What BTUs Mean and Why Sizing Your Garage Heater Matters
BTU (British Thermal Unit) measures heat output per hour. For a garage or workshop, picking the right BTUs keeps the space warm without wasting energy or stressing equipment. Undersize it and the heater runs nonstop yet you still shiver. Oversize it and you’ll short-cycle (quick on/off), burn extra fuel, and wear parts early. In garages, heat loss is higher than inside the home because of big doors, concrete floors, and thinner insulation. The good news: a simple calculator approach gets you very close. We’ll measure your room, estimate insulation, choose a realistic temperature rise, and run a quick formula. Along the way, we’ll point to heater types that fit real garages—like unit heater options, ductless mini-split heat pumps, or through-the-wall heat pumps—so you can match the BTUs to gear you can actually buy and install.
Start with Room Size: Calculate Cubic Feet
Grab a tape measure. You need cubic feet, not just square footage, because ceiling height matters.
Formula:
Cubic Feet = Length × Width × Height
Example: a 20 ft × 22 ft garage with an 8 ft ceiling: 20 × 22 × 8 = 3,520 cu ft. Add in a small storage nook? Measure and include it. If your ceiling is sloped, estimate an average height. Jot the number down; we’ll use it shortly. If you’d like more context beyond garages, The Furnace Outlet’s general HVAC Sizing Guide explains why volume and heat loss drive system size. Tip: if you plan to insulate or seal doors later, size for today’s build and note you can lower BTU demand once improvements are done. It’s better to fit the current heat loss than gamble on “future upgrades” that might slip.
Judge Insulation Honestly (It Changes BTUs a Lot)
Insulation quality is the biggest swing factor after size. Use these Insulation Factors for our calculator:
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No insulation (very poor): ~0.17
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Minimal (poor): ~0.15
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Standard (moderate): ~0.13
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Well-insulated (good): ~0.11
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Fully insulated (excellent): ~0.09
Be honest about the garage door—uninsulated metal panels leak heat fast. If walls have batts but the ceiling is bare, treat the space as moderate to poor. Weatherstripping the door and sealing obvious gaps pays back immediately by cutting required BTUs. If your long-term plan is to add insulation, consider installing a modulating heat source like a ductless mini-split: it can throttle down later instead of short-cycling.
Pick a Realistic Temperature Rise (ΔT)
ΔT is how much warmer you want the garage than outdoors.
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Mild climates: 30–40°F rise is typical.
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Moderate: 40–45°F rise.
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Cold: 50–70°F rise (depends on comfort goals and task time).
Example: It’s 25°F outside and you want 60°F inside to work comfortably. ΔT = 35°F. If you’re in a colder region and plan evening work when temps plunge, set ΔT higher—better to plan for the actual coldest times you’ll use the space. For rough checks, many pros also reference BTU per square foot: ~30–40 (warm), ~40–45 (moderate), up to 60 BTU/sq ft (cold). We’ll still do the cubic-feet method because it reacts better to ceiling height and insulation differences.
Run the Calculator: The Core BTU Formula
Here’s the simple sizing formula that ties it all together:
BTUs Needed = Cubic Feet × Insulation Factor × Temperature Rise (°F)
Example A (1,000 sq ft shop): 1,000 sq ft × 8 ft = 8,000 cu ft.
Moderate insulation (~0.133) and a 40°F rise:
8,000 × 0.133 × 40 = 42,560 BTU/h.
Example B (typical 2-car, 4,608 cu ft):
Moderate insulation (0.133), 70°F rise for cold nights:
4,608 × 0.133 × 70 ≈ 42,900 BTU/h.
That’s your target output. If choosing electric, remember 1 kW ≈ 3,412 BTU/h. So ~43,000 BTU/h is about 12.6 kW. If you’d like a sanity check or a second pass, you can message The Furnace Outlet team via Help Center or request a Quote by Photo.
Quick Size Ranges by Garage Volume (Reality Check)
These ranges help confirm your math. Adjust upward for poor insulation or larger ΔT; downward for excellent insulation or smaller ΔT.
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5,000–10,000 cu ft: ~15,000–25,000 BTU/h
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10,000–15,000 cu ft: ~25,000–30,000 BTU/h
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15,000–20,000 cu ft: ~30,000–40,000 BTU/h
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> 20,000 cu ft: 40,000+ BTU/h
Compare this to your formula result. If you’re far outside the band, recheck inputs—especially ceiling height and insulation. When in doubt, pick equipment with stages or variable output so it can adapt. For many garages that double as hobby shops, a right-sized unit heater or a cold-climate mini-split heat pump hits the sweet spot.
Match BTUs to the Right Heater Type
Unit heaters (gas or electric): Great for quick warm-ups and simple installs. Gas models need venting and combustion air; electric models need adequate amperage. Browse practical unit heater options.
Ductless mini-split heat pumps: Efficient, quiet, and provide cooling in summer. Choose a model rated for low-ambient heating if winters bite. See ductless mini-split systems or ceiling cassette and concealed-duct styles.
Through-the-wall & PTAC heat pumps: Compact, self-contained, and popular for single rooms/garages. Check through-the-wall heat pumps and PTAC heat pump units.
If you also want summer cooling, heat pumps (mini-split, TTW, PTAC) give you two seasons of value. For parts like line sets and wall sleeves, see line sets and accessories.
Fuel & Power: Gas vs Electric vs Heat Pump (Costs & Practicalities)
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Natural gas/propane unit heaters: Strong BTU output and fast recovery. Require safe venting, combustion air, and clearances. Good when you already have gas service and want quick warm-ups.
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Electric unit heaters: Simple install; no venting. Check panel capacity—bigger units can demand 10–60A+ circuits. Operating cost depends on electric rates.
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Heat pumps (mini-split, TTW, PTAC): Very efficient in mild/moderate cold. In deeper cold snaps, capacity drops; pick models with low-ambient ratings and defrost strategies.
Rule of thumb for cost comparisons: translate everything to BTU/h (or kW) and check local rates. If upfront cost is a barrier, The Furnace Outlet offers HVAC financing. Still unsure? Share a photo and details through Quote by Photo to get options that fit your service panel, gas line, and space.
Installation Basics: Venting, Airflow, and Controls
For gas unit heaters, follow manufacturer venting specs—length, elbows, termination clearances—and add a CO detector in the garage and the adjacent interior space. Keep combustibles clear of the heater and respect mounting heights above vehicles and workbenches. For mini-splits, plan the line-set route, condensate drain, and outdoor unit placement where snow, debris, and cars won’t block airflow. A simple programmable or smart control helps avoid overheating the space when you’re not working. Need a second set of eyes on layout? The Furnace Outlet’s Design Center can review load assumptions, equipment choices, and accessory lists before you buy, so you don’t miss a vent kit, sleeve, or bracket.
Common Sizing Mistakes to Avoid (Learn from the Field)
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Guessing square footage only: Tall ceilings change everything—always calculate cubic feet.
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Ignoring the garage door: Thin, leaky doors can add thousands of BTU/h. Weatherstrip or insulate.
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Over-optimistic ΔT: If you’ll work during the coldest hours, size for that reality.
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Oversizing “just in case”: Leads to short-cycling and noise. Prefer staged/variable equipment when uncertain.
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Skipping safety/clearances: For gas units, venting and CO protection aren’t optional.
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Buying parts piecemeal: Plan the full bill of materials—mounting, line sets, disconnects, sleeves, and accessories.
If you’d like a quick review of your numbers before purchasing, reach out via the Help Center or explore product-specific categories like unit heaters and ductless mini-splits.
