How to Size a Garage Heater Properly
When it comes to garage heaters, “just guess” isn’t a sizing method.
Choosing the wrong heater — too small or too big — means wasted energy, uneven heat, or even early equipment failure.
In this guide, we’ll show you how to size a garage heater properly for Calgary’s freezing winters, including BTU calculations, insulation factors, and when to consider gas vs. electric models.
Get it right the first time — your tools, your car, and your toes will thank you.
Why Size Matters
Here's the thing about garage heaters: too small and you're still freezing, too big and you're burning money.
Oversized heaters short-cycle (on/off like a nervous tic), undersized ones run nonstop and never quite get the job done. Either way, you lose. The secret? Finding that sweet spot where your garage is comfortable without wasting energy.
Let's break down how to size a garage heater properly for Calgary homes.
The Golden Rule of Thumb
For garages in cold climates like Calgary:
- 50–60 BTU per square foot for uninsulated garages.
- 30–45 BTU per square foot for insulated garages.
So:
- 400 sq ft (single-car): ~30,000 BTU
- 600–700 sq ft (two-car): ~45,000 BTU
- 900–1,000 sq ft (three-car): ~60,000 BTU+
Start With Square Footage
Step one is simple: measure your garage floor.
- Width × length = total sq ft.
- Example: 20 ft × 20 ft = 400 sq ft.
Easy math, but it's just the start.
Don't Forget Ceiling Height
Most sizing charts assume an 8 ft ceiling. If you've got higher ceilings, adjust upward:
8 ft ceiling (baseline)
normal calculation.
10 ft ceiling
add ~20% BTU.
12 ft ceiling
add ~30%.
Why? Heat rises. More air volume = more BTUs needed to warm it.
Insulation — The Game Changer
Insulation is the single biggest swing factor in garage heater sizing.
Uninsulated garage
Leaks heat like a screen door in January. Size on the higher end (50–60 BTU per sq ft).
Partially insulated
Walls insulated, ceiling not? Split the difference.
Fully insulated
Size on the lower end (30–40 BTU per sq ft).
Pro tip: Insulating first may save you hundreds on heater size and thousands in gas/electric bills.
Garage Use Profile & Heater Types
Not every garage is used the same way:
- Daily workshop or gym: Size higher to recover heat faster when doors open.
- Car storage only: Size lower you just need to keep the chill off.
- Occasional weekend project space: Middle ground is fine.
Gas vs Electric Sizing
- Gas heaters (30k, 45k, 60k BTU): Better for larger garages or frequent use.
- Electric heaters (5kW–10kW ≈ 17,000–34,000 BTU): Work well for single-car garages or light use, but struggle in big spaces.
Rule of thumb:
- Single-car insulated → electric can do it.
- Double/triple-car → gas is usually the only practical choice.
Sizing Examples (Calgary Homes)
30K
400 sq ft, insulated, 8 ft ceiling
30,000 BTU gas or 5kW electric.
45K
600 sq ft, insulated, 10 ft ceiling
45,000 BTU gas.
75K
900 sq ft, uninsulated, 12 ft ceiling
75,000 BTU gas.
45K
450 sq ft, uninsulated, 8 ft ceiling
45,000 BTU gas (30k would underperform).
The Door Factor & Heater Types
Garage doors = giant moving walls of heat loss.
- Insulated overhead door: Can cut required BTUs by 10–15%.
- Uninsulated thin steel door: Adds load.
- Frequent openings (kids, projects, multiple cars): Oversize slightly for quick recovery.
Infrared vs Forced Air
- Forced air heaters: Heat the whole garage volume. Great for all-around comfort.
- Infrared heaters: Radiant heat warms objects/people directly. Great for workshops.
Sizing is similar, but infrared feels warmer at lower BTU levels since it doesn't waste as much on unused air volume.
Calgary-Specific Considerations & When to Call a Pro
- -30°C cold snaps: Always size for the worst-case week of the year, not the mild chinook days.
- Wild swings: Gas heaters recover faster when temps swing 20°C in 24 hours.
- Building code: City of Calgary requires properly vented gas heaters and clearance from combustibles.
DIY Safe Checks
- Measure garage sq ft and ceiling height.
- Check insulation levels.
- Decide usage profile (storage vs workshop).
- Compare gas vs electric availability.
Stop there. Leave load calculations, gas line sizing, and venting to the pros.
When to Call a Pro
- If your garage is unusually shaped (L-shaped, vaulted ceilings).
- If you want to run multiple heaters/zones.
- If you're unsure about insulation or air leaks.
- If you're leaning toward gas (always pro-installed).
CONCLUSION
Sizing a garage heater isn't rocket science — but it does take more than a guess. Measure your space, account for ceiling height and insulation, and pick the right BTU range.Get it right and you'll have a garage that's warm, efficient, and ready for projects yearround. Get it wrong and you'll either be sweating (in January!) or freezing while your heater runs its heart out.In Calgary, the magic numbers are usually 30k, 45k, or 60k BTU. Find your sweet spot, and let a pro handle the install.
About HVACHELP.pro
At HVACHELP.pro, our mission is to empower Calgary homeowners with the knowledge and tools to achieve optimal comfort and efficiency, especially when it comes to heating their garages. We understand the unique challenges of Alberta winters and provide tailored insights to ensure your space stays warm without unnecessary energy waste. From detailed garage heater sizing guides to comprehensive installation best practices and ongoing maintenance tips, we're dedicated to helping you heat smarter, save money, and extend the lifespan of your heating system.
Explore More Resources
Visit HVACHELP.pro for expert heating guides, cost breakdowns, and real-world advice built specifically for the harsh demands of Alberta winters. Our resources cover everything from BTU calculations and insulation impact to selecting the right heater type for your specific needs, ensuring you make informed decisions for a comfortable and efficient garage environment.