How garage heater sizing works
The calculator separates two major heating loads: conduction through walls, ceiling, and garage doors, plus air infiltration. The conduction portion uses BTU/hr = area × U-value × ΔT, where U = 1/R. Air leakage uses a planning air-change assumption and the room volume.
Important: the outdoor temperature is entered directly. ACCA Manual J uses location-specific winter design conditions; an IECC climate-zone number is not a substitute for a local design temperature because cities in the same climate zone can have different winter design conditions.
Outdoor design-temperature input
| Outdoor design input | ΔT to 60°F | Planning interpretation |
|---|
| 25°F | 35°F | Mild-winter design condition |
| 15°F | 45°F | Moderate winter |
| 5°F | 55°F | Cold winter |
| −5°F | 65°F | Colder winter |
| −15°F | 75°F | Very cold winter |
| −25°F | 85°F | Extreme cold-weather design input |
These rows are examples of the math only; they are not mapped to IECC zones or cities. Use the winter design temperature for your actual location.
Envelope assumptions used in the calculator
| Surface | Uninsulated | Partially insulated | Well insulated |
|---|
| Walls | R-3 planning value | R-11 | R-19 |
| Ceiling | R-2 planning value | R-11 | R-38 |
| Garage door | Selected separately: R-2, R-7, or R-15 planning value |
| Air leakage | Selected separately: 0.5, 1.0, or 2.0 ACH planning assumption |
These are simplified planning inputs, not measured assembly R-values. If you know the actual insulation or infiltration performance, use a professional load calculation for final equipment selection.
Electric vs gas — what the calculator compares
Electric resistance heat can be compared directly in thermal output because 1 watt of resistance heat is about 3.412 BTU/hr. Large electric unit heaters commonly require dedicated circuits, but breaker/wire sizing must come from the actual heater nameplate, voltage, manufacturer instructions, and local electrical code — not from the BTU conversion alone. For other electric-heating comparisons, see the baseboard heater calculator.
Gas and propane unit heaters are commonly sold by BTU input or output depending on the product. When shopping, compare the calculator’s load with the heater’s delivered/output capacity and efficiency data, and follow venting/combustion-air requirements.
Recommended garage-heater categories
Use your result to compare:
Also compare local availability: Home Depot · Lowe's.
Garage heater wattage — BTU to watts conversion
For electric resistance heat, the thermal conversion is straightforward: watts = BTU/hr ÷ 3.412. This conversion tells you the heat output equivalent; it does not by itself determine breaker size, wire gauge, or whether a specific heater is permitted on a circuit.
| BTU/hr heat output | Equivalent electric resistance kW | Equivalent watts | Electrical planning note |
|---|
| 5,000 | 1.5 kW | 1,465 W | Small resistance-heat class; verify nameplate |
| 10,000 | 2.9 kW | 2,931 W | Dedicated circuit may be required |
| 15,000 | 4.4 kW | 4,396 W | Common fixed electric-heater range |
| 20,000 | 5.9 kW | 5,862 W | Usually a substantial dedicated load |
| 25,000 | 7.3 kW | 7,327 W | Verify voltage, current, breaker and wiring |
| 30,000 | 8.8 kW | 8,793 W | Large electric resistance load |
| 40,000 | 11.7 kW | 11,723 W | Compare installed electric vs fuel-fired options |
| 50,000 | 14.7 kW | 14,654 W | Very large electric resistance load |
| 60,000 | 17.6 kW | 17,585 W | Professional electrical review essential |
Do not select a breaker from this conversion table. Fixed electric space-heating equipment must be installed according to its nameplate, manufacturer instructions, and applicable electrical code. Have a qualified electrician size the circuit.
