Garage Heater BTU Calculator

About Garage Heater BTU Calculator

Heating a garage is the hardest case in a domestic building, and this calculator combines the two factors that make it so: heating mode, which uses higher rates than cooling, and the garage space type, which assumes poor insulation. Together that gives 45 BTU/h per square foot, nearly a third more than a standard insulated room. The problem is not usually the size of the heater but the rate at which the heat leaves. An uninsulated sectional door is typically the largest single surface in a garage and often the worst performing part of the building envelope, and an uninsulated concrete slab draws heat continuously into the ground. Sealing the gaps around the door frequently makes more difference to comfort than moving up a heater size, because air leakage bypasses the insulation entirely. For workshops, add capacity beyond the result if you are working with the door open, since that changes the problem from heating a room to heating an outdoor space. For cooling the same space see Garage BTU Calculator.

How to use Garage Heater BTU Calculator

  1. Enter room area in m² or ft²
  2. Set ceiling height, sunlight exposure, and number of occupants
  3. See recommended BTU, kW, and AC unit type

Frequently Asked Questions

How many BTU to heat a 2 car garage?
A 400 to 600 square foot garage at 45 BTU/h per square foot needs roughly 18,000 to 27,000 BTU/h. That assumes it is uninsulated. Insulating the walls, ceiling and door can bring it close to the standard room rate.
Why does a garage need more BTU to heat than a bedroom?
Insulation and air leakage. The sectional door is a large uninsulated panel, the walls and ceiling are often bare, and the slab loses heat to the ground. Heat leaves a garage far faster than it leaves a finished room.
Is insulating the garage door worth it?
Usually yes, since it is often the largest and worst performing surface. Sealing air gaps around the door is worth doing first, because leaked air bypasses insulation altogether and no amount of heater capacity compensates for it efficiently.
Can I heat a garage with an electric heater?
Divide the BTU result by 3,412 to get kilowatts. A 20,000 BTU/h requirement is close to 6 kW, which is beyond a standard plug socket and needs a dedicated circuit. That is why larger garages often use gas or a heat pump instead.
What if I work with the garage door open?
Then you are heating outdoor air and no reasonable sizing applies. Radiant or infrared heaters are the usual answer, since they heat people and surfaces directly rather than trying to warm air that immediately leaves.

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