Garage Heater Size Calculator: BTU, Watts and Amps

Size a garage heater from your own garage instead of a rule of thumb. The calculator adds the heat lost through the walls, ceiling and garage door, plus the air that leaks in, on a cold day. It converts the total to BTU per hour, watts and the amps an electric heater would draw on a 240-volt circuit.

When this tool helps most

Use it before you buy any heater, electric or fuel-burning. Oversized heaters cost more and cycle on and off; undersized heaters run constantly and never catch up. The result also shows where most heat escapes, which tells you what to insulate first.

How to use this tool

1. Enter the inside length, width and ceiling height.
2. Set how much of the wall area faces outdoors. Attached garages share part of their walls with the heated house.
3. Enter R-values for the walls, ceiling and garage door from your insulation or the door maker’s rating.
4. Set the temperature you want inside, a cold-day outdoor temperature and how drafty the garage is, then read the size.

Worked example

The default garage is 24 by 24 feet with a 9-foot ceiling, 75 percent of the walls exposed, a 112-square-foot door, R-13 walls, an R-30 ceiling and an R-6 door, heated to 55 F when it is 10 F outside with 1 air change per hour. Heat loss works out to 1,855 BTU per hour through the walls, 864 through the ceiling, 840 through the door and 4,199 from air leakage. With a 20 percent margin the calculator recommends about 10,000 BTU per hour, or 3,000 W electric, which is 11.4 amps at full output on a 240-volt circuit. The biggest loss is air leakage, so sealing the door perimeter would help more than adding ceiling insulation.

Understanding your results

Look for a heater rated at or slightly above the BTU figure. For electric heaters, compare the watt rating and confirm the circuit size with a licensed electrician. The chart shows which surface loses the most heat, which is where an upgrade pays back first.

Common mistakes to avoid

• Ignoring air leakage. Gaps around a garage door can lose more heat than the walls.
• Using the house’s outdoor design temperature for a garage you only heat occasionally. Pick the coldest day you will actually work.
• Running a fuel-burning heater not rated for garage use, or without the venting its manual requires.
• Putting a heater near fuel, solvents or anything that can burn.

How to get the most accurate result

• Measure your own garage with a tape measure instead of using typical sizes.
• Use the numbers printed on your product labels, manuals and data sheets.
• Change one input at a time to see which assumption matters most.
• Print or save the result and bring it to your installer or contractor.

Next steps

Read the garage heater guide to compare electric, propane and natural gas, then the garage insulation guide to cut the load.

Frequently asked questions

How many BTUs do I need to heat a two-car garage?

It depends on insulation, air leakage, how warm you want it and how cold it gets. A well-sealed, insulated garage can need a fraction of the heat an uninsulated, drafty one needs. Enter your own dimensions and R-values to get a number for your garage.

How do I convert BTU to watts?

Divide BTU per hour by 3.412 to get watts. A 10,000 BTU per hour heater is about 2,930 watts.

Should I heat my garage all the time?

Most people heat a garage only while they use it. Insulation and air sealing make heating faster and cheaper either way.

This tool is for planning and education only. It does not store your entries. Garage door springs and cables are jobs for a trained technician, and new circuits are jobs for a licensed electrician. Read our disclaimer.