For estimation and sales-phase use. A simplified heat loss estimate for one space, not a Manual J load calculation. Compare the result with a heater's rated output and install any heater to the manufacturer's instructions and local code.

Maintained by the BuildSolver engineering team · Updated

Calculator · Garage heater

Garage heater size calculator

Enter the garage size, insulation, garage door and the cold you have to heat against. The calculator adds up heat loss surface by surface, then shows the heat output needed in BTU/h, the same heat as electric kW and the input a gas or propane heater needs at your efficiency.

  1. Enter the inside size, wall height and which walls face outside.
  2. Choose wall and ceiling insulation, then enter the garage door, windows and any entry door.
  3. Enter the winter design temperature for your location and the temperature you want to hold.
  4. Compare the BTU/h line with a heater's rated output, or use the kW line for electric and the input line for gas or propane.

Heating only, one garage, steady losses with the door closed. Walls shared with heated rooms are left out.

Inside dimension.

Inside dimension.

Floor to ceiling. The calculator uses it for wall area and air volume.

Long and short refer to the length and width above. Walls against heated rooms are left out, and the garage door has to fit in the longest outside wall.

Pick heated room above for a garage under a bonus room. That ceiling then adds no heat loss.

Add the widths for two doors in the same wall, for example 9 + 9 = 18. Enter 0 for no garage door.

Enter the tested U-factor if the door is listed in the DASMA program. An insulated door's label R-value will not give you this number, because DASMA says the panel R-value and the tested U-factor of the installed door are not reciprocals.

All windows in exterior walls, glass and frame.

A service door in an exterior wall. A 3 x 6.8 ft door is about 20 sq ft. The door into the house sits in a shared wall, so leave it out.

Use the 99% heating design temperature for your location. Look it up on the HVAC design temperatures page at /hvac-design-temperatures.

A workshop is often held around 55 to 65°F. Keeping pipes and paint from freezing takes much less.

Rated efficiency from the heater's spec sheet. Used only for the gas input line.

Example result for the default inputs

24 x 24 ft garage, 9 ft walls, 3 exterior walls, a long side against the house, held at 55°F with 5°F outside.

Heat output needed

12,434 BTU/h

Electric heater equivalent

3.64 kW

Gas or propane input needed at 80% efficiency

15,543 BTU/h

Calculation details
Floor area576 sq ft
Volume5,184 cu ft
Exterior wall length72 ft
Temperature difference50°F
Walls, net of openings2,038 BTU/h
Garage door2,800 BTU/h
Windows147 BTU/h
Entry door0 BTU/h
Ceiling1,516 BTU/h
Slab edge3,600 BTU/h
Air leakage, 0.50 ACH, 43 CFM2,333 BTU/h

For estimation purposes only. Consult a licensed engineer for final designs.

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How the heat output needed is calculated

Each surface loses heat in proportion to its area, how poorly it insulates and the gap between the garage and outside. Air leakage adds its own share. The total is the heat output needed to hold your target on a design-cold day.

