What size heater do I need for a two-car garage?

It depends on insulation, air leakage and your climate, more than on floor area. Enter your R-values and design temperature above. An uninsulated garage can need several times the heat of an insulated one.

How many BTU per square foot for a garage?

There is no fair single figure. Per-square-foot rules ignore insulation and air leakage, which are the two things that matter most.

How do I convert BTU to watts?

One kilowatt is about 3,412 BTU per hour. Divide the BTU result by 3,412 for kilowatts.

Does an insulated garage door help heating?

Yes, if the door is a large share of the wall. A better bottom seal often helps more, because leaking air is a big share. See insulation evidence .

Is a gas garage heater safe?

When it is vented and installed to the maker’s manual and local code. Installation is a job for a licensed professional. See garage heater installation .

HyreGarage tools

Garage heater size calculator

Heat lost through the walls, ceiling and door, plus heat lost to leaking air, at your coldest design day.

0.018 BTU to warm 1 cu ft of air by 1°F The infiltration constant in University of Minnesota Extension’s heat-loss method.

This garage heater size calculator, a garage heater BTU calculator, adds two losses. Conduction is each surface’s area divided by its R-value, times the temperature difference.

Air leakage is 0.018 × volume × air changes per hour × the temperature difference (University of Minnesota Extension). The sum is the BTU per hour the heater must supply. Divide by 3,412 for kilowatts.

Size, insulation, then climate

Nothing is emailed. R-values and air changes are not preset.

— Heater size

— In kilowatts

— Where the heat goes

What this assumed —

A sizing estimate, not a Manual J load calculation. HyreGarage does not install heaters.

How is garage heater size calculated?

Add the heat lost through the walls, ceiling and door to the heat lost to air leaking in. The heater must replace both at your coldest design temperature.

Conduction. Each surface loses heat equal to its area divided by its R-value, times the temperature difference. The garage door is often the weakest surface, so its R-value matters. See garage door R-value.

Air leakage. University of Minnesota Extension gives the infiltration loss as 0.018 × volume × air changes per hour × temperature difference. The 0.018 is the heat needed to warm one cubic foot of air by one degree Fahrenheit.

The answer. The sum is in BTU per hour. Divide by 3,412 for kilowatts, which is how electric heaters are sold.

Where do I find each input?

From your building and your climate. None are preset, because a guess here throws the answer off more than anything else.

Wall and ceiling R-value: From the insulation label or the plans. An uninsulated stud wall is very low; ask an insulation contractor if unsure.
Door R-value: From the door maker’s data sheet. Makers measure it differently, so treat it as approximate (see R-value claims).
Air changes per hour: UMN Extension cites 0.2 to 2.0 for houses, and 0.5 or less for new tight construction. Garages usually leak more than houses, mostly around the door.
Outdoor design temperature: A cold-day figure for your area. Your local building department or an HVAC contractor uses one for heating loads.

What should I check before installing a garage heater?

Fuel, venting, clearance and the wiring. A heater that fits the load can still be the wrong unit.

Gas and propane heaters must be vented and installed to the maker’s manual and local code.
Keep the clearances the manual lists from cars, shelves and anything stored.
An electric heater may need its own circuit. That is an electrician’s job.
Seal the garage door first. A new bottom seal can cut the air-leak share more cheaply than a bigger heater.
See garage heater installation for what the job involves.

Questions this calculator answers

What size heater do I need for a two-car garage?: It depends on insulation, air leakage and your climate, more than on floor area. Enter your R-values and design temperature above. An uninsulated garage can need several times the heat of an insulated one.
How many BTU per square foot for a garage?: There is no fair single figure. Per-square-foot rules ignore insulation and air leakage, which are the two things that matter most.
How do I convert BTU to watts?: One kilowatt is about 3,412 BTU per hour. Divide the BTU result by 3,412 for kilowatts.
Does an insulated garage door help heating?: Yes, if the door is a large share of the wall. A better bottom seal often helps more, because leaking air is a big share. See insulation evidence.
Is a gas garage heater safe?: When it is vented and installed to the maker’s manual and local code. Installation is a job for a licensed professional. See garage heater installation.

Sources and methodology

Figures dated October 26, 2026. Last reviewed August 16, 2026.

Home heat loss calculation (infiltration 0.018 × volume × ACH × ΔT) (University of Minnesota Extension, retrieved 2026-10-26. Infiltration formula and the 0.2 to 2.0 air-change range.)

Garage heater installation What the job involves.
Garage insulation Cut the load first.
Garage door R-value How door ratings work.
Insulated garage doors When a new door helps.

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