Garage Heater Size Calculator

A garage loses heat differently from a house: the overhead door is a large panel of almost nothing, the floor is an uninsulated slab that leaks around its edge, and the building leaks air at two or three times the rate of a finished room. This page works the load surface by surface from the garage dimensions, the insulation you actually have and the temperature you want to hold, then turns it into a kilowatt rating for an electric heater and an input rating for a gas or propane unit heater. It reports each loss separately, so the door and the slab edge that whole-house rules of thumb leave out are visible, and it shows what one full air change costs every time you raise the door. A second action puts electric resistance, a heat pump, natural gas and propane side by side at your own fuel prices.

Calculator Type
Dimensions In
The building only. R-values, BTU per hour, the therm and the propane gallon stay as they are, because that is how the insulation and the heaters are sold; the kilowatt output is on the page either way.
Window U-Value In
The two scales differ by a factor of 5.678, so a 0.5 window and a 0.5 W/sq m.K window are not the same window at all.
Temperatures In
Both temperature boxes below, and every temperature difference in the answer, are read and printed in whichever you pick.

Dimensions, what is insulated and what is not, and the temperature you want to hold on your coldest design day.

To the ceiling in feet, or to the top plate if it is open to the roof. Blank means 9 ft.
A single door is usually 8 or 9 feet wide, a double 16 or 18. Blank means 16 ft.
Blank means 7 ft, which is the usual single-car door height.
Square feet, or square metres in metric. Counting the frame. A personnel door is close enough to the wall it sits in to ignore.
Single glazing about 1.0, ordinary double glazing 0.5, which are 5.7 and 2.8 in W per sq m per K. Blank means 0.5, or 2.8 in the metric scale.
A garage is not a house. 50 to 55 F, which is 10 to 13 C, keeps pipes and a car happy; 60 to 65 F, or 16 to 18 C, is a workshop you stand about in, and every degree costs. Blank means 55 F, which is 12.8 C.
The winter design temperature for your area, which is the temperature about one percent of winter hours fall below. Not the record low. Read in whichever scale you picked above.
A sealed and drywalled garage 0.5 to 0.8, a typical one 1.0 to 1.5, a bare stud garage with a gappy door 2 or more.
A vented unit heater is about 80. A condensing one is 90 to 95. Electric resistance is 100 by definition.
Leave at 0 to skip the running cost lines.
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The same heat delivered four ways. Put the output from the sizing action in the first box.

Output, not the input rating on a burner nameplate.
A heater cycles, so this is equivalent hours at full output, not hours the switch is on.
A therm is 100,000 BTU. If your bill is in CCF, a CCF is close enough to a therm for this.
The US gallon of 91,502 BTU, which is how retail propane is priced in North America. A price per litre becomes a price per gallon by multiplying by 3.785411784, and a price per kilogram by about 1.9, since a gallon of propane weighs roughly 1.9 kg.
Applied to both the gas and the propane figure. A vented unit heater is about 80, a condensing one 90 to 95.
A mini-split makes 2 to 3 units of heat per unit of electricity on a cold day and 3 to 4 in mild weather. A COP of 1 is a resistance heater.
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