Garage Heater Sizing: How to Determine Wattage or BTU Requirements

Whether you use the garage as a place to park your car or as a place of work. You may end up needing a heater for your garage.

As such, determining the right heater capacity based on the sizing of your garage will make the space more comfortable and functional for working throughout the year.

Luckily, you aren't limited by the number of garage heaters available. They come in a variety of fuel types, shapes, sizes, and mounting options. With that said, the main thing you need to keep in mind is the sizing of your garage. You should pick an option that can heat the space effectively.

How to Determine the Size of Your Garage

How many cars can fit in our garage? An average single car garage is about 12-14 feet wide and 16-22 feet long. With these numbers you can assume that a typical one-car garage is 12' x 22' (or 264 square feet). Additionally, a two-car garage is 18' x 20', whereas a 32' x 22' feet garage could fit three cars.

If you want to obtain the total volume of the space that should be heated, you need to multiply the square footage by the ceiling height (e.g., if you have a 10-foot ceiling, the calculation is 264 x 10 = 2,640).

Thermal Output

The next thing to determine is the size of the heater and the thermal output you should look for. Each heater has its thermal output measured in BTU (British Thermal Units) for propane heaters, or in Watts for electric. As expected, bigger garages require greater BTUs or Watts.

Our general guideline for garage heater sizing is 10 watts per each square foot. To calculate BTU per hour for gas heaters, you need to multiply the wattage by 3.41. For unit conversions sake, 1 btu/hr is equal to 3.41 watt.

For instance, a single-car 12’ x 22’ garage is 264 sq ft, so the simplest way to calculate the wattage is to multiply it by 10 (264 x 10 = 2,640 watts). As a result, this garage would require a 2640-watt electric heater or 9,002 BTU/hr propane (2,640 x 3.4 = 9,002).

According to the “10-watt rule,” here are some illustrative examples of garage heater sizing:

  • Single-car garages (0-450 sq ft) require a 2000-3000 W unit (or 6,800-9,000 BTUs/hr)
  • Two-car garages (450-700 sq ft) require a 3600-7000 W unit (electric heaters) or 12,000-24,000 BTUs/hr (for the propane ones)
  • Three-car or bigger garages (700-900 sq ft) need a 7000-9000 W unit (or 24,000-31,000 BTUs/hr).

However, for more precise calculations, you may need to consider other factors.

Temperature Rise

How warm would you like your garage to be? This can depend on the climate and seasonal temperatures of your region.

For example, an average winter temperature is 32°F, so a comfortable indoor temperature in your garage should be around 65°F. The heater should be able to generate this difference of 33°F, which is, in fact, recognized as temperature rise.

Therefore, if you choose the right size, your garage will stay warm even in the coldest winter days.


Are the walls and the ceiling insulated? In most cases, heater manufacturers state how many square feet their units can handle. However, they sometimes forget to take insulation into account.

Poorly insulated garages generally require bigger heaters to maintain a comfortable temperature, while a better-insulated garage may require a smaller heater.

To avoid losing heat through walls or doors, you can determine the insulation quality and use the following numbers when calculating the size of the heater. For instance, we use "0.5" for well-insulated garages, "1.5" for little, and "5" for those without insulation.

How to Calculate Garage Heater Capacity

More precise calculations will include the ceiling height as well, which is usually between 8-12 feet.

Other than volume, the BTU output calculations involve the insulation level and the temperature rise. If garage walls are well insulated, you can buy a heater with a lower thermal output. But what should we do in cases of poor insulation?

Let us try to recalculate the BTU for a 12’x22’ garage with an 8 ft ceiling and poor ventilation (insulation level is “1”). The temperature rise is 33°F, and the garage volume is 2,112 cubic feet.

First, we multiply 2,112 x 1 x 33 to get 69,696 BTU. You then divide that number by a btu factor of 1.6 to get 43,560 BTU. To convert this number into watts we divide by 3.41 and get 12,774 W.

When we round up the numbers, we can conclude that a 12’x22’ garage with rather poor insulation requires a 12,800 W or 44,000 BTU heater to stay warm in colder seasons.

To sum up, you can use either of these methods to solve the dilemma of perfect garage heater sizing. For cost-effective solutions, choose heaters that suit the needs of your garage and don't forget to have a glance at the unit specifications. Also consider ways to improve insulation as it goes a long way with heat retention.

Richard Reed
I've been a General Manager of a moving company and I've also been a Professional Mover for over 30 years. I've driven flat beds, reefers, dropdecks, moving vans, heavy machinery, etc. In my time as a Mover I've driven over 1,000,000 safe miles. My days of moving and driving truck are past me but The Vehicle Lab allows me to share the knowledge I've gained over the past 40 years.
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