How Many BTUs Do I Need to Heat a Garage?
Updated January 29, 2026
Just want the number? Use the whole-home heat load calculator.
A typical 2-car garage, somewhere around 400 to 600 square feet, usually needs about 30,000 to 60,000 BTU per hour to heat. The low end is for a mild climate with a finished, insulated space. The high end is for a cold climate with bare block walls, an uninsulated door, and a high ceiling.
The reason the range is so wide is that a garage is a much harder space to heat than the same square footage inside your house. Here is what drives the number up or down.
Garages leak heat in ways living spaces do not
Most homes are designed to hold heat. Most garages are not. Three things make a garage a heat sink.
- Poor insulation. Many garages have uninsulated block or stud walls, no attic insulation above them, and a slab floor with no thermal break. Heat escapes through every one of those surfaces.
- The big door. A garage door is a large, thin, often uninsulated panel, and the gaps around it let cold air in. It is usually the single biggest source of heat loss in the room.
- Ceiling height. You heat volume, not floor area. A garage with a 10 or 12 foot ceiling, or an open truss roof, holds far more air than a standard 8 foot room of the same footprint, so the load climbs.
Sealing the door gaps and adding even modest insulation can cut the required BTU noticeably, which is why the same garage can sit at either end of that 30,000 to 60,000 range.
Climate sets the baseline
The colder your winters, the more BTU per square foot you need. A rough guide for a garage, which runs higher than a finished home because of the heat loss above:
- Mild climate: about 45 to 55 BTU per square foot
- Moderate climate: about 60 to 75 BTU per square foot
- Cold climate: about 90 to 100 BTU per square foot
For a 500 sq ft garage that spans roughly 25,000 BTU in a mild zone up to about 50,000 BTU in a cold one, before you adjust for insulation and ceiling height. The U.S. Department of Energy publishes climate zone guidance you can use to place yourself.
Match the heater type to the space
Garages tend to suit a few heater styles, and the type you pick affects how the BTU translate into comfort.
- Forced-air heaters blow warm air and heat the whole volume quickly. Good for a space you use for a few hours at a time. Available in propane, natural gas, and electric.
- Infrared (radiant) heaters warm objects and people directly rather than the air, so they work well in a drafty garage or one with a high ceiling where heated air would otherwise rise out of reach.
- Electric unit heaters are simple to install and need no venting, but electricity is often the more expensive fuel per unit of heat, so size them carefully and check the running cost first.
Before you commit to electric, it is worth comparing fuels for your area with the energy cost calculator, because a cheaper unit can cost more to run over a winter.
Get your number
Square footage gets you in the ballpark, but insulation, the door, and ceiling height are what decide where you land. Plug your dimensions, climate, and insulation level into the whole-home heat load calculator and treat the garage as a room with poor insulation and a lot of exposed surface. That gives you a far more honest figure than a flat BTU-per-square-foot rule.
If you are sizing a heater for the whole structure rather than just topping up an attached space, the furnace size calculator works the same way. If your heater is already installed and short-cycles rather than running in steady bursts, is my furnace oversized covers the symptoms and what to do. Run your own numbers before you buy, and round to the nearest available heater size rather than oversizing for headroom.