Roomtherm

Garage heater size

A two-car garage of 20 × 24 ft (480 sq ft) with little or no insulation needs about 24,000 BTU/h of heat (7,034 W). Insulated to the standard of a newer home, the same garage needs 9,827 BTU/h.

Updated

Measure your room — these are examples.

What are you sizing?
Units

Heat to look for

24,000 BTU/h 7,034 W

Radiator rating to look for at ΔT50: 7,034 W (24,001 BTU/h)

Roomtherm sizes heat output only. It does not cover wiring, circuits, gas or other fuel-burning heaters; those follow the heater's instructions and local code.

How well is it insulated?

Not used for little or no insulation.

Sizing radiators?

ΔT is the average of the flow and return temperatures minus the room temperature. Radiators are rated at ΔT50; a system that runs cooler gets less heat from the same radiator. Work your ΔT out from the temperatures:

Show the math
  1. 20 ft × 24 ft = 480 sq ft
  2. 480 sq ft × 50 BTU/h = 24,000 BTU/h × 0.29307 = 7,034 W (rounded up)

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Heat by garage size and insulation

The garage sizes are examples; measure yours. The first two columns use King Electric's 6 and 10 W per sq ft, the third uses 50 BTU/h per sq ft from NDSU Extension.

Heat needed in BTU/h and watts
GarageInsulated (newer-home rate)Lightly insulated (older-home rate)Little or no insulation
One-car garage12 × 20 ft, 240 sq ft 4,913 BTU/h
1,440 W		8,189 BTU/h
2,400 W		12,000 BTU/h
3,517 W
Two-car garage20 × 24 ft, 480 sq ft 9,827 BTU/h
2,880 W		16,378 BTU/h
4,800 W		24,000 BTU/h
7,034 W
Three-car garage30 × 24 ft, 720 sq ft 14,740 BTU/h
4,320 W		24,567 BTU/h
7,200 W		36,000 BTU/h
10,551 W

Where the rates come from

King Electric's heater sizing sheet says it recommends approximately 6 watts per square foot for newer homes and 10 watts per square foot for older homes. A newer home there means R19 walls and R30 ceilings, mostly built from the 1980s on; an older one, R11 walls and single-pane windows. A garage insulated and sealed to those levels can use the same rates.

For a garage with bare walls and an uninsulated door, Roomtherm uses 50 BTU/h per sq ft. That figure is from NDSU Extension's AE1620, Heating Farm Shops (2025): a few years ago the rule of thumb was that farm shop heating should provide around 50 Btu per sq ft per hour, and with better insulation shops now commonly need only 20. NDSU gives 50 as the former general rule, not as a rate for uninsulated buildings; applying it to little or no insulation is our assumption. The insulated column is close to NDSU's newer figure: 6 W per sq ft is 20.5 BTU/h per sq ft.

These are rules of thumb. NDSU itself adds that you need heat loss calculations to properly size a farm shop heater; a garage with a large door, a high ceiling or a long cold season can need more than the table shows.

Reading the result

The number is a heat requirement: how much heat output the space needs, in BTU per hour and in watts. Compare it with the heat output printed on a unit. If it falls between two sizes, King Electric's advice is to pick the larger. Cold-climate and large-window adjustments apply to the two insulated levels only; the calculator greys them out for little or no insulation.