Choosing the right heater for your garage is essential for staying warm without wasting energy or money. The key factor in selecting the right heater size is BTUs (British Thermal Units), which measure the amount of heat output required to warm a given space efficiently.
If your heater is too small, it won’t keep your garage warm. If it’s too large, you’ll waste energy and risk overheating. So, how many BTUs do you actually need to heat your garage? Let’s break it down.
What Are BTUs and Why Do They Matter?
A BTU (British Thermal Unit) is a measurement of heat energy. One BTU is the amount of heat needed to raise one pound of water by one degree Fahrenheit.
For garage heating, BTUs help determine how much heat is required to maintain a comfortable temperature. The size of the garage, insulation quality, and temperature difference all affect how many BTUs you’ll need.
How to Calculate the BTUs Needed for Your Garage
The number of BTUs required depends on three key factors:
- Garage size – Measured in cubic feet (length × width × height)
- Insulation level – Well-insulated garages require fewer BTUs
- Desired temperature increase – The difference between the outdoor temperature and your target indoor temperature
BTU Calculation Formula:
BTUs Needed = Cubic Feet × Insulation Factor × Temperature Difference
This formula accounts for the total air volume in your garage and the energy required to heat it.
Step 1: Measure Your Garage Size
To determine your garage's heating needs, you must measure your own space, as garages vary in size. Use the formula:
Cubic Feet = Length × Width × Height
Below are estimates of common garage sizes. These are only rough guidelines—be sure to measure your actual space for accurate results.
| Garage Type | Estimated Dimensions (L × W × H) | Estimated Cubic Feet |
| 1-car garage | 20 ft × 12 ft × 8 ft | 1,920 cu. ft. |
| 2-car garage | 24 ft × 24 ft × 8 ft | 4,608 cu. ft. |
| 3-car garage | 32 ft × 24 ft × 8 ft | 6,144 cu. ft. |
| 4-car garage | 40 ft × 24 ft × 8 ft | 7,680 cu. ft. |
If your garage has a higher or lower ceiling, use your actual height in the calculation.
Step 2: Select Your Insulation Level
| Insulation Level | Description | Insulation Factor |
| No Insulation (Very Poor) | Bare walls, uninsulated doors, major air leaks, no ceiling insulation. | 0.173 (+30%) |
| Minimal Insulation (Poor) | Thin or partial insulation, unsealed gaps, no insulated door, exposed ceiling. | 0.153 (+15%) |
| Standard Insulation (Moderate) | Insulated walls, but an uninsulated door or some air leaks. | 0.133 (Baseline) |
| Well-Insulated (Good) | Insulated walls, insulated door, sealed air gaps, insulated ceiling. | 0.113 (-15%) |
| Fully Insulated (Excellent) | High-efficiency insulation, weather-sealed doors/windows, insulated ceiling, minimal air loss. | 0.093 (-30%) |
Step 3: Determine the Temperature Difference
The greater the difference between the outdoor and indoor temperature, the more BTUs you will need.
For example, if it’s 0°F outside and you want the garage to be 70°F, the temperature difference is:
70°F - 0°F = 70°F
Step 4: Calculate the BTUs Needed
Now, apply the formula:
BTUs Needed = Cubic Feet × Insulation Factor × Temperature Difference
Example Calculations (Using a 70°F Temperature Difference)
1-Car Garage (1,920 cu. ft.)
| Insulation Level | Equation | BTUs Needed |
| No insulation (very poor) | 1,920 × 0.173 × 70 | 23,200 BTUs |
| Minimal insulation (poor) | 1,920 × 0.153 × 70 | 20,500 BTUs |
| Standard insulation (moderate) | 1,920 × 0.133 × 70 | 17,850 BTUs |
| Well-insulated (good) | 1,920 × 0.113 × 70 | 15,200 BTUs |
| Fully insulated (excellent) | 1,920 × 0.093 × 70 | 12,500 BTUs |
2-Car Garage (4,608 cu. ft.)
| Insulation Level | Equation | BTUs Needed |
| No insulation (very poor) | 4,608 × 0.173 × 70 | 55,600 BTUs |
| Minimal insulation (poor) | 4,608 × 0.153 × 70 | 49,200 BTUs |
| Standard insulation (moderate) | 4,608 × 0.133 × 70 | 42,800 BTUs |
| Well-insulated (good) | 4,608 × 0.113 × 70 | 36,400 BTUs |
| Fully insulated (excellent) | 4,608 × 0.093 × 70 | 30,000 BTUs |
3-Car Garage (6,144 cu. ft.)
| Insulation Level | Equation | BTUs Needed |
| No insulation (very poor) | 6,144 × 0.173 × 70 | 74,200 BTUs |
| Minimal insulation (poor) | 6,144 × 0.153 × 70 | 65,800 BTUs |
| Standard insulation (moderate) | 6,144 × 0.133 × 70 | 57,200 BTUs |
| Well-insulated (good) | 6,144 × 0.113 × 70 | 48,600 BTUs |
| Fully insulated (excellent) | 6,144 × 0.093 × 70 | 40,000 BTUs |
4-Car Garage (7,680 cu. ft.)
| Insulation Level | Equation | BTUs Needed |
| No insulation (very poor) | 7,680 × 0.173 × 70 | 92,700 BTUs |
| Minimal insulation (poor) | 7,680 × 0.153 × 70 | 82,400 BTUs |
| Standard insulation (moderate) | 7,680 × 0.133 × 70 | 71,500 BTUs |
| Well-insulated (good) | 7,680 × 0.113 × 70 | 60,700 BTUs |
| Fully insulated (excellent) | 7,680 × 0.093 × 70 | 50,000 BTUs |
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