Size a garage heater by calculating cubic footage, then adjusting for insulation, climate, and the temperature rise you need.
For the full breakdown, see our best Wall Mount Garage Heater guide.
Most of the guidance on how to size a garage heater agrees on one starting point: cubic feet, not square feet. For a typical U.S. residential garage or workshop, the reliable sequence is measure the volume, factor the heat loss, convert to BTU or watts, then verify the power or fuel supply.
Sizing a Garage Heater: Start With Cubic Feet
The sizing math starts with one measurement: length × width × ceiling height, all in feet. Multiply the three numbers and you get the cubic footage the heater actually has to work on.
Cubic footage is the floor, not the final answer. The heat load depends on how much of that heat escapes. Uninsulated or drafty walls, uninsulated garage doors, large door openings, high ceilings, and colder local climates all push the requirement up. An attached garage shares a conditioned wall with the house and tends to need less than a detached one sitting in the wind.
When the measuring is done, you’ll have a cubic-foot figure to carry into the BTU math.
From Cubic Feet to BTU: The Sizing Method
Turning cubic feet into a BTU rating means factoring in insulation quality, your climate, and the temperature rise you need — your target indoor temperature minus the coldest outdoor temperature you expect to heat against. The most consistent published approach multiplies cubic footage by a factor tied to insulation and climate.
They work as a sanity check, but they ignore ceiling height — exactly how heaters end up undersized in tall garages.
How Many BTU Does a Two-Car Garage Need?
Published estimates put a two-car garage between 12,000 and 30,000 BTU/hr, with insulation and climate deciding where you land.