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Garage and Workshop Heater Sizing Calculator

Find the heater output that holds your garage or shop at the temperature you want on a design-cold day, with each part of the heat loss shown.

Method reviewed October 26, 2026. Version 1.0.0. How it works

This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.

Your inputs

Unit system

A workshop is often kept cooler than a house. Each degree lower reduces the heater size needed.

Your own figure for the whole wall. It is lower than the insulation label because studs bridge the insulation; uninsulated walls are far lower still.

Width × height of every overhead door. 112 ft² is one 16 × 7 ft door — enter yours.

From the door maker’s literature. Uninsulated single-skin doors are much lower than insulated ones, and the label describes the panel, not the gaps around it.

From the NFRC label on the window or door.

Heat loss per foot of exposed slab edge per °F. 0.58 is ENERGY STAR’s example for R-5 edge insulation 2 ft deep; an uninsulated slab edge loses more.

Garages are usually leakier than houses, mainly around the overhead door. 1.5 is the leaky-house example in the WSU heat-loss factsheet; use a blower-door result if you have one.

Result

Heater output needed
17,042BTU/h
Electric heater size
4.99kW

Holding 55 °F at 0 °F outside needs about 17,042 BTU/h of heat output (5 kW for an electric heater).

  • For a fuel-fired heater, this is the output it must deliver; its input rating is higher (output ÷ efficiency). Unvented combustion heaters are not suitable for enclosed garages and workshops — follow the heater maker’s venting and clearance instructions.
  • Air leakage is the largest part of the loss. Weatherstripping the overhead door is often cheaper than a bigger heater.
Electric heater current at 240 V
20.8A
Heat loss per square foot of floor
29.6BTU/h·ft²

Heat loss breakdown

Walls
3,660BTU/h
Garage door
1,027BTU/h
Windows and entry doors
539BTU/h
Ceiling
1,056BTU/h
Slab edge
3,062BTU/h
Air leakage
7,698BTU/h
Show the working
  1. Design temperature difference
    55 − (0) = 55 °F
  2. Exterior wall length and net wall area
    96 ft × 9 ft − 132 ft² openings = 732 ft²
  3. Walls = net area ÷ R × ΔT
    732 ÷ 11 × 55 = 3,660 BTU/h
  4. Garage door = area ÷ R × ΔT
    112 ÷ 6 × 55 = 1,027 BTU/h
  5. Windows and entry doors = U × area × ΔT
    0.49 × 20 × 55 = 539 BTU/h
  6. Ceiling = floor area ÷ R × ΔT
    1,056 BTU/h
  7. Slab edge = F × exposed perimeter × ΔT
    0.58 × 96 × 55 = 3,062 BTU/h
  8. Air leakage = 0.018 × ACH × volume × ΔT
    0.018 × 1.5 × 5,184 × 55 = 7,698 BTU/h

What the estimate includes

The calculator adds up the steady heat loss at your outdoor design temperature:

  • Walls, garage door, windows and ceiling: area ÷ R-value × temperature difference (or U-factor × area × ΔT)
  • Slab edge: F-factor × exposed perimeter × ΔT
  • Air leakage: 0.018 × air changes per hour × volume × ΔT

An attached garage loses nothing through the wall it shares with the heated house, and a heated room above means no ceiling loss. Climate presets use one representative station per climate zone; choose Custom to enter your own design temperature.

Why garages need more heat than their size suggests

Garage doors are large, often poorly insulated and leaky around the edges, and walls and ceilings are frequently uninsulated. Air leakage is usually the largest part of the loss — weatherstripping the overhead door can cut the heater size more cheaply than buying a bigger heater.

Concrete floors and parked vehicles also soak up heat when you warm the space from cold, so allow time to warm up, or choose a somewhat larger heater if you heat only when you are working.

Choosing a heater safely

For an electric heater, the kW result is the size to look for; hard-wired heaters need a dedicated circuit installed by a licensed electrician. For a gas or propane heater, the result is the output it must deliver — its input rating is higher. Use only heaters listed for garage use and vented as the maker specifies; unvented combustion heaters are not suitable for enclosed garages and workshops.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Length
24 ft
Width
24 ft
Wall height
9 ft
Garage type
Detached (all four walls outdoors)
Climate
Zone 5 — cool (e.g. Chicago)
Temperature to hold inside
55 °F
Wall R-value (whole wall)
11 h·ft²·°F/BTU
Ceiling
Ceiling below a vented attic or roof
Ceiling or roof R-value
30 h·ft²·°F/BTU
Garage door area (total)
112 ft²
Garage door R-value
6 h·ft²·°F/BTU
Window and entry-door area
20 ft²
Window and entry-door U-factor
0.49 BTU/h·ft²·°F
Slab edge F-factor
0.58 BTU/h·ft·°F
Air changes per hour
1.5

Result

Heater output needed
17,042 BTU/h
Electric heater size
4.99 kW
Electric heater current at 240 V
20.8 A
Heat loss per square foot of floor
29.6 BTU/h·ft²

Holding 55 °F at 0 °F outside needs about 17,042 BTU/h of heat output (5 kW for an electric heater).

Assumptions

  • Steady-state heat loss at the design temperature: conduction Q = A × ΔT ÷ R (or U × A × ΔT) for each surface, slab edge Q = F × exposed perimeter × ΔT, air leakage Q = 0.018 × ACH × volume × ΔT.
  • An attached garage loses no heat through the one width wall it shares with the heated house; a heated room above means no ceiling loss.
  • Climate presets use the 99 % heating design temperature of one representative station per climate zone (NOAA Engineering Weather Data).

Limitations

  • Opening the overhead door dumps the heated air; the heater then needs time to recover, which a steady-state estimate does not cover.
  • Concrete floors and stored vehicles absorb heat when the space is warmed from cold, so warm-up from an unheated state takes longer than the steady output suggests.
  • R-values and leakage are your inputs; the result is only as good as they are.

Frequently asked questions

How many BTU do I need for a two-car garage?

It depends far more on insulation, the door and air leakage than on size. With this page’s example inputs — an insulated, detached 24 × 24 ft garage held at 55 °F with 0 °F outside — the result is about 17,000 BTU/h; poorer insulation or a leakier door raises it quickly. Enter your own details.

What temperature should I heat a workshop to?

Whatever you are comfortable working at. Each degree lower reduces the heater size and running cost. Some finishes and glues need a minimum temperature — check their labels.

Can I use the electric kW figure for a heat pump?

Use the BTU/h figure and compare it with the heat pump’s capacity at your design temperature, which is lower than its rated capacity in mild weather.

Sources

  1. Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
  2. Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
  3. F-Factor Additional Guidance (SFNH Item 3.2 / MFNC Item 3.4), revised 2023-12-01 — ENERGY STAR (U.S. EPA). The F-factor is the perimeter heat-loss factor for slab-on-grade floors, expressed per unit length of slab perimeter (similar to a U-factor per unit area); R-5 edge insulation to a depth of 2 ft on an unheated slab gives an F-factor of 0.58.
  4. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.

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