How garage heater sizing works
Garage heat loss comes from three main sources: walls and ceiling (conduction through surfaces),
garage doors (the biggest single loss factor in most garages), and air infiltration
(cold air leaking through gaps). This calculator estimates each separately and sums them using the formula:
BTU/hr = Surface Area × U-value × ΔT — where U-value is the inverse of R-value (U = 1/R) and
ΔT is the temperature difference between your target indoor temp and the design outdoor temperature for your
climate zone.
Design temperatures by climate zone
| IECC Zone |
Example Cities |
Design Outdoor Temp |
ΔT (to 60°F) |
| Zone 2 | Houston, Phoenix, Miami | 25°F | 35°F |
| Zone 3 | Atlanta, Dallas, Los Angeles | 15°F | 45°F |
| Zone 4 | NYC, DC, Seattle, Memphis | 5°F | 55°F |
| Zone 5 | Chicago, Boston, Denver | −5°F | 65°F |
| Zone 6 | Minneapolis, Burlington | −15°F | 75°F |
| Zone 7 | Duluth, Fairbanks | −25°F | 85°F |
Insulation R-values used in this calculator
| Surface | Uninsulated | Partially Insulated | Well Insulated |
|---|
| Walls | R-3 | R-11 | R-19 |
| Ceiling | R-2 | R-11 | R-38 |
| Floor (slab — fixed) | R-2 (assumed, heat loss via perimeter) |
Electric vs gas — when to choose each
Electric heaters (240V unit heaters, infrared tube heaters, baseboard) are the easiest to
install and need no venting. Running cost is higher per BTU, but for occasional use — a few hours a week — the
simplicity and lower installation cost often wins. A 240V/30A circuit supports up to about 7,200 watts (≈ 24,600
BTU), enough for a 1-car garage in Zone 5.
Gas and propane unit heaters output 30,000–125,000 BTU and cost significantly less to run in
cold climates. They require proper venting (horizontal through a wall or vertical flue) and a gas or propane
supply. For a 2-car or 3-car garage in Zone 5–7, a 45,000–60,000 BTU unit heater is the most common choice.
Garage heater wattage — BTU to watts conversion
Electric garage heaters are rated in watts or kilowatts (kW). Gas and propane heaters are rated in BTU.
To compare or convert: watts = BTU ÷ 3.412, or roughly
1 kW = 3,412 BTU/hr.
The table below shows common garage sizes, the BTU needed in Zone 5 with average insulation,
and the equivalent wattage — including the 240V circuit you’ll need.
| Garage size |
BTU needed
(Zone 5, avg. insulation) |
Equivalent watts |
240V circuit required |
Electric heater option |
| 1-car (200–240 sq ft) |
18,000–22,000 BTU |
5,300–6,400 W |
240V / 30A |
240V electric heater |
| 1-car large (240–300 sq ft) |
22,000–28,000 BTU |
6,400–8,200 W |
240V / 40A |
240V electric heater |
| 2-car (440–528 sq ft) |
30,000–45,000 BTU |
8,800–13,200 W |
240V / 50–60A or gas |
Gas/propane heater recommended |
| 3-car (600–750 sq ft) |
45,000–65,000 BTU |
13,200–19,000 W |
Gas/propane strongly recommended |
Gas/propane unit heater |
Why gas wins for large garages: A 45,000 BTU gas heater draws about 1–2 amps of electricity
(for ignition and controls only), while delivering the same heat as a 13,000-watt electric heater
requiring a 240V/60A circuit. The electrical upgrade alone often costs more than the gas heater itself.
Quick BTU ↔ watts conversion reference
| BTU/hr |
Kilowatts (kW) |
Watts |
Typical 240V circuit |
| 5,000 BTU | 1.5 kW | 1,465 W | 120V / 15A |
| 10,000 BTU | 2.9 kW | 2,931 W | 240V / 15A |
| 15,000 BTU | 4.4 kW | 4,396 W | 240V / 20A |
| 20,000 BTU | 5.9 kW | 5,862 W | 240V / 30A |
| 25,000 BTU | 7.3 kW | 7,327 W | 240V / 40A |
| 30,000 BTU | 8.8 kW | 8,793 W | 240V / 40–50A |
| 40,000 BTU | 11.7 kW | 11,723 W | 240V / 60A — consider gas |
| 50,000 BTU | 14.7 kW | 14,654 W | Gas/propane recommended |
| 60,000 BTU | 17.6 kW | 17,585 W | Gas/propane recommended |
Circuit sizes assume the 80% NEC continuous-load rule (e.g., a 30A breaker is derated to 24A usable).
Always have a licensed electrician install and size any 240V circuit for a garage heater.
