What size garage heater do I need?
Put in your dimensions and your electricity rate. You get BTU, watts, cost per hour and cost per winter, with the arithmetic shown.
Heads up: the buy links on this page are affiliate links. If you buy through one, we earn a commission at no extra cost to you. It does not change what we recommend or the order we rank things in - here is how we keep those separate.
Heater listings answer this question with a square-footage claim, which is not a specification and is not derived from one. Square feet says nothing about ceiling height, nothing about how cold it gets where you live, nothing about what temperature you want, and nothing at all about how well the space closes up — which is the term that matters most.
So here is the calculation, with every input exposed.
Work out your size
Everything below runs in your browser. Nothing is sent anywhere and nothing is stored.
Insulated walls, uninsulated door, no obvious daylight around the opening. The common case.
You need roughly
22100 BTU/hr
which is about 6500 watts of electric heat.
This needs a 240 V circuit on its own breaker. That is an electrician's visit, and it is the real cost of heating this space.
- Cost per hour at full output
- $1.19
- Cost per 120-day winter at 2 h/day
- $285.36
- Volume being heated
- 4,752 cu ft
The sum, so you can check it
4,752 cu ft × 35 F rise × 0.133 × 1 (air-change factor) = 22,121 BTU/hr. Divided by 3.412 that is 6,483 W, or 6.48 kW. At 18.34 cents per kWh that is $1.19 for every hour the heater runs at full output. A thermostat cycling at roughly half duty halves it.
The air-change factor is doing most of the work in that sum. Moving from a sealed door to a gappy one multiplies the answer by five. That is not a rounding error, and it is why the first thing to buy is a garage door seal, not a heater.
What to buy, given all that
Dr. Infrared Heater
Dr. Infrared Heater DR-988A Garage Shop Heater
Roughly $180-$280 for the heater alone
A 240-volt electric shop heater, and the reason it is here is the plug: it needs a dedicated 30-amp 240-volt circuit with a NEMA 6-30R outlet. That is an electrician's visit, and nobody selling this heater leads with it.
Skip it if: You do not have a 240 V circuit and are not prepared to pay to have one run. Then this is a heater you cannot plug in.
#ad We hold no verified live price for this product, so we show none. How this site is funded.
M-D Building Products
M-D Building Products Garage Door Bottom Seal
Roughly $20-$40 for a door width
The cheapest fix on this entire site, and the one with the biggest effect. A worn bottom seal leaves a gap the width of the door open to the outside all winter, and no amount of heating overcomes an open gap.
Skip it if: Your existing seal is intact and your door still closes flush. Replacing a good seal changes nothing.
#ad We hold no verified live price for this product, so we show none. How this site is funded.
The formula, and where it comes from
This is the standard temperature-rise method used in published electric heater sizing tables:
Heat requirement
BTU/hr = volume (cu ft) × temperature rise (F) × 0.133 × air-change factor
0.133 is the constant for air — density multiplied by specific heat, expressed per degree Fahrenheit per cubic foot per hour. It does not vary.
To convert to electric watts: watts = BTU/hr ÷ 3.412. To get cost: cost per hour = watts ÷ 1000 × rate per kWh.
The air-change factor is doing most of the work
Everything else in that formula is a measurement. The air-change factor is a judgment, and it swings the answer by a factor of five between the best and worst cases. It is worth being honest with yourself about which row you are in.
| Factor | Describes | What you would see |
|---|---|---|
| 0.5 | Insulated walls and ceiling, sealed door | A finished garage. Door closes flush, bottom seal intact, no daylight anywhere. |
| 1.0 | Some insulation, seal in reasonable shape | Insulated walls, uninsulated door panel, no visible gaps. The common case. |
| 1.5 | Uninsulated but properly closed up | Bare studs or block, no insulation, but the door and jambs actually seal. |
| 2.5 | Gaps around the door | Daylight visible under or beside the closed door. Fix this before buying a heater. |
If you land on 2.5, the calculator will tell you that you need an amount of heat that no domestic circuit can supply. That is the correct answer, and the fix is not a bigger heater. It is a $30 door seal, which moves you to 1.0 or 1.5 and roughly quarters the requirement.
Common cases, worked
These are the standard sizes people search for, all at a 35 F temperature rise (20 F outside to 55 F inside), a 9-foot ceiling and an air-change factor of 1.0 — some insulation, door sealing properly.
| Garage | Volume | BTU/hr needed | Watts | Cost/hour at 18.34c |
|---|---|---|---|---|
| Single, 12 x 22 ft | 2,376 cu ft | About 11,100 | About 3,250 W | About $0.60 |
| Two-car, 20 x 20 ft | 3,600 cu ft | About 16,800 | About 4,900 W | About $0.90 |
| Two-car, 24 x 22 ft | 4,752 cu ft | About 22,100 | About 6,500 W | About $1.19 |
| Three-car, 30 x 24 ft | 6,480 cu ft | About 30,200 | About 8,850 W | About $1.62 |
Notice that every one of those exceeds the roughly 1,500 W a standard 120-volt outlet can supply. That is the real finding in this category and it is why the electrical question comes before the shopping. The heaters that can actually deliver these numbers are here.
Two ways to make the answer smaller
Lower your target temperature. Heating a garage to 55 F because that is comfortable to work in is a different project from heating it to 40 F to keep tools and stored liquids above freezing. Dropping the target from 55 to 45 cuts the requirement by nearly 30 percent, because it is the rise that drives the formula.
Improve the air-change factor. Cheaper, more permanent, and it improves every hour you ever heat the space rather than just this one.
How this was ranked
Every site in this category says it tested twenty of these. We have not tested any, and we say so. What we did instead: built the candidate set from what Amazon actually stocks, compiled the published specifications from each manufacturer’s own pages, did the arithmetic on running cost and on what the thing prevents, and read owner reviews for failure patterns rather than star averages. Every number on this page has a link to where it was published, so you can check the work.
- Units we claim to have tested
- 0
- Sponsored placements
- 0
- Free products accepted
- 0
Frequently asked
What size heater do I need for a 2-car garage?
For a 24 × 22 ft garage with a 9 ft ceiling, raised from 20 F to 55 F, with some insulation and a sealing door: about 22,100 BTU/hr, or roughly 6,500 watts. That requires a 240-volt circuit. Lowering the target temperature or improving the seal both reduce it substantially.
How many BTU per square foot for a garage?
BTU per square foot is the wrong unit, and it is why the listings that use it are unreliable. Heat requirement scales with volume, not floor area, so a 9-foot ceiling needs 50 percent more than a 6-foot one over the same floor. It also scales with the temperature rise you want and with how leaky the space is. Use the volume formula.
Can I run a garage heater on a normal outlet?
Only up to about 1,500 W, which a standard 120-volt 15-amp circuit can carry. That is roughly 5,100 BTU/hr — enough for a small sealed insulated garage, not enough for a two-car space. Anything larger needs a dedicated 240-volt circuit, and that is an electrician’s job.
Does insulating the garage door reduce the heater size I need?
Some, but far less than sealing it does. Insulation improves the R-value of the panel and reduces conduction through it. Sealing closes an open aperture and reduces the air exchange, which is the dominant term in the formula. Seal first, then insulate if you still want to.
Sources
- EIA, Electric Power Monthly, Table 5.3 - average retail price of electricity (June 2026: 18.34 cents per kWh residential)
- Dr. Infrared Heater DR-988A - published specifications
- Formula: standard temperature-rise sizing method (volume x rise x 0.133 x air-change factor), as used in published electric heater sizing tables