Calculator
Garage Heater BTU Calculator
A 22 x 22 ft two-car garage with a 9 ft ceiling, R-13 walls, an uninsulated ceiling and a steel door needs about 12,000 BTU per hour to hold 55F when it is 20F outside, which is a 3.5 kW electric heater on a 20 A, 240V circuit. At 0F outside the same garage needs about 19,000 BTU per hour. The calculator adds heat lost through walls, ceiling and door to the heat needed for air leaking in, so a tight insulated garage needs a fraction of that.
Garage heater BTUUpdates as you type
12,000 BTU per hour to hold 55F at 20F outside
- Walls / ceiling / door / air leakage
- 18 percent / 36 percent / 23 percent / 23 percent
- Electric heater
- 3.5 kW (3,500 W), 14.6 A at 240V
- Circuit (125 percent, continuous load)
- 20 A breaker, 12 AWG copper
- Vented gas heater input to look for
- 15,000 BTU per hour at 80 percent efficiency
- Volume
- 4,356 cu ft
Heat loss = U x area x temperature difference for walls, ceiling and door, plus 0.018 x air changes x volume x difference for leakage. U-values and air changes are conventions, and slab edge loss is ignored. Never run an unvented combustion heater in a closed garage, and keep fuel and fumes away from a parked car. Have a licensed electrician confirm the circuit.
Heaters and insulation matched to the load
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Where the heat goes
A garage loses heat three ways: through the walls and ceiling, through the overhead door, and through the cracks that let cold air in. The calculator multiplies the area of each surface by its U-value and the temperature difference, then adds a term for air leakage: 0.018 x air changes per hour x volume x temperature difference. For most garages the uninsulated ceiling and the steel overhead door dominate, so a few hundred dollars of ceiling insulation and a door kit cuts the load more than a bigger heater would. Attached garages share one or two walls with the house, which is why the exposed-wall option matters.
Safety with a car and fuel in the room
Never run an unvented fuel heater in a closed garage, and keep gasoline, solvents and paints away from any flame or heating element. Electric ceiling heaters avoid combustion in the space; check the clearance to the ceiling and to the garage door. Carbon monoxide and smoke alarms belong in a garage that holds a heater.
Size the circuit with the amps calculator before you buy a heater that needs a 40 A breaker. Per-garage answers are in the heater sizing pages.
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Frequently asked questions
What size heater for a 2 car garage?
For a 22 x 22 ft garage with R-13 walls and a bare ceiling, about 12,000 BTU per hour holds 55F when it is 20F outside, a 3.5 kW heater. In a deep freeze at -10F it is closer to 22,500. Insulating the ceiling reduces both.
Is a propane heater safe in a garage?
Only a vented unit heater with proper combustion air and a carbon monoxide alarm. Unvented radiant propane heaters release moisture and combustion gases into a closed bay and do not belong there.
Does a 240V garage heater need its own circuit?
Yes. A heater is a continuous load, so the breaker and wire are sized at 125 percent of its running current, and it should have its own circuit.
This page is researched guidance, not professional electrical, engineering or safety advice. Circuits, loads and clearances depend on your house, your service and your town: have a licensed electrician confirm any wiring, and check local code and each manufacturer's manual before you act.
