One rule of thumb for estimating the size of an electric garage heater is to use the β€œlaw of 10.” This means using 10 watts of heating power per square foot of floor space in your garage.

Garage Electrical Heater Size Calculator

For an electric heater, the best way is to abide by the law of 10. That is 10 watts per square foot of floor space.

For example, if your garage is 500 square feet, you need a heater with a minimum output of 5,000 watts (500 x 10 = 5,000).

Assuming a 20 ft. x 25 ft. garage with a ceiling height of 8 ft., the total volume of the space is 4000 cubic feet (20 x 25 x 8). To estimate the required wattage for heating the garage, we can use the formula:

Wattage = Volume (cubic feet) x Temperature Difference (Fahrenheit) / 3

When estimating the required BTU for a gas garage heater, the process is similar to that for an electric heater. The main difference is that instead of using watts, we use BTU per hour (BTU/hr) as the unit of measurement.

To convert watts to BTU/hr, we can use the following formula:

1 watt = 3.412 BTU/hr

So, if you know the wattage of the electric heater, you can convert it to BTU/hr by multiplying it by 3.412. For example, if the electric heater has a wattage of 5,000 watts, the equivalent BTU/hr would be:

5,000 watts x 3.412 = 17,060 BTU/hr

How to Estimate the Size of Heater Needed for Your Garage

The following is based on a study of estimated wattage or BTU (British Thermal Unit) requirements for electric, gas, and infrared heaters based on the size of the garage, ranging from 240 to 900 square feet, as well as an option for a ceiling height of 10 feet.
Garage Size (ft)Square FootageElectric HeaterGas HeaterInfrared Heater
12 x 202402,400 W12,000 BTUs1,500 W
20 x 204004,000 W20,000 BTUs2,500 W
22 x 245285,280 W26,400 BTUs3,500 W
24 x 245765,760 W28,800 BTUs4,000 W
24 x 307207,200 W36,000 BTUs5,000 W
30 x 309009,000 W45,000 BTUs6,000 W
24 x 24 (10')8646,480 W48,960 BTUs5,000 W

For the above table, the recommended wattage for infrared heaters was set to 7.5 watts per square foot to provide a conservative estimate. The calculation criteria for infrared heaters is multiplying the square footage by 7.5 to determine the needed wattage.

Infrared heaters are measured in watts, like electric heaters, and the calculation criteria is similar: multiply the square footage by a certain number of watts per square foot. The recommended wattage for infrared heaters is typically higher than for electric heaters, ranging from 10-20 watts per square foot.

Heat loss calculation: Determine the amount of heat loss from your garage through various sources and choose a heater with the appropriate BTU output to heat your garage effectively.

The calculation criteria for electric heaters remain the same: multiply the square footage by 10 to determine the wattage needed. For gas heaters, the calculation criteria are to multiply the square footage by 50 to determine the BTUs needed.

For the option with a ceiling height of 10 feet, the calculation criteria for electric heaters is to multiply the square footage by 10 and the ceiling height divided by 8 to determine the wattage needed. For gas heaters, the calculation criteria are to multiply the square footage by 50 and the ceiling height divided by 8 to determine the BTUs needed.

Factors To Conisder While Chosing Heater For Garage

FactorDescription
Garage sizeThe size of your garage is the most important factor in determining the size of the heater you need. A larger garage will require a larger heater to adequately heat the space.
InsulationThe level of insulation in your garage is the second most important factor, as it can have a significant impact on the amount of heat required to keep it warm. A well-insulated garage will require less heating than a poorly insulated one.
Heating speedThe heating speed is the third most important factor, as most people prefer to have their garage heated quickly after turning on the heater.
ClimateThe climate in your area is the fourth most important factor, as it will affect the size of heater you need. Cold climates will require a larger heater than mild climates.
Types of fuelThe type of fuel you choose for your garage heater is the fifth most important factor, as it can affect the size of the heater required. For example, natural gas and propane heaters typically require a smaller unit than electric heaters to produce the same amount of heat.
Purpose of the garageThe purpose of your garage can impact the size of heater required. A workspace may require a larger heater to keep the space warm and comfortable while you work.
Ceiling heightThe height of your garage ceiling can impact the amount of heat required. Taller ceilings require more heat to maintain a comfortable temperature.
Windows and doorsThe number and size of windows and doors in your garage can impact the amount of heat required. More windows and larger doors can result in greater heat loss.
Garage door openersThe frequency of garage door opening and closing can cause heat loss and may require a larger heater to compensate.
Heat toleranceThe amount of heat required to keep your garage warm may also depend on your personal heat tolerance.
Humidity levelsHigh humidity levels can make a garage feel colder than it actually is, and may require a larger heater to compensate for extra heat loss due to moisture in the air.
Number of cars/vehiclesThe number of cars or vehicles in your garage can impact the size of the heater needed, as they can absorb some of the heat and make it more difficult to keep the garage warm. However, this factor may not have as significant an impact as some of the other factors previously mentioned.

