The short answer
An attached 2-car garage with insulated walls and ceiling and a plain steel door, kept at 50°F, needs about 15,400 BTU/h in Minneapolis, where the winter design temperature is -6°F: a 5,000 W, 240 V electric heater or a 30,000 BTU gas or propane unit heater. In Houston (37°F) the same garage loses only 3,600 BTU/h. Insulation, the door and how warm you keep it matter more than the floor area.
The same garage in every state
An attached 2-car garage (22 × 22 ft, 9 ft ceiling), insulated walls and ceiling, plain door, kept at 50°F, at the winter design temperature of each state's largest city.
| Alabama Huntsville | 3A | 23 | 7,400 | 4,000 | 30,000 |
|---|---|---|---|---|---|
| Alaska Anchorage | 7 | -4 | 14,900 | 5,000 | 30,000 |
| Arizona Phoenix | 2B | 43 | 1,900 | 4,000 | 30,000 |
| Arkansas Little Rock | 3A | 25 | 6,900 | 4,000 | 30,000 |
| California Los Angeles | 3B | 47 | 800 | 4,000 | 30,000 |
| Colorado Denver | 5B | 7 | 11,800 | 4,000 | 30,000 |
| Connecticut Bridgeport | 5A | 16 | 9,400 | 4,000 | 30,000 |
| Delaware Wilmington | 4A | 18 | 8,800 | 4,000 | 30,000 |
| District of Columbia Washington | 4A | 23 | 7,400 | 4,000 | 30,000 |
| Florida Jacksonville | 2A | 42 | 2,200 | 4,000 | 30,000 |
| Georgia Atlanta | 3A | 27 | 6,300 | 4,000 | 30,000 |
| Hawaii Honolulu | 1A | 66 | 0 | None | None |
| Idaho Boise | 5B | 16 | 9,400 | 4,000 | 30,000 |
| Illinois Chicago | 5A | 6 | 12,100 | 4,000 | 30,000 |
| Indiana Indianapolis | 4A | 9 | 11,300 | 4,000 | 30,000 |
| Iowa Des Moines | 5A | 0 | 13,800 | 5,000 | 30,000 |
| Kansas Wichita | 4A | 13 | 10,200 | 4,000 | 30,000 |
| Kentucky Louisville | 4A | 16 | 9,400 | 4,000 | 30,000 |
| Louisiana New Orleans | 2A | 40 | 2,800 | 4,000 | 30,000 |
| Maine Portland | 6A | 7 | 11,800 | 4,000 | 30,000 |
| Maryland Baltimore | 4A | 19 | 8,500 | 4,000 | 30,000 |
| Massachusetts Boston | 5A | 14 | 9,900 | 4,000 | 30,000 |
| Michigan Detroit | 5A | 9 | 11,300 | 4,000 | 30,000 |
| Minnesota Minneapolis | 6A | -6 | 15,400 | 5,000 | 30,000 |
| Mississippi Jackson | 3A | 29 | 5,800 | 4,000 | 30,000 |
| Missouri Kansas City | 4A | 9 | 11,300 | 4,000 | 30,000 |
| Montana Billings | 6B | -4 | 14,900 | 5,000 | 30,000 |
| Nebraska Omaha | 5A | 4 | 12,700 | 4,000 | 30,000 |
| Nevada Las Vegas | 3B | 37 | 3,600 | 4,000 | 30,000 |
| New Hampshire Manchester | 5A | 9 | 11,300 | 4,000 | 30,000 |
| New Jersey Newark | 4A | 17 | 9,100 | 4,000 | 30,000 |
| New Mexico Albuquerque | 4B | 23 | 7,400 | 4,000 | 30,000 |
| New York New York | 4A | 18 | 8,800 | 4,000 | 30,000 |
| North Carolina Charlotte | 3A | 27 | 6,300 | 4,000 | 30,000 |
| North Dakota Fargo | 6A | -13 | 17,300 | 7,500 | 30,000 |
| Ohio Columbus | 4A | 11 | 10,700 | 4,000 | 30,000 |
| Oklahoma Oklahoma City | 3A | 19 | 8,500 | 4,000 | 30,000 |
| Oregon Portland | 4C | 30 | 5,500 | 4,000 | 30,000 |
| Pennsylvania Philadelphia | 4A | 18 | 8,800 | 4,000 | 30,000 |
| Rhode Island Providence | 5A | 11 | 10,700 | 4,000 | 30,000 |
| South Carolina Charleston | 3A | 32 | 5,000 | 4,000 | 30,000 |
| South Dakota Sioux Falls | 6A | -4 | 14,900 | 5,000 | 30,000 |
| Tennessee Nashville | 3A | 21 | 8,000 | 4,000 | 30,000 |
| Texas Houston | 2A | 37 | 3,600 | 4,000 | 30,000 |
| Utah Salt Lake City | 5B | 20 | 8,300 | 4,000 | 30,000 |
| Vermont Burlington | 6A | 0 | 13,800 | 5,000 | 30,000 |
| Virginia Virginia Beach | 3A | 27 | 6,300 | 4,000 | 30,000 |
| Washington Seattle | 4C | 30 | 5,500 | 4,000 | 30,000 |
| West Virginia Charleston | 4A | 16 | 9,400 | 4,000 | 30,000 |
| Wisconsin Milwaukee | 5A | 4 | 12,700 | 4,000 | 30,000 |
| Wyoming Cheyenne | 5B | 3 | 12,900 | 4,000 | 30,000 |
How it works
The formula
We add up the heat the garage loses on a design-temperature night at the temperature you keep it, then pick the smallest common heater that covers it.
