Garage Heater Cost Calculator
| BTU/h needed | Insulated (10/sq ft) | Average (18) | None (35) |
|---|---|---|---|
| 1-car, 400 sq ft | 4,000 | 7,200 | 14,000 |
| 2-car, 576 sq ft | 5,800 | 10,400 | 20,200 |
| 3-car, 864 sq ft | 8,600 | 15,600 | 30,200 |
| Shop, 1,200 sq ft | 12,000 | 21,600 | 42,000 |
Enter the garage size, how honest you are about its insulation, and your fuel price. The calculator sizes the heater in BTU per hour, converts it to the watt class or gas unit you would actually buy, and prices an hour, a month and a working winter of heat.
The framing to keep: sizing and fuel cost are two different questions. Insulation moves the BTU number more than anything you can buy - an uninsulated two-car garage asks three and a half times the heat of an insulated one - and the 1,500-watt ceiling of a plug-in electric heater covers only a fraction of it. Sizing first, then fuel.
How to use
- Floor area in square feet: one-car ~400, two-car ~576, three-car ~864.
- Insulation honesty matters most: insulated (walls+ceiling, sealed door) runs about 10 BTU/h per sq ft at a 45-degree rise, average about 18, uninsulated or drafty door about 35.
- Fuel price comes off your bill - dollars per kWh for electric, per gallon for propane, per therm for natural gas.
- A cold-weather work winter is about 60 to 100 heater hours a month; tinkerers run less, workshops run more.
Frequently asked questions
How many BTU do I need to heat a 2-car garage?
A 576 sq ft garage asks roughly 5,800 BTU/h if fully insulated, 10,400 at average insulation, and 20,000+ if the door leaks air at a 45-degree rise over outside. The calculator scales with your area and honesty setting - and every degree of target temperature adds about 2 percent to the load.
Is an electric garage heater enough?
Only for insulated small garages or spot work. A 1,500-watt plug-in unit delivers 5,118 BTU - sized for an insulated one-car. Real garage furnaces run 10,000 to 30,000 BTU, which means hardwired 5-10 kW electric or a vented gas/propane unit. The electric is 100 percent efficient at the wall but the slowest to recover after the door opens.
Do propane garage heaters need ventilation?
Yes - unvented propane heaters dump water vapor and combustion byproducts into the room, and any garage running a fuel burner needs fresh air and a CO alarm. Ventless is why they are cheap to install and why the safety manual keeps repeating the two words: ventilation, alarm.
What is the cheapest fuel per BTU for a garage?
At typical rates natural gas wins outright - roughly a third of electric per delivered BTU, with propane in between and nearest to gas. The break-even shifts with local prices, which is why the calculator prices your actual rates instead of quoting a national average.
Why does opening the door matter so much?
A single car-door opening exchanges most of the garage air volume in under a minute - the heater then re-heats it from scratch, which is why an hour of door-swinging work can cost as much as three quiet hours. Strip curtain or a windbreak on the door beats any heater upgrade on payback.