Again, please note that this table only provides a general estimate, and other factors such as insulation, climate, and desired temperature may also affect the size of the heater needed.

1. Start By Measuring Your Garage Size

When it comes to selecting a heater for your garage, the size of your garage is an important factor to consider. A heater that is too small will struggle to heat the entire space, while a heater that is too large will consume more energy than necessary and result in higher energy bills. Choosing a heater that is appropriately sized for your garage is important.

A Garage of 20*20 feet.

A 5000 watt electric heater would be sufficient for a garage of size 20*20 feet.

The size of your garage is measured in square footage, and the larger your garage, the more heating power you will need to heat the entire space effectively.

A small garage, such as one that measures 10 feet by 10 feet, will require a heater with a lower BTU output and wattage output compared to a larger garage, such as one that measures 30 feet by 30 feet or more.

Garage SizeRecommended BTU OutputRecommended Wattage OutputAverage Energy Cost Per Month*
Small (up to 300 sq ft)5,000 – 10,0001,500 – 3,000$15 – $30
Medium (300 – 600 sq ft)20,000 – 30,0005,900 – 8,800$60 – $90
Large (over 600 sq ft)45,000 – 60,00013,200 – 17,600$120 – $180

In general, a small garage will require a heater with a BTU output of 5,000 to 10,000 and wattage output of 1,500 to 3,000.

A medium-sized garage will require a heater with a BTU output of 30,000 to 40,000 and wattage output of 8,800 to 11,700. A large garage will require a heater with a BTU output of 50,000 or higher and a wattage output of 14,600 or higher.

2. Ceiling Height

The height of the ceiling affects the size of the space that needs to be heated and the heat distribution within that space.

For electric heaters, the wattage and coverage area of the heater should be chosen based on the height of the ceiling. If the ceiling is high, a heater with a higher wattage and larger coverage area will be required to provide sufficient warmth to the space.

This is because heat rises and can accumulate at the ceiling, leaving the lower areas of the garage colder.

Ceiling HeightElectric Heater WattageGas Heater BTU Output
8-10 feet10 watts/sq. ft.45,000 BTUs
10+ feet13 watts/sq. ft.60,000 BTUs

A higher-wattage heater can produce more heat to overcome this problem and provide warmth to the entire garage.

Gas heaters also need to be chosen based on the ceiling height of the garage.

The BTU (British Thermal Unit) output of the heater is an important consideration. For a high ceiling, a gas heater with a higher BTU output will be required to provide sufficient heat to the entire space. The type of gas used in the heater is also a consideration, with natural gas and propane being the most common options.

3. Insulation Level

The insulation level in your garage can vary depending on the age of your garage, the type of construction, and the type of insulation used.

A well-insulated garage will have insulation in the walls, ceiling, and possibly even the floor. In addition, the insulation should be thick enough to provide a sufficient thermal barrier. If you have a garage with good insulation, you will require less heating power to maintain a comfortable temperature compared to a poorly insulated garage.

For example, a small garage with good insulation may only require a heater with a BTU output of 5,000 to 10,000, while a small garage with poor insulation may require a heater with a BTU output of 15,000 or more to effectively heat the space.

Insulation LevelRecommended BTU OutputRecommended Wattage OutputAverage Energy Cost Per Month*
Good (R-value of 15 or higher)5,000 – 10,0001,500 – 3,000$15 – $30
Average (R-value of 10-14)10,000 – 20,0002,900 – 5,900$30 – $60
Poor (R-value of less than 10)15,000 or higher4,400 or higher$45 – $90
R-value is a measure of the thermal resistance of insulation. It measures how well a material resists the flow of heat through it. The higher the R-value, the better the insulation's ability to resist heat transfer.

The R-value is calculated by dividing the thickness of the insulation (in inches) by the thermal conductivity of the insulation (in BTUs per hour per square foot per degree Fahrenheit). The resulting number is the R-value.

For example, if you have insulation that is 6 inches thick and has a thermal conductivity of 0.25 BTUs per hour per square foot per degree Fahrenheit, the R-value would be calculated as follows:

R-value = 6 inches / 0.25 BTUs per hour per square foot per degree Fahrenheit
R-value = 24

This means that the insulation has an R-value of 24.

The R-value of insulation is an important factor to consider when selecting a heater for your garage. A garage with good insulation, which typically has an R-value of 15 or higher, will require less heating power compared to a garage with poor insulation, which typically has an R-value of less than 10.

