For a standard, well-insulated two-car garage (approximately 400-600 square feet), a heater with 18,000 to 30,000 BTUs is typically sufficient to raise the temperature to a comfortable working level. However, the exact BTU requirement heavily depends on your climate zone and the garage's insulation quality. In very cold regions or poorly insulated spaces, you may need up to 45,000 BTUs or more.
The primary factor is the temperature difference (ΔT) between the outside air and your desired indoor temperature. A larger ΔT requires more heating power. Other critical considerations include the garage's ceiling height and how well-sealed it is against drafts.
Here’s a quick reference table based on common scenarios:
| Climate Zone & Insulation Level | Garage Size (sq. ft.) | Recommended BTU Range |
|---|
| Mild Climate (e.g., Southern US), Well-Insulated | 400 - 500 | 18,000 - 24,000 BTU |
| Moderate Climate (e.g., Midwest), Average Insulation | 500 - 600 | 24,000 - 30,000 BTU |
| Cold Climate (e.g., Northern US), Poorly Insulated | 400 - 600 | 30,000 - 45,000 BTU |
| Very Cold Climate (e.g., Canada), Well-Insulated | 500 - 600 | 36,000 - 50,000 BTU |
| Any Climate, Detached Garage with High Ceilings | 500 - 600 | 30,000 - 60,000 BTU |
Before purchasing a heater, it's wise to perform a manual J calculation, which is the industry standard for sizing HVAC equipment. This complex calculation considers all variables, but for a rough DIY estimate, you can use 30-45 BTUs per square foot for a well-insulated attached garage and 50-60 BTUs per square foot for a detached or poorly insulated one. Always prioritize safety by ensuring the heater is approved for garage use and has adequate ventilation if it's a combustion model.
