Garage Heat Lab

Garage Heater Calculator

Find the right garage heater size in BTU/hr and watts. This garage heater BTU calculator uses your garage size, winter low and insulation.

Your garage

Garage size
ft
ft
ft

Width is the side with the garage doors.

Climate
Enter a ZIP or FSA for a nearby station estimate, or type a temperature. Coverage limits
°F
Target

Keep it at

°F
Insulation

Some insulation, but typical garage doors and air leaks.

Doors, windows & shared wall · optional

Assumed-envelope scenario. See model assumptions.

Garage doors
2
Windows
1

What to buy for this garage

USD · 6 hours/day, 30 days/month. Assumes heating runs half the time you use the garage.

Edit prices & hours

Electric

7,500 W heater

Covers 7,200 W heat equivalent

  • Selected heater: 240 V, about 31.3 A.
  • 125% load example: 40 A dedicated double-pole circuit. Confirm the product's nameplate and instructions.
$112/mo646 kWh/mo

Natural gas

Lowest in this scenarioLowest

45,000 BTU/hr input

Needs ≥ 30,700 BTU/hr input at 80%

  • Compare delivered output, not just the input label.
  • Check venting, clearances and fuel supply for an enclosed garage.
$41/mo28 therms/mo

Propane

45,000 BTU/hr input

Needs ≥ 30,700 BTU/hr input at 80%

  • Compare delivered output, not just the input label.
  • Check venting, clearances and fuel supply for an enclosed garage.
  • Selected class: about 0.49 gal/hour at full fire.
$77/mo30 gal/mo

Editable references: electricity 2025 annual average; gas 2025 annual average; propane October 2025–March 2026 weekly mean. These are USD references, including for Canadian locations. Sources and conversion.

Where the heat goes

Assumed-envelope shares, not measured losses.

Air leaks
28%
Walls
22%
Slab edge
20%
Garage door
18%
Ceiling / roof
11%
Windows
1%

The model estimates about 14% less heat with this change.

R-values, ACH and calibration assumptions →

Next steps

Estimate the running costAdjust heating time, fuel and local pricesEstimate installation costWiring, ventilation and gas-line work

Planning estimate from a published method. Confirm wiring, venting and clearances with a licensed installer. How we calculate.

Open the full Garage Heat Lab calculator · Formula and limitations