4. Heating Speed

The heating speed of a heater refers to how quickly the heater can warm up your garage to a comfortable temperature.

In general, larger garages will require heaters with a higher heating speed compared to smaller garages. This is because larger garages will take longer to warm up and require more heating power to maintain a comfortable temperature.

Additionally, garages with poor insulation will also require heaters with a higher heating speed, as they will lose heat more quickly and take longer to warm up.

5. Climatic Conditions

Areas with high humidity or extreme temperatures may require heaters with additional features to effectively maintain a comfortable temperature.

Climate ZoneWeather CharacteristicsHeating Considerations
NorthernLong and harsh wintersRequire heaters with higher heating power; consider insulation level and select a heater with appropriate BTU output and wattage output
SouthernHot and humid summersMay require air conditioning in addition to heating; consider climate control needs and select appropriate heater and/or air conditioning system
MountainCold winters and mild summersRequire heaters with higher heating power during winter months; consider insulation level and select a heater with appropriate BTU output and wattage output
CoastalMild and wet wintersMay require heaters with additional features to effectively maintain a comfortable temperature during wet and cold weather; consider humidity level and insulation of garage, and select a heater with appropriate features and specifications

6. Fuel Type

The type of fuel used for heating a garage can play a significant role in determining the size of the heater required. Different types of fuel have different heating capacities, efficiencies, and costs.

  • Natural gas heaters are among the most efficient and cost-effective options, with an average efficiency rating of 80-90%. The cost of natural gas varies by region, but it is generally cheaper than other fuel sources.
  • Propane heaters have an efficiency rating of around 70-80%. The cost of propane varies depending on location, but it is typically more expensive than natural gas.
  • Electric heaters are the least efficient and most expensive option, with an average efficiency rating of 95%. However, electric heaters are often favored for their ease of installation and maintenance.

7. Purpose Of Garage

Let’s take a closer look at each type of garage and how it can impact your heater selection:

Purpose of GarageHeater ConsiderationsPreferred HeaterRecommended Heater Models
Parking vehiclesConsider a heater with high heating capacity and quick warmth to make the garage comfortable for entering and exiting vehicles. A natural gas or propane heater can be a good option.Forced-air natural gas or propane heatersModine Hot Dawg Heater, Dyna-Glo RMC-FA60DGP, Mr. Heater F260550 Big Maxx
Storing tools and equipmentA heater with consistent, gentle heat can help protect your tools and equipment from damage due to temperature changes. Electric heaters can be a good choice for this purpose.Electric heatersCadet RCP402S, Dr. Infrared Heater DR-988, Fahrenheat FUH54
Working on DIY projectsYou'll want a heater that can efficiently heat the space while also being safe to use around tools and equipment. A natural gas or propane heater with appropriate safety features can be a good choice.Forced-air natural gas or propane heatersModine Hot Dawg Heater, Mr. Heater F260550 Big Maxx, Dyna-Glo RMC-FA60DGP
Housing recreational vehicles (RVs)An RV garage requires a heater that can accommodate the large space while also being safe to use around the vehicle. Consider a forced-air natural gas or propane heater with appropriate safety features.Forced-air natural gas or propane heatersMr. Heater MH125FAV, Dyna-Glo RMC-FA125DGD, Heatstar HS125NG
Storing boats and other watercraftA boat garage requires a heater that can keep the space at a consistent temperature to protect your boat from freezing temperatures and humidity. An electric heater can be a good option, with a built-in thermostat to maintain a safe temperature for your boat.Electric heatersNewAir G73 Hardwired Electric Garage Heater, Fahrenheat FUH54, Cadet RCP402S
Providing additional storage spaceConsider a heater with a moderate heating capacity, depending on the size of your storage space. Natural gas or propane heaters can be a good option for their efficiency and cost-effectiveness.Forced-air natural gas or propane heatersMr. Heater F299730, Dyna-Glo RMC-FA150DGD, Modine Hot Dawg Heater
Serving as a workshop or studioYou'll want a heater that can efficiently heat the space while also being safe to use around tools and equipment. A natural gas or propane heater with appropriate safety features can be a good choice.Forced-air natural gas or propane heatersModine Hot Dawg Heater, Mr. Heater F260550 Big Maxx, Dyna-Glo RMC-FA60DGP
Converting into additional living spaceConsider a heater that can provide consistent heat to create a comfortable living space. Electric heaters can be a good option for their ease of installation and maintenance.Electric heatersDimplex DGWH4031G, Dr. Infrared Heater DR-988, Cadet Com-Pak Twin
Providing a space for home gym or exercise equipmentYou'll want a heater that can efficiently heat the space while also being safe to use around your exercise equipment. A natural gas or propane heater with appropriate safety features can be a good choice.Forced-air natural gas or propane heatersDyna-Glo RMC-FA60DGP, Mr. Heater F260550 Big Maxx, Modine Hot Dawg Heater
Serving as a playroom or game room for childrenConsider a heater that can provide consistent, gentle heat to create a comfortable space for children to play. Electric heaters can be a good option for this purpose.Electric heaters
Dimplex DGWH4031G, Dr. Infrared Heater DR-988, Cadet Com-Pak Twin

8. Windows and doors

The number and quality of windows and doors in the garage affect the insulation and air circulation of the space.