walls, door, ceiling = U × area × ΔT
slab edge = F × exposed perimeter × ΔT
air leaks = 1.08 × CFM × ΔT
electric watts = BTU/h ÷ 3.412
gas input = BTU/h ÷ 0.8
Assumptions
- U-factors, in BTU/h per sq ft per °F: uninsulated walls 0.25, ceiling 0.3; insulated walls 0.075 (R-13) and ceiling 0.033 (R-30); a plain steel garage door 1.1, an insulated one 0.35 once its gaps are counted. Garage doors are 7 ft high: one 9 ft door per bay or a 16 ft double.
- Air leaks: 1 air changes an hour uninsulated, 0.6 insulated, 0.5 with a sealed, insulated door. Opening the door lets the heat out; budget extra time to warm back up, not a bigger heater.
- An attached garage shares one wall with the house, which we count as no loss. A wall to a heated room loses heat the other way, which this ignores.
- Slab edge loss along the exposed walls: 0.8 BTU/h per foot per °F, for an uninsulated slab, whatever else is insulated; few garage slabs are. Windows are counted as wall.
- Electric heaters come in 4,000, 5,000, 7,500, 10,000 W at 240 V. Vented gas and propane unit heaters in 30k, 45k, 60k, 75k, 100k, 125k BTU input, delivering about 80% of it.
Worked example: 2-car garage in Minneapolis
Attached, 22 × 22 ft, 9 ft ceiling, insulated walls and ceiling, plain 16 × 7 ft door. Kept at 50°F; winter design -6°F, so ΔT = 56°F.
- Walls: 22 + 22 + 22 = 66 ft × 9 ft − 112 sq ft door = 482 sq ft × 0.075 × 56 = 2,000
- Door: 112 × 1.1 × 56 = 6,900; ceiling: 484 × 0.033 × 56 = 900
- Slab edge: 66 × 0.8 × 56 = 3,000; air: 1.08 × 43.6 CFM × 56 = 2,600
- Total 15,400 BTU/h = 4,516 W, so a 5,000 W heater (30 A breaker), or 15,400 ÷ 0.8 = 19,300 BTU input, so a 30,000 BTU gas heater.
Too big or too small
- Too small: it never gets there. On the coldest days the heater runs nonstop and the garage stays well below the setting.
- Too big: wasted money up front. A larger electric heater needs a bigger breaker and wire; a larger gas heater cycles on and off. Running cost depends on the heat lost, not the heater size.
- Insulate before you size up. Insulating and sealing the door often saves more than buying the next size.
- Heating only while you work? Size for the temperature you actually want; warming a cold slab takes time, so start the heater an hour or two early.
Questions people ask
What size heater do I need for a 2-car garage?
For an attached 2-car garage (22 × 22 ft, 9 ft ceiling) with insulated walls and ceiling and a plain steel door, kept at 50°F: about 15,400 BTU/h in Minneapolis (winter design -6°F), so a 5,000 W electric heater or a 30,000 BTU gas or propane unit heater. In Houston (37°F) it needs only 3,600 BTU/h. An uninsulated, detached garage in Minneapolis needs about 32,900 BTU/h.
How many watts to heat a garage?
It depends far more on insulation and how warm you keep it than on square feet. The example 2-car garage needs about 4,500 W at 50°F in Minneapolis, and 6,000 W to keep it at 68°F. Most 240 V garage heaters are 4 kW, 5 kW, 7.5 kW, 10 kW.
Can I heat a garage with a 120 V space heater?
Only a small, insulated one in a mild climate. A plug-in heater tops out at 1,500 W (about 5,100 BTU/h), while even a 1-car garage in Minneapolis needs about 10,200 BTU/h at 50°F. For more, use a 240 V heater on its own circuit, or a vented gas or propane unit heater.
What breaker does a garage heater need?
A heater runs for hours, so the circuit must carry 125% of its current. At 240 V: 4,000 W draws 16.7 A and needs a 25 A breaker; 5,000 W, 20.8 A on 30 A; 7,500 W, 31.3 A on 40 A; 10,000 W, 41.7 A on 60 A. Use the heater's instructions and your local code for the wire size; an electrician should install it.
Does insulating the garage door help?
A lot. A plain steel door loses about 3 times as much heat per square foot as an insulated one, and in a garage with insulated walls it's often the biggest loss. Insulating the door and sealing its edges takes the example garage in Minneapolis from 15,400 to 10,300 BTU/h.
Is it safe to use a propane heater in a garage?
Use a vented unit heater, installed to the maker's instructions and local code, for a garage you heat regularly. Unvented (ventless or "buddy" style) heaters put carbon monoxide and moisture into the space and are meant for short, supervised use with fresh air. Keep any flame away from gasoline and solvents, and fit a CO alarm.