Temperature difference
dT = target garage temperature - winter outdoor design temperatureIf the target is at or below the design temperature, every line below is zero. The calculator never reports a negative heat loss.
Exterior walls
Q = (exterior wall length x wall height - window, door and garage door area) x (1 / R) x dTExterior wall length follows the walls you pick. A long side against the house removes one long side, a corner removes one long and one short side. The R-value you pick is treated as the whole wall, the same way the BuildSolver load engine does it. For a wall with no insulation this calculator assumes R-4.
Garage door
Q = door width x door height x U x dTFor an uninsulated steel door this calculator assumes U 1.2. A bare steel panel is little more than its two air films, which DASMA Technical Data Sheet 163 puts at R 0.85, so its U is close to 1 / 0.85 = 1.18 before joints and perimeter gaps. An insulated door is different. DASMA says the calculated panel R-value and the tested U-factor of the installed door are not reciprocals, so this calculator assumes U 0.5 unless you enter a tested U-factor.
Windows and entry door
Q = area x U x dTWindow U by type is 1.04 single pane, 0.49 double clear, 0.32 double low-e and 0.22 triple. An opaque entry door uses U 0.40. These are BuildSolver engine defaults, used here as assumptions.
Ceiling
Q = floor area x (1 / R) x dTAn open rafter ceiling with no insulation is taken as R-3, this calculator's assumption. With a heated room above, the ceiling line is zero.
Slab edge
Q = F2 x exterior wall length x dTA slab loses most of its heat at the exposed edge, so the loss runs by perimeter, not area. F2 is 1.0 with no edge insulation, 0.7 with R-5 and 0.5 with R-10, BuildSolver engine defaults used here as assumptions.
Air leakage
CFM = ACH x garage volume / 60, Q = 1.08 x CFM x dTAir changes per hour are 0.20 tight, 0.30 average and 0.50 leaky, from the BuildSolver engine. The very leaky choice at 1.0 ACH is this calculator's assumption for a garage with visible gaps at the door. 1.08 is the standard-air sensible heat factor at sea level (60 min/h x 0.075 lb/cu ft x 0.24 BTU/lb-°F, ASHRAE Handbook of Fundamentals).
Electric and gas equivalents
kW = Q / 3,412, gas or propane input needed = Q / efficiencyThe U.S. Energy Information Administration lists 1 kilowatthour as 3,412 Btu, so 1 kW of resistance heat delivers 3,412 BTU/h. A fuel-burning heater only delivers its rated efficiency times its input, so the input needed is larger than the heat output needed.

Worked examples for four garages

Row one is the default on the form. Row two keeps the same garage and strips the insulation, the door insulation and the weatherstripping. In the last two, the size, the walls against the house, the target and the climate change.

Heat output needed, electric kW and gas input needed for four example garages
GarageHeat output neededElectricGas input needed at 80%
24 x 24 attached on a long side, R-13 walls, R-19 ceiling, insulated door, 55°F12,434 BTU/h3.64 kW15,543 BTU/h
Same garage, no insulation, uninsulated door, very leaky31,358 BTU/h9.19 kW39,197 BTU/h
12 x 22 one-car at a corner of the house, R-13 walls, R-30 ceiling, 50°F4,922 BTU/h1.44 kW6,153 BTU/h
30 x 40 detached shop, R-19 walls, R-38 ceiling, -10°F outside, 60°F25,942 BTU/h7.60 kW32,427 BTU/h

Taking the insulation out of the same 24 x 24 ft garage multiplies the heat loss by 2.5. In that bare garage the ceiling is the largest single line at 31% of the total, and the uninsulated garage door is 21%. In the insulated default, the slab edge carries the most, 29% of the loss. Check the line-by-line result for your own garage before deciding what to insulate first.

Standards and data sources

  • U.S. Energy Information Administration, British thermal units (Btu) - the conversion 1 kilowatthour = 3,412 Btu behind the electric kW line.
  • DASMA Technical Data Sheet 163, U-factor and R-value for Residential and Commercial Garage Doors - the air film figure of R 0.85 and the note that steel facings add almost nothing, behind the uninsulated door assumption. DASMA also explains why a tested U-factor beats a calculated section R-value.
  • BuildSolver load engine defaults - window and door U-values, slab edge F2 factors and air change rates. The engine code names ACCA Manual J 8th edition and ASHRAE Fundamentals tables as their origin, and BuildSolver adjusted several values after comparing them against completed Manual J reports. This page uses them as assumptions, not as quoted ACCA figures.
  • Winter outdoor design temperature - entered by you. The 99% heating design temperature for your location is on the BuildSolver HVAC design temperatures page.

