Calculate the heater size needed to warm a garage.
A garage is sized on volume rather than floor area because the ceiling is usually high and the space is a single uninsulated box. Heat loss is volume multiplied by the temperature rise you want and by a loss coefficient that captures how leaky the shell is — around 0.25 for bare block with a thin steel door, 0.10 once the walls, ceiling and door are insulated. A typical two-car garage at 22 by 20 by 9 ft holding 50 degrees above outside therefore lands near 32,000 BTU/hr if partly insulated, and 20,000 once properly insulated. That is the honest argument for insulating the door first: it is usually the single largest leak and it halves the heater you need to buy.
Garage Heater Size
Heat loss BTU/hr = volume x temperature rise x insulation factor; kW = BTU/hr / 3412
A sizing estimate only. Fuel-burning heaters in garages require correct combustion air, flueing and clearance from vehicles and flammable storage — have installation designed and inspected to your local code.
Above about 20,000 BTU/hr, vented gas or propane is much cheaper to run — the same output in electric resistance heat is 6 kW, which needs a dedicated 240 V circuit and costs three to four times as much per hour.
With any fuel-burning heater, yes — combustion air and flue venting are mandatory, and unvented models are not appropriate where a car is running. Electric heaters need no venting but still want a CO alarm in the space.