Garage heater types compared
| Type | Typical market capacity | Installation | Best fit |
|---|
| Electric unit heater | Small to large kW classes | Often needs a dedicated electrical circuit | Simple combustion-free heating where electrical capacity is available |
| Gas / propane unit heater | Commonly tens of thousands of BTU/hr | Fuel supply plus approved venting/combustion-air provisions | Larger garages and frequent heating |
| Infrared radiant | Model-specific | Electric or fuel-fired depending on product | Work zones and spot comfort |
| Mini split heat pump | Nominal capacities vary by model | Professional HVAC/electrical installation is common | Heating + cooling where low-temperature capacity is verified |
Heat-pump heating capacity changes with outdoor temperature. For cold-weather garage heating, verify the selected model’s manufacturer/AHRI heating capacity at or near your local design temperature rather than relying only on its nominal BTU label. Use the mini split calculator for a planning load.
Frequently asked questions
How many BTU do I need to heat a 2-car garage?
There is no reliable BTU answer from car count alone. A 2-car garage can vary greatly with outdoor design temperature, insulation, door area, air leakage, ceiling height, and target temperature. Use the calculator above with your local winter design temperature; broad retail heater classes for two-car garages often span roughly 25,000–60,000 BTU/hr.
How many BTU for a 1-car garage?
A 1-car garage often shops in roughly the 15,000–30,000 BTU/hr range, but the actual load can be well below or above that. Enter the garage dimensions, door construction, insulation, leakage, and outdoor design temperature rather than sizing from square footage alone.
What size garage heater do I need for a 3-car garage?
Three-car garages have enough variation that a single rule-of-thumb number is especially unreliable. Larger door area, tall ceilings, frequent openings, and a low outdoor design temperature can push the load much higher. Use the calculated heat-loss target and then select a heater whose delivered output meets it.
What outdoor design temperature should I use?
Use the location-specific winter heating design temperature from ACCA Manual J/ASHRAE weather data or your HVAC contractor. Do not substitute the IECC climate-zone number or the coldest temperature ever recorded; those are different concepts.
Should I choose electric or gas for a garage heater?
Electric resistance heaters avoid combustion and venting but can require substantial electrical capacity at higher outputs. Gas and propane units require fuel supply, combustion air, and approved venting. Compare installed cost, local energy prices, available electrical service, and how often the garage is heated. Electric unit heaters and gas/propane heaters are common categories.
Do garage doors significantly affect BTU requirements?
Yes. Garage doors can represent a large low-R-value surface and also create air leakage around their perimeter. The calculator separately accounts for door area/insulation and an air-leakage assumption so those effects are not hidden inside one square-foot rule.
How many watts is 30,000 BTU/hr of electric heat?
For electric resistance heat, 30,000 BTU/hr is about 8,793 watts because 1 watt is approximately 3.412 BTU/hr. That is a thermal-output conversion only. Use the heater nameplate and a qualified electrician to determine the required voltage, breaker, and wiring.
How many watts does a garage heater need?
For electric resistance heat, divide the required BTU/hr by 3.412. For example, 20,000 BTU/hr is about 5.9 kW and 30,000 BTU/hr is about 8.8 kW. Heat pumps are different: their electrical input is not equal to their delivered thermal output.
What size propane heater for a garage?
Choose a propane heater by its delivered/output capacity relative to the calculated heat load, not by garage-car count alone. Also verify whether the product lists input BTU or output BTU, its efficiency, venting requirements, and approved fuel-supply configuration.
Does ceiling height change the BTU requirement?
Yes. Higher ceilings increase wall area and room air volume, and therefore can increase both conductive and infiltration heat loss. The calculator uses the entered ceiling height in both parts of the estimate.
Can I use a mini split to heat a garage?
Yes, if the specific heat pump has enough heating capacity at your local winter design temperature. ENERGY STAR cold-climate criteria evaluate low-temperature performance at 5°F, but actual capacity still varies by model and temperature. Check manufacturer/AHRI extended performance data rather than assuming a nominal 12,000 or 18,000 BTU label is the delivered capacity on the coldest design day.