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Garage Heater BTU Calculator

A garage heater is sized in BTU per hour. Too small and it runs all day without getting the garage comfortable; too big costs more to buy and run. Enter the garage size, insulation and climate zone, and the calculator estimates the BTU you need, the electric equivalent and the circuit size, and the heater types that fit.

How the calculation works

  1. Volume. Length x width x ceiling height, in cubic feet.
  2. Temperature rise. How much warmer than outside you want the garage, in °F. Picking a climate zone fills in a typical rise: a 55°F target minus the zone’s typical winter low.
  3. BTU per hour. BTU = volume x temperature rise x insulation factor. The factors are 0.10 for an insulated garage, 0.133 for some insulation and 0.20 for none.
  4. Electric equivalent. kW = BTU / 3,412. Amps at 240V = kW x 1,000 / 240. A heater is a continuous load, so the circuit should be rated for at least the amps / 0.8.

Worked example

A two-car garage is 24 x 24 ft with a 9 ft ceiling: 5,184 cubic feet. It has some insulation (factor 0.133) and you want it 40°F warmer than outside. BTU = 5,184 x 40 x 0.133 = about 27,600 BTU per hour. That is 8.1 kW, or about 33.7 A at 240V, so the circuit needs at least 43 A, which means the next standard breaker size up and wire rated for it, per the heater’s manual. A 7.5 to 10 kW electric heater or a vented gas unit heater fits.

The same garage with no insulation (factor 0.20) needs about 41,500 BTU per hour, which points to a vented natural gas or propane unit heater.

Assumptions and limits

This is a planning estimate, not a full heat load calculation. It does not count air leaks around the garage door, windows, a heated house wall or how often the door opens. Insulation factors and climate zone temperatures are typical values. Size up if your door leaks or you open it often, and follow the heater’s manual and local code for venting and wiring.

Heater safety

Never run a fuel-burning heater, generator or engine in a closed garage: carbon monoxide builds up fast and has no smell. Fit a CO alarm, keep clearances to cars and storage, use vented gas heaters installed to code, and put 240V electric heaters on a dedicated circuit. See CPSC carbon monoxide guidance. Read our Safety Disclaimer.

Frequently Asked Questions

How many BTU do I need to heat a 2-car garage?

For a typical 24 x 24 ft garage with a 9 ft ceiling, about 20,000 to 42,000 BTU per hour for a 40°F rise, depending on insulation. Insulated garages sit at the low end; uninsulated garages with a thin door need the high end.

Can a 120V space heater heat a garage?

A 1,500 W heater on 120V puts out about 5,100 BTU per hour. That can take the chill off a small, well insulated space or a work area, but it will not heat a typical two-car garage in winter. That job needs 240V electric or a vented gas heater.

Is a propane or kerosene heater safe in a garage?

Unvented fuel-burning heaters release carbon monoxide and moisture into the space. Never run one in a closed garage. Use a vented heater installed to code, follow the manual’s ventilation rules and fit a carbon monoxide alarm.

What size breaker does a 240V garage heater need?

Divide the heater’s watts by 240 to get amps, then divide by 0.8 because a heater is a continuous load. A 5,000 W heater draws about 20.8 A, so it needs at least a 26 A circuit, which means a 30 A breaker. The heater’s manual states the exact breaker and wire size.

Does insulation really make that much difference?

Yes. In this estimate, going from no insulation to an insulated garage cuts the heat needed in half. Insulating the garage door is often the cheapest place to start.

Garage heater buying guides

Next: read all garage upgrade guides.