Garage Heater Sizing Calculator
Heating a garage depends on the air volume, how cold it gets, and how well the space is sealed and insulated. This calculator finds the volume as length times width times height, multiplies by the temperature rise you want to hold and by a heat factor for the construction, and reports the heater output needed in BTU per hour and watts. Because the heat factor depends on your specific garage, it is a value you enter rather than a guessed constant, keeping the result honest.
Garage heater sizing formula
Volume = length * width * height
Output (BTU/hr) = volume * temperature rise * heat factor
Output (watts) = BTU per hour / 3.412142
Output (kilowatts) = watts / 1,000
The conversion 1 watt equals 3.412142 BTU per hour is exact. The heat factor is your input for the garage construction, so the output reflects your actual insulation and air-tightness rather than a default.
Garage heating notes
- Volume-based sizing accounts for tall garage ceilings that floor-area methods miss.
- Temperature rise is your target indoor temperature minus the coldest outdoor design temperature.
- A lower heat factor suits sealed, insulated garages; a higher one suits drafty, uninsulated ones.
- One watt equals 3.412142 BTU per hour, the exact thermal-to-electrical power conversion.
- Electric heaters are rated in watts; gas heaters in BTU per hour. Both are shown here.
Garage heater sizing: frequently asked questions
How do I size a garage heater?
A practical method multiplies the garage volume in cubic feet by the desired temperature rise in degrees Fahrenheit and by a heat factor that reflects insulation, then the heater must deliver that many BTU per hour. This calculator computes the volume for you and applies the temperature rise and heat factor you enter, then converts the result to watts.
What heat factor should I use?
The heat factor captures how leaky and poorly insulated the garage is, and it is not a single fixed number. Better-insulated, sealed garages use a lower factor; drafty, uninsulated ones use a higher factor. Use a value from a heating reference appropriate to your construction. Because it varies, this calculator takes the factor as your input.
What temperature rise should I target?
Temperature rise is the difference between the coldest outdoor temperature you want to heat against and your target indoor temperature. For example, holding 60 degrees Fahrenheit inside when it is 10 outside is a 50 degree rise. Enter the rise that matches your climate and comfort goal.
Why is the answer also in watts?
Electric garage heaters are often rated in watts. One watt equals 3.412142 BTU per hour, an exact conversion, so the calculator divides the BTU per hour figure by 3.412142 to show the equivalent wattage. This lets you compare gas heaters rated in BTU with electric heaters rated in watts.
Does garage ceiling height matter?
Yes. Many garages have tall ceilings, and a volume-based method accounts for that air, which a floor-area-only estimate would miss. The calculator multiplies length, width, and height for the volume, so a high-bay or workshop garage is sized for the larger air space it actually contains.
Official sources
- U.S. Department of Energy: Home Heating Systems.
- U.S. National Institute of Standards and Technology: Unit conversion (BTU, watt).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.