Garage Heating Load Calculator

Measure floor area and ceiling height for the room volume. Enter only walls exposed to outdoors, excluding the overhead door area. A wall adjoining a heated home needs its own temperature difference and should be calculated separately.

Use whole-assembly U-factors and local winter design weather. The defaults are a sample garage, not an insulation prescription. The roof area is assumed equal to floor area.

Design ACH represents winter operating air exchange, not a blower-door ACH50 reading. This steady-state estimate assumes the door is closed. Slab loss, open-door recovery and warming cold vehicles need separate assessment.

This is the same design-temperature method used in the heat load calculator, but with separate U-factors for the walls, roof, and overhead door because those are the surfaces that differ most in a garage. To work out a U-factor from layers, use the U-value calculator. If you are sizing a furnace for the house instead, use the furnace size calculator.

Using these inputs, the results are heating load: 9,952 BTU/h; conduction: 7,360 BTU/h; infiltration: 2,592 BTU/h; equivalent electric heat: 2.917 kW; temperature difference: 40 F. Defaults are example assumptions, not universal design conditions.

The arithmetic: UA = 560 × 0.15 + 400 × 0.05 + 160 × 0.5 = 84 + 20 + 80 = 184 BTU/(h·°F), so conduction is 184 × 40 = 7,360 BTU/h. Infiltration CFM is 400 × 9 × 1 ÷ 60 = 60, and 1.08 × 60 × 40 = 2,592 BTU/h. Together that is 9,952 BTU/h, about 2.9 kW if supplied by electric resistance heat. Notice that the door is a smaller area than the walls but carries nearly as much conductive loss as all of them.

Holding the default garage constant, the load tracks the temperature difference linearly. These rows change only the indoor target or the outdoor design temperature:

The next table changes the envelope while keeping the 60°F target and 20°F outdoor design. The U-factors are illustrative whole-assembly values chosen to show the size of the effect, not a recommendation or a code requirement; use your own measured or documented values.

The spread between the best and worst rows is nearly four to one, which is why a quoted size based on floor area alone can be badly wrong in either direction. A smaller or larger garage scales the same way: with the default U-factors and ACH, a one-car garage of 256 sq ft (520 sq ft of wall, 56 sq ft of door) comes to about 6,400 BTU/h, and a 720 sq ft three-car garage (762 sq ft of wall, 210 sq ft of door) to about 14,900 BTU/h.

• Do not count door area again within the wall area.
• Compare heater output with the complete load; fuel-fired input ratings are larger than useful output.
• Ask a qualified installer to check combustion safety, slab losses, door-opening recovery and installation clearances.
• Treating a garage like a living space: assuming 70°F instead of the 50–60°F many people actually use raises the load by 25% or more.
• Ignoring the overhead door. An uninsulated, non-weatherstripped door is often the single largest loss, both through its U-factor and through air leakage around its edges.
• Forgetting ceiling height. A taller ceiling adds air volume that the infiltration term scales with, and more wall area to enter.
• Sizing a 120 V plug-in electric heater for a whole garage. A 1,500 W unit is only about 5,100 BTU/h.
• Buying a hugely oversized gas unit 'to be safe'. It short-cycles, wastes fuel, and heats unevenly.
• Using a vent-free heater in an enclosed garage. It releases combustion moisture and gases indoors and is restricted by many local codes.

Once you know the BTU/h the space demands, the next decision is the heater itself, and the mainstream options heat a garage in different ways. A forced-air gas unit heater (natural gas or propane) is the workhorse for attached garages: it hangs from the ceiling, blows warm air across the room, and recovers a cold space quickly, which matters when an overhead door has just let the heat out. A low-intensity infrared tube heater warms objects and floors rather than the air, so it handles door openings better and suits high-ceiling shops and detached workshops where heating the full air volume would be wasteful. Electric resistance unit heaters avoid combustion, venting, and gas lines entirely, but at 3.412 BTU/h per watt a 60,000 BTU/h load draws about 17.6 kW, which means a hardwired 240 V circuit rather than a plug-in. Finally, a cold-climate ductless mini-split heat pump can deliver roughly two to three units of heat per unit of electricity in moderate weather, with lower efficiency in extreme cold, and adds summer cooling, which makes it attractive for an insulated, finished garage that doubles as a gym or studio. See the mini-split calculator and heat pump calculator if that route interests you.

Operating cost usually decides between gas and electric. Where natural gas is inexpensive, a vented gas unit heater is often the cheapest to run for a large or frequently used shop. Where electricity is cheap or gas is unavailable, a heat pump wins on efficiency and a resistance heater wins on install cost. Match the equipment to the load you calculated: undersize and the heater never satisfies the thermostat on the design day; grossly oversize a gas unit and it short-cycles, wears faster, and heats unevenly. Convert between watts, BTU/h, and tons with the wattage to heat load calculator if you are comparing electric options.

The cheapest BTU is the one you never have to produce. Because conduction loss is U × area × ΔT, envelope upgrades cut the load directly. The overhead door is often the largest single weak point; a door insulation kit and fresh weatherstripping can lower both the door's U-factor and the air-change rate the garage leaks, as the tables above show. Adding wall and ceiling insulation lowers the other conduction terms. Air-sealing the wall-to-slab joint, the door perimeter, and any attic access reduces infiltration, which the model treats as 1.08 × CFM × ΔT. The U-value calculator shows how conduction loss responds to better assemblies, and the heat load calculator applies a related design-temperature method to the whole house.

Safety is not optional with any fuel-burning appliance in a garage. A vented or direct-vent gas unit heater exhausts carbon monoxide and combustion moisture outdoors; vent-free heaters release both into the space and are restricted or prohibited for garages in many jurisdictions. Mount heaters at the manufacturer's required clearance from combustibles, and keep ignition sources at the height your local fuel-gas code requires (commonly 18 inches above the floor in garages) so they cannot ignite gasoline vapors. Install a carbon monoxide alarm. A gas unit heater also needs a properly sized gas line and, depending on the input, may require a permit and inspection. The what size furnace guide covers the sizing logic that carries over to larger equipment. Plan the heater type, the fuel, the venting, and the electrical or gas supply together, and the BTU/h figure above becomes a buying decision you can hand to an installer.

ASHRAE Handbook - Fundamentals provides heat-transfer, air-property and load-calculation background. Example inputs are stated scenarios, not prescribed handbook values.

For residential design, ACCA Manual J, S and D address load calculation, equipment selection and duct design. This calculator does not certify a complete design or code compliance.