakage, and add a 25% margin.
Worked example: a 20 × 20 × 8 ft garage has 1,040 ft² of surface and 3,200 ft³ of air. With insulated walls and ceiling and a plain door (U = 0.2, one air change an hour) it loses about 266 BTU per hour per degree. Holding 50°F against 20°F outside, a 30 degree rise, needs 7,970 BTU, or about 9,960 BTU with the 25% margin. That is 2,920 W, so the next standard electric size is 3,000 W, drawing 12.5 A at 240 V.
Heater size by garage size and insulation (30°F rise)
BTU per hour including a 25% margin, 8 ft ceiling.
| Garage | No insulation | Walls and ceiling insulated | Well insulated |
|---|---|---|---|
| 12 × 20 ft | 14600 BTU | 6900 BTU | 3600 BTU |
| 20 × 20 ft | 20800 BTU | 10000 BTU | 5200 BTU |
| 24 × 24 ft | 27300 BTU | 13200 BTU | 6900 BTU |
| 30 × 30 ft | 38700 BTU | 18800 BTU | 9900 BTU |
Tips before you buy
- Insulating costs less than heating. A poorly insulated garage needs a heater about twice the size, and the door is usually the biggest leak.
- A 240 V electric heater of 3,000 to 5,000 W is the usual choice for a 2-car garage. Hardwire it on a dedicated circuit sized by an electrician.
- A radiant (infrared) heater warms you and objects directly, and can feel warm at a lower air temperature. A forced-air heater warms the air and works for a whole garage.
- Propane and natural gas heaters give a lot of heat for the money, but they need venting and the right clearances, and a heater in a garage that stores gasoline needs to be one designed for it.
- Keep the door closed and seal the gaps. Draft stripping around the door is cheap and cuts the heat loss a lot.
- Never heat a garage with a fuel-burning heater not rated for indoor use, and fit a carbon monoxide alarm.
Tools & materials for this project
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Frequently asked questions
What size heater do I need for a 2 car garage?
For a 20 × 20 ft garage with insulated walls and ceiling and a 30 degree rise, about 10,000 BTU (3,000 W). For a bare, uninsulated garage the same rise needs roughly double that, and a bigger rise needs more.
How many watts to heat a garage?
Roughly 3,000 W for an insulated 2-car garage, 5,000 W for a larger or poorly insulated one, and 7,500 W or more for a big shop with a high ceiling.
What size breaker do I need for a 5,000 W garage heater?
A 5,000 W heater draws 20.8 A at 240 V, and the circuit must be sized at 125% of that for a continuous load, so a 30 A breaker with 10 AWG wire. Have an electrician install it.
How much does it cost to run a garage heater?
A 3,000 W heater costs about 51 cents an hour at 17 cents per kWh if it runs non-stop, and a thermostat cycles it on and off, so less in practice.
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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.