The garage is 20 feet by 20 feet with 10-foot ceilings. The walls are uninsulated stud cavities. The ceiling has no insulation. The garage door is a sheet of metal. In January, the temperature inside the garage is the same as the temperature outside. You want to heat it to 65 degrees. The question is how many BTUs the heater needs to produce to raise the temperature and hold it there. The answer is not a single number. It depends on the garage volume, the insulation level, the climate zone, and the desired temperature rise above the outside temperature. An uninsulated two-car garage in Minnesota needs roughly twice the BTUs of an insulated garage of the same size in Virginia.
According to wikiHow’s garage heating guide, which has been viewed over 41,000 times, insulating the garage is the first and most important step before installing any heater. A heater in an uninsulated garage represents a tremendous waste of energy because the heat escapes as quickly as it is produced. This guide covers the BTU calculation, heater sizing by garage size and climate, and the mistakes that result in a garage that never gets warm or a heater that runs constantly and consumes fuel without producing comfort.
The BTU Calculation Formula
The basic formula for garage heating BTUs is:
BTUs = Cubic Feet × Desired Temperature Rise (°F) × Insulation Factor
Cubic feet: length × width × ceiling height. A standard two-car garage is 20 × 20 × 10 = 4,000 cubic feet. A one-car garage is 12 × 22 × 10 = 2,640 cubic feet.
Desired temperature rise: the difference between the outside temperature on the coldest typical day and the desired indoor temperature. If the garage should be 65°F and the coldest day is 10°F, the temperature rise is 55°F. In a cold climate where the coldest day is -10°F and the target is 65°F, the rise is 75°F.
Insulation factor: a multiplier that accounts for how well the garage holds heat. The factor is based on the insulation level of the walls, ceiling, and garage door. Per wikiHow’s guide, an R-value of at least 19 in the walls and ceiling is recommended before installing a heater.
| Insulation Level | Factor | Description |
| Well-insulated (R-19+ walls, R-30+ ceiling, insulated door) | 1.0-1.5 | Garage converted to living space standards |
| Moderately insulated (R-13 walls, R-19 ceiling, uninsulated door) | 2.0-2.5 | Standard garage insulation upgrade |
| Uninsulated (no wall or ceiling insulation, metal door) | 3.0-4.0 | Original uninsulated garage |
BTU Examples for Common Garage Sizes
| Garage Size | Cubic Feet | Insulated (40°F Rise, Factor 1.5) | Uninsulated (40°F Rise, Factor 3.5) | Cold Climate (70°F Rise, Factor 2.0) |
| One-car (12×22×10) | 2,640 | 12,000-18,000 | 30,000-40,000 | 25,000-35,000 |
| Two-car (20×20×10) | 4,000 | 18,000-25,000 | 45,000-60,000 | 40,000-55,000 |
| Large two-car (24×24×12) | 6,912 | 30,000-45,000 | 75,000-100,000 | 65,000-85,000 |
An uninsulated two-car garage in a moderate climate needs a 45,000 to 60,000 BTU heater. The same garage fully insulated needs 18,000 to 25,000 BTUs. The insulation reduces the heater size by more than half. The cost difference between a 25,000 BTU heater and a 50,000 BTU heater is $200 to $500 for the unit plus the ongoing fuel cost of running a larger heater. The insulation pays for itself in heater cost savings alone, before accounting for the fuel savings over the life of the garage.
Heater Types and Their BTU Ranges
| Heater Type | BTU Range | Best For | Approximate Cost |
| Electric space heater (120V) | 5,000 | Small workshop, spot heating | $50-100 |
| Electric space heater (240V) | 5,000-25,000 | One-car insulated garage | $200-500 |
| Natural gas forced-air | 25,000-100,000 | Two-car garage, insulated | $500-1,500 |
| Natural gas infrared tube | 30,000-100,000 | High-ceiling garages, workshops | $800-2,000 |
| Propane forced-air | 30,000-80,000 | Garages without natural gas service | $400-1,200 |
Common BTU and Garage Heating Mistakes
- Buying an oversized heater. A heater that is too large for the space cycles on and off frequently. Each cycle has a startup period where the heater is not producing full heat. Frequent cycling reduces efficiency and shortens the heater’s lifespan. A properly sized heater runs for longer cycles at steady state, which is more efficient and more comfortable.