Garage heater types compared
| Type |
Typical BTU Range |
Installation |
Running Cost |
Best For |
| Electric unit heater (240V) |
5,000–34,000 BTU
(1.5–10 kW) |
Electrician needed for 240V circuit |
Higher per BTU |
1-car garages, occasional use |
| Gas / propane unit heater |
30,000–125,000 BTU |
Gas line + venting required |
Lower per BTU |
2–3 car garages, daily use, cold climates |
| Infrared radiant (electric) |
5,000–17,000 BTU
(1.5–5 kW) |
Easy — plug-in or 240V |
Higher per BTU |
Spot heating, workshops, spot tasks |
| Infrared radiant (gas) |
30,000–80,000 BTU |
Gas line + ceiling mount + vent |
Lower per BTU |
Large garages, commercial shops |
| Mini split (heat pump) |
9,000–36,000 BTU |
Professional install required |
Lowest (COP 2–4) |
Mild climates, dual heat/cool need |
Note: Mini splits lose efficiency in very cold temperatures (below 20°F). For Zone 6–7 garages, a gas unit heater
or high-output electric heater is typically the more reliable primary heat source.
Use our mini split calculator if a heat pump is your preferred option.
Frequently asked questions
How many BTU do I need to heat a 2-car garage?
A standard 2-car garage (roughly 440–528 sq ft, 8 ft ceiling) in a cold climate (Zone 5) with partial insulation
and one uninsulated double-wide door typically needs 30,000–45,000 BTU. If the garage is
uninsulated and you're in Zone 6, that figure can rise to 55,000–65,000 BTU. Use the calculator above with your
exact dimensions.
How many BTU for a 1-car garage?
A 1-car garage (roughly 200–250 sq ft) in Zone 5 with partial insulation typically needs
18,000–25,000 BTU. A well-insulated 1-car garage in Zone 4 may only need 12,000–15,000 BTU,
which a 240V electric unit heater can cover easily.
What size garage heater do I need for a 3-car garage?
A 3-car garage (600–900 sq ft) generally requires 45,000–75,000 BTU depending
on insulation quality, ceiling height, number of garage doors, and climate zone. Very cold climates or
uninsulated garages may need 80,000–100,000 BTU. Use the calculator above
with your exact dimensions.
Should I choose electric or gas for a garage heater?
Electric heaters are easier to install, require no venting, and are ideal for smaller garages or occasional use.
Electric unit heaters
work on a dedicated 240V circuit. Gas and propane unit heaters cost less to run in cold climates and are better
for large garages or frequent heating. The calculator recommends both options so you can compare.
Gas/propane heaters
are also available.
Do garage doors significantly affect BTU requirements?
Yes. Garage doors are typically the largest source of heat loss in a garage. An uninsulated metal garage door
has an R-value of roughly 2–4, while an insulated door can reach R-16 or higher. This calculator accounts
for door count and insulation level when estimating total heat loss. Upgrading from an uninsulated to an insulated
door is often the single highest-ROI improvement before buying a larger heater.
How many watts is a 30,000 BTU garage heater?
30,000 BTU is approximately 8,790 watts (since 1 watt = 3.412 BTU/hr). For a continuous electric heater, that
requires roughly a 240V/40A circuit. This is why gas or propane unit heaters are popular for larger garages —
they avoid the need for heavy-duty electrical upgrades while delivering the same heat output.
How many watts does a garage heater need?
Convert BTU to watts using this formula: watts = BTU ÷ 3.412. So a 30,000 BTU requirement
equals approximately 8,790 watts (≈ 9 kW). That's beyond what a standard 120V circuit can supply — it requires
a 240V/40A circuit. This is why gas or propane heaters are popular for larger garages: a 45,000 BTU gas unit
heater draws only about 1–2 amps of electricity (for ignition and controls) while delivering the same heat as a
13,000-watt electric heater.
What size propane heater for a garage?
Propane unit heaters are commonly sold in 30,000, 45,000, 60,000, and 80,000 BTU models. For a 2-car garage in
Zone 5–6 with average insulation, a 45,000–60,000 BTU propane heater is the most common fit.
Propane tanks for garage use are typically 100–500 lb cylinders (20–100 lb if portable).
Does ceiling height change the BTU requirement?
Yes — significantly. A 10 ft ceiling has 25% more volume than an 8 ft ceiling for the same footprint, and more
ceiling area for heat loss. The calculator above adjusts for ceiling height automatically. If you have a vaulted
garage ceiling over 12 ft, add an additional 15–20% buffer to the result.
Can I use a mini split to heat a garage?
A mini split works well in mild climates (Zone 2–4) where it rarely drops below 10–15°F. In Zone 5–7, a
cold-climate mini split ("hyper heat" rated to −13°F or lower) can handle garage heating, but costs more upfront
than a unit heater. See our mini split calculator for sizing.
How many watts do I need to heat a 300–400 sq ft garage?
A 300 sq ft garage in Zone 5–6 typically needs 15,000–20,000 BTU (≈ 4,400–5,900 watts).
A standard 240V/20A circuit (4,800W) covers most 1-car garages in moderate climates. For 400 sq ft in Zone 6,
plan for 20,000–28,000 BTU (5,900–8,200 watts) — a 240V/30A or 40A circuit. Use the calculator above with
your exact dimensions for a precise figure.