What Size Heater for a Garage?

Size it by the garage's volume and how much you need to raise the temperature. The common formula is cubic feet x 0.133 x temperature rise (for average insulation). A 2-car garage of about 3,840 cubic feet, raising the temperature 55 degrees, needs roughly 28,000 BTU; in a cold climate at a 70-degree rise it needs about 36,000 BTU. As a quick guide, plan 30-40 BTU per square foot in a warm climate, 40-45 in a moderate one, and up to 60 in a cold climate.

About 28,000 BTU in a moderate climate, ranging from roughly 20,000 BTU in a mild climate to 36,000 in a cold one, for a typical 480 sq ft (20 x 24) two-car garage with average insulation. A poorly insulated garage needs about 30% more, so a cold-climate uninsulated two-car garage can want 45,000 BTU or more. For an electric heater that is about a 7,500-10,000 watt unit; for gas or propane, a 30,000-45,000 BTU unit heater.

Multiply the garage's cubic feet (length x width x ceiling height) by 0.133, then by the temperature rise you want (indoor target minus the coldest outdoor temperature). For example, a 24 x 24 garage with an 8 ft ceiling is 4,608 cubic feet; to hold 60 degrees when it is 0 outside is a 60-degree rise, so 4,608 x 0.133 x 60 = about 36,800 BTU. Add about 30% if the garage is uninsulated, and round up to the next standard heater size.

Yes, meaningfully. An uninsulated garage loses heat much faster, so it needs roughly 30% more capacity than a well-insulated one of the same size, the difference between about a 40,000 and a 52,000 BTU heater. Insulating the walls, ceiling, and especially the garage door is often cheaper than the extra heater capacity and fuel, and it makes the space far more comfortable. Size to your actual insulation, not the best case.

Both work; the choice is fuel and capacity. Electric heaters are simple to install (no venting) but are limited by your circuit: a 240V heater is rated in watts, and 1 watt is about 3.41 BTU, so a 10,000 watt heater delivers about 34,000 BTU and needs a large circuit. Gas and propane unit heaters deliver more heat cheaply and are common for larger or colder garages, but need a gas line or tank and venting. For a big or cold garage, gas is usually more practical; for a small insulated one, electric is easiest.