Garage Heater Size Calculator: BTU by Garage Size and Climate
Garage heater size is the heat needed to offset wall, roof, door, window, and air-leakage losses at the desired indoor and outdoor design temperatures. Large garage doors and infiltration often matter more than floor area alone.
Garage Heater Load Calculator
Enter project-specific values, calculate, then change an input to compare scenarios. The result stays in this page and sends no visitor data anywhere.
Air leakage can dominate garage loads. Use realistic door-area, assembly and ACH values.
How the garage heat-loss estimate works
Opaque wall, ceiling, overhead-door, and window/personnel-door losses use U × A × ΔT. The calculator subtracts entered door and window areas from gross exposed wall area. Air leakage uses garage volume, entered ACH, and 1.08 × CFM × ΔT.
The attached/detached selection appears in the result, but it does not apply an arbitrary multiplier. Instead, measure only the perimeter exposed to outdoors or unconditioned space. An attached wall shared with conditioned living space should not be counted as an exterior wall.
Measure the garage doors
Enter the total area of all overhead doors and their effective R-value. Two 9-by-7-foot doors total 126 square feet; a 16-by-7-foot door is 112 square feet. Product-labeled panel R-value may not represent the full installed assembly because joints, perimeter seals, tracks, and glazing affect performance.
Door operation also exchanges a large volume of air. Represent ordinary leakage and expected door use with a defensible ACH input. A busy workshop door opened repeatedly needs a project-specific infiltration or recovery calculation rather than a casual multiplier.
Desired temperature changes the answer
Keeping a garage at 45°F for freeze protection creates a smaller temperature difference than maintaining 65°F for occupied work. Enter the actual design goal. If the garage is heated intermittently, steady-state heat loss does not include the extra capacity and time needed to warm a cold slab, vehicles, tools, and contents.
Attached versus detached garages
An attached garage often has fewer outdoor-facing walls, but it also creates air-sealing and safety boundaries next to the home. Do not use a heating system or air-distribution arrangement that moves vehicle exhaust or garage contaminants into living space. Combustion appliances require approved location, clearances, venting, and combustion-air provisions.
A detached garage usually has four exposed walls and may have more wind exposure. Measure the actual perimeter, ceiling boundary, and door construction instead of assuming a fixed BTU-per-square-foot rate.
Example interpretation
If the result is 28,000 BTU/h at a 45°F temperature difference, that is the approximate steady-state output required under the entered assumptions. It is not automatically a 30,000-input gas heater. Check the appliance output rating, efficiency, altitude, fuel, venting, clearances, mounting height, air throw, and listed application.
Electric, gas, and radiant options
Electric resistance equipment converts output BTU/h to watts at about 3,412 BTU per kWh, but branch-circuit design is a separate electrical task. Unit heaters distribute warm air and may need destratification in tall spaces. Radiant heaters warm surfaces and occupants differently from forced air, so steady-state air heat loss alone does not design emitter layout or comfort.
What is outside this calculator
The estimator does not model slab-edge and ground coupling, warm-up recovery, latent moisture from snow-covered vehicles, exhaust fans, makeup air, combustion air, infiltration from each door cycle, or stratification. It also cannot determine safe appliance location in a garage.
Use a qualified HVAC professional for gas-fired equipment, frequent door operation, tall ceilings, hazardous or flammable storage, attached-home air-sealing concerns, or any permanent electrical installation. Follow the exact equipment instructions and local requirements.
Air sealing can reduce required capacity
Before buying a larger heater, inspect weatherstripping at overhead doors, the bottom seal, personnel doors, wall-to-roof joints, and penetrations. Correcting a large leak lowers the infiltration term every hour the garage is heated. Insulation and air sealing solve different paths; a high-R door with failed perimeter seals can still have a large air-leakage load.
Moisture and ventilation
Snow and rain carried in by vehicles can create a substantial moisture source. Heating raises the air’s moisture-holding capacity but does not remove water from the building. Safe ventilation, drainage, and envelope moisture control may be needed. Never block required combustion air or use the house HVAC system to pull garage air into living spaces.
Garage heater FAQ
How many BTUs for a two-car garage?
There is no universal number. Size, ceiling height, door area, insulation, leakage, target temperature, and outdoor design temperature determine the load.
Should I size for 70°F?
Only if that is the real design goal. Freeze protection or occasional work may use a lower maintained temperature with a separately evaluated warm-up strategy.
Does an insulated door solve leakage?
No. Panel insulation reduces conduction, while perimeter seals and door operation control separate air-leakage effects.