- Heating an uninsulated garage. The heat escapes as fast as the heater produces it. The fuel bill is enormous and the garage never feels warm because the surfaces, the walls, the floor, the ceiling, are all cold and radiating cold to the occupant. Insulate first, then size the heater for the insulated space.
- Ignoring ceiling height. BTU calculators that use square footage instead of cubic feet underestimate the heat loss for garages with high ceilings. A 20×20 garage with 8-foot ceilings is 3,200 cubic feet. The same footprint with 12-foot ceilings is 4,800 cubic feet, a 50 percent increase in volume. The heater must be sized for the volume, not the floor area.
- Forgetting the garage door. An uninsulated metal garage door is an R-1 thermal hole that represents roughly 20 to 30 percent of the wall area in a standard garage. Insulating the garage door adds R-6 to R-9 and reduces the BTU requirement by 5,000 to 10,000 BTUs for a two-car garage. The door insulation kit costs $50 to $150.
BTU Guidelines by Climate Zone
The temperature rise in the BTU formula depends on your location. Here are typical design temperatures and the resulting BTU range for a 4,000 cubic foot insulated garage (factor 1.5):
| Climate Zone | Typical Coldest Day | Temp Rise to 65°F | BTUs Needed (Insulated) |
| Warm (Florida, Texas Gulf, SoCal) | 30-40°F | 25-35°F | 10,000-18,000 |
| Moderate (Mid-Atlantic, Pacific NW) | 10-25°F | 40-55°F | 18,000-25,000 |
| Cold (Midwest, Northeast) | -5 to 10°F | 55-70°F | 25,000-35,000 |
| Very Cold (Upper Midwest, Northern New England) | -15 to -5°F | 70-80°F | 35,000-45,000 |
These numbers assume an insulated garage. For an uninsulated garage, multiply the BTU range by 2 to 2.5. A 4,000 cubic foot uninsulated garage in a cold climate needs 50,000 to 85,000 BTUs. Insulating first reduces that to 25,000 to 35,000 BTUs, which can be served by a smaller, less expensive heater with lower operating costs.
Frequently Asked Questions
Should I use an electric or gas heater for my garage?
Electric heaters are cheaper to buy and install but more expensive to operate. Natural gas heaters cost more upfront but are roughly one-third the operating cost of electric resistance heat per BTU in most of the United States. For a garage heated occasionally, a few hours on weekends, an electric heater is sufficient. For a garage heated daily or used as a full-time workshop, natural gas pays for itself in operating savings within one to two heating seasons. Propane is the alternative where natural gas is not available. Propane is more expensive than natural gas but still less expensive than electric resistance heat in most regions.
Can I just buy a bigger heater instead of insulating the garage?
A bigger heater will raise the air temperature, but the garage will never feel warm in the way an insulated garage does. The air will be warm while the heater is running, and cold within minutes of the heater turning off. The surfaces, walls, floor, and ceiling, will remain cold and radiate cold to anyone in the space. The fuel cost will be two to three times higher than heating an insulated garage. The bigger heater is a short-term solution that costs more every month it operates. Insulate first. Size the heater second.
The Garage That Stays Warm
The correct BTU calculation for a garage heater is cubic feet times temperature rise times insulation factor. The insulation factor is the variable that halves or doubles the result. Spending $550 to $1,350 on insulation reduces the required heater size from 50,000 BTUs to 25,000 BTUs, saving $200 to $500 on the heater purchase and hundreds more per year in fuel. The insulation is not an additional cost. It is the foundation that makes the heater affordable to run. A warm garage is an insulated garage with a correctly sized heater. An uninsulated garage with an oversized heater is a cold garage with a high fuel bill.