Limitations of this calculator

  • This is a simplified heat loss estimate for one space. It is not a Manual J load calculation and not a room-by-room calculation of the house.
  • Heat that moves in from the house through a shared wall is not credited, and neither is heat from a parked car, lights or tools. That keeps the number on the high side.
  • An open garage door lets the warm air out. The estimate covers steady losses with the door closed, so heat at that rate takes a while to recover the space after the door has been open.
  • Nominal R-values are used as the whole wall or ceiling, with no allowance for studs, air films or insulation gaps. Block and uninsulated metal buildings can lose more than the assumptions here.
  • A cathedral or sloped ceiling is taken at the floor area. A steep roof has more surface than that.
  • This page does not pick a heater. The kW and gas input lines are raw numbers to compare with a heater's rated output and input.
  • A gas or propane heater burns fuel inside the garage. Follow the manufacturer's installation instructions and local code for venting, combustion air, clearances and carbon monoxide safety.

Common garage heating mistakes

  1. Reading a gas heater's input as its output. Gas and propane unit heaters are usually listed by input. At 80% efficiency, every 1,000 BTU/h of input puts about 800 BTU/h into the garage. Compare the heat output needed to the heater's output, or divide it by the efficiency as this page does.
  2. Leaving the garage door out of the math. A two-car door is over 100 sq ft of thin panel in one wall, and an uninsulated one loses heat several times faster per square foot than an insulated wall.
  3. Designing a shop for 70°F when 55°F will do. Heat loss scales with the temperature difference. Dropping the target from 70 to 55°F with 5°F outside cuts the design difference from 65 to 50°F, almost a quarter less heat.
  4. Using an average January temperature instead of the design temperature. An average day runs well above the cold nights the heat has to cover. Use the 99% heating design temperature for the location, or the garage will fall behind on the coldest nights.
  5. Ignoring the slab. Most garage slabs are poured without edge insulation. The exposed edge of an uninsulated slab can lose as much heat as an insulated wall of the same length.

Common questions about heating a garage

How much heat does a two-car garage need?

It depends on the insulation, the door and how cold it gets. In the default example on this page, a 24 x 24 ft attached garage with a long side against the house, R-13 walls, an R-19 ceiling and an insulated door, held at 55°F with 5°F outside, needs about 12,400 BTU/h of heat output. That is 3.64 kW of electric heat, or about 15,500 BTU/h of gas input at 80% efficiency.

How do I convert BTU/h to kW for an electric garage heater?

Divide BTU/h by 3,412. The U.S. Energy Information Administration lists 1 kilowatthour as 3,412 Btu, so a 1 kW resistance heater running steadily delivers 3,412 BTU/h.

Why is the gas input number bigger than the heat output needed?

Fuel-burning heaters are rated by input, and only part of that input ends up as heat in the garage. At 80% efficiency the input needed is the heat output needed divided by 0.80.

Does an insulated garage door make a big difference?

It cuts the door's own loss a lot. This calculator uses U 1.2 for an uninsulated door and U 0.5 for an insulated one, so the same door loses less than half the heat once it is insulated. Whether the door is the biggest loss in your garage depends on the walls, ceiling and slab, which the result breaks out line by line. If the door has a tested U-factor from the DASMA program, enter it for a closer number.

What temperature should I use for outside?

Use the 99% winter heating design temperature for your location, not the record low and not a monthly average. The HVAC design temperatures page on BuildSolver lists it by location.

Can I use this for a detached shop or pole barn?

For a framed, insulated building, yes. Pick 4 exterior walls and enter the real wall height. For bare metal siding or an uninsulated roof, the R-4 and R-3 assumptions here are likely optimistic, so treat the result as a floor.

Is this a Manual J calculation?

No. It is a simplified heat loss estimate for one space, using U times area times temperature difference plus air leakage. A room-by-room Manual J for a whole house is a separate BuildSolver Pro and Team feature.

Used on the same job

Run the whole-house load in chat

The chat has no garage or single-space tool, so the garage number comes from this page. Describe the house in chat and the assistant runs the free whole-house Manual J heating and cooling load for the same job.

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