What Size Garage Heater Do I Need? BTU Calculator and Sizing Chart
Most garage heaters get bought on a guess. You grab a 1,500W plug in unit, run it all evening, and the shop never gets warm enough to work in. Sizing a garage heater takes one formula and about two minutes.
Enter your garage size below and the calculator gives you the BTU output, the matching electric wattage, and whether you need 120V, 240V or gas.
Garage Heater BTU Calculator
Size your heater from your garage dimensions, insulation and climate.
Formula: cubic feet x insulation factor x temperature rise. One watt equals about 3.412 BTU per hour. Treat the result as an estimate and size up slightly for drafty doors.
The garage heater sizing formula
Heater size depends on three things: how much air you heat, how many degrees you want to raise it, and how fast the building loses heat. The formula combines all three.
BTU per hour = length x width x ceiling height x temperature rise x insulation factor
Temperature rise is your target temperature minus the coldest outdoor temperature you plan to work in. The insulation factor is 0.133 for an average insulated garage, higher for bare studs and lower for a well sealed shop.
| Insulation level | Factor |
|---|---|
| None, bare studs and an uninsulated door | 0.173 |
| Poor, thin or partial insulation | 0.153 |
| Average, insulated walls | 0.133 |
| Good, insulated walls and ceiling | 0.113 |
| Excellent, everything insulated including the door | 0.093 |
A worked example
Take a 24 by 24 foot two car garage with a 10 foot ceiling. That is 5,760 cubic feet of air.
To go from 15°F outside to 65°F inside, you need a 50 degree rise. With average insulation, 5,760 x 50 x 0.133 gives about 38,300 BTU per hour, or roughly 11,200 watts of electric heat.
Quick sizing chart
These figures assume a 9 foot ceiling, average insulation and a 40 degree rise, such as 25°F outside to 65°F inside.
| Garage | Size | BTU per hour | Electric watts |
|---|---|---|---|
| 1 car | 12 x 22 ft | About 12,600 | About 3,700W |
| 2 car | 24 x 24 ft | About 27,600 | About 8,100W |
| 3 car | 36 x 24 ft | About 41,400 | About 12,100W |
120V, 240V or gas?
A standard 120V plug in heater tops out at 1,500 watts, which is about 5,100 BTU. That is enough for spot heat at a workbench, not for a whole two car garage in winter.
- Under 5,100 BTU: a 120V plug in heater on its own 20 amp outlet.
- 5,100 to 34,000 BTU: a hardwired 240V electric heater, from about 1,500 to 10,000 watts.
- Above 34,000 BTU: a vented natural gas or propane unit heater is usually cheaper to run and easier to wire.
A 240V heater needs a dedicated double pole breaker and wire sized for its amps. A 5,000W heater draws about 21 amps at 240V, which typically means a 30 amp circuit. Hire an electrician if you are not sure.
Safety before you switch it on
Any heater that burns propane, kerosene or natural gas makes carbon monoxide. Fit a carbon monoxide alarm, keep fresh air coming in, and never run an unvented heater with the door shut.
Keep flammable liquids, sawdust and rags well away from any heater. If you also run a generator during outages, check the best portable generators for home backup power guide for safe placement.
Pick your heater
Once you have your number, compare models in our best garage heaters for a cold workshop guide. It lists each heater’s wattage or BTU output, fuel type and coverage.
Working in the cold either way? A heated work jacket keeps you warm while the shop heats up. Cold also drains cordless packs, so read how to store lithium tool batteries in winter.
Frequently Asked Questions
How many BTU do I need to heat a two car garage?
A typical 24 by 24 foot two car garage with a 9 foot ceiling and average insulation needs about 27,600 BTU for a 40 degree rise. Colder climates or poor insulation can push that past 40,000 BTU.
Will a 1,500 watt heater heat a garage?
Only a small, well insulated garage on a mild day. A 1,500W heater gives about 5,100 BTU, so in most garages it works as spot heat near your bench rather than heating the whole space.
How do I convert watts to BTU?
Multiply watts by 3.412 to get BTU per hour. To go the other way, divide BTU by 3.412, so a 34,000 BTU need equals about 10,000 watts.
Is it better to oversize a garage heater?
Go up one size if your door is drafty or you open it often. Going far bigger wastes money and makes the heater cycle on and off more.
Also Read On:
Best Garage Heaters for a Cold Workshop (2026 Picks)