Poorly insulated windows and doors can result in heat loss and higher energy consumption, while proper insulation can help retain heat and minimize energy use.

9. Garage door openers

The presence of a garage door opener can also affect the heating needs of a garage. Garage door openers can allow cold air to enter the garage, resulting in higher heating requirements. It is important to consider the insulation of the garage door and the frequency of use when choosing a garage heater.

10. Heat tolerance

The heat tolerance of equipment and vehicles stored in the garage is also a consideration. Some equipment and vehicles may require a specific temperature range to function properly. It is important to select a garage heater that can provide the required temperature range without overheating the space.

11. Humidity levels

Humidity levels in the garage can affect the efficiency and performance of a heater. High humidity can reduce the effectiveness of a heater and increase the risk of corrosion and mold growth. It is important to ensure proper ventilation and consider a dehumidifier when selecting a garage heater.

12. Number of cars/vehicles

Garage TypeIntended UseRecommended BTU OutputRecommended Wattage Output
Parking garagePrimarily for parking cars. Must be warm enough to prevent damage to the cars.5,000 - 10,0001,500 - 3,000
RV garageUsed to store recreational vehicles (RVs). RVs require a lot of heating power to maintain a comfortable temperature, especially during cold weather months.30,000 - 60,0008,800 - 17,600
Boat garageUsed to store boats. Boats may require heating during cold weather months to prevent damage to the boat's interior and systems.20,000 - 30,0005,900 - 8,800

Calculating BTUs for Garage Heaters

Whereas electric garage heaters are rated according to wattage, gas garage heaters are rated according to BTUs (British Thermal Units).

A Propane heater of 18,000 BTUs running in a garage.

While it all sounds exceptionally high-tech, it’s actually quite straightforward. Each BTU is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.

Climate ZoneCold Climate BTUs Per Sq FtModerate Climate
BTUs Per Sq Ft
Warm Climate BTUs Per Sq Ft
Climate Zone 130-40 BTUs/sq. ft.18-30 BTUs/sq. ft.10-20 BTUs/sq. ft.
Climate Zone 240-60 BTUs/sq. ft.24-40 BTUs/sq. ft.14-28 BTUs/sq. ft.
Climate Zone 360-80 BTUs/sq. ft.36-60 BTUs/sq. ft.20-40 BTUs/sq. ft.
Climate Zone 480-100 BTUs/sq. ft.48-80 BTUs/sq. ft.26-52 BTUs/sq. ft.
Climate Zone 5100-150 BTUs/sq. ft.60-100 BTUs/sq. ft.34-68 BTUs/sq. ft.
Climate Zone 6150-200 BTUs/sq. ft.90-150 BTUs/sq. ft.50-100 BTUs/sq. ft.
Climate Zone 7200-250 BTUs/sq. ft.120-200 BTUs/sq. ft.68-136 BTUs/sq. ft.

Even though gas heaters are heating air and not water, this is the standard method of referring to heating capacity.

To work out the size of a gas garage heater, you multiply the wattage by 3.41. Using 4500 as an example, the measurement would be as follows: 4500 x 3.41 = 15,345 BTUs.

Another more exact way to estimate the heater size for your garage is to calculate the number of watts or BTUs required to correlate with your desired temperature rise.

For example, a garage measuring 20 feet by 20 feet with an 8-foot ceiling and moderate insulation requires approximately 30,000 BTUs to raise the temperature by 30 degrees Fahrenheit.

Use a rating of 5 if your garage space has no insulation, 1.5 if it has minor insulation, one if it has average insulation, and 0.5 if it has reliable insulation.

Once your measurements are in order, you can determine the required number of BTUs by following the below equation:

By implementing the previous example of a 20 x 22 garage (average insulation,4,400 cubic ft. and a 25F desired temperature rise) the formula would be:

(1 x 4,400 x 25) / 1.6 = 68,750 BTU.

To convert into watts, you would then divide by 3.41.

68,750 / 3.41 = 20.161 watts.

In this example, it would be unlikely that you could find an electric garage heater rated for 20.161 watts. You would either need to purchase and install a gas heater or multiple electric heaters.

The only other option would be to increase the volume of insulation in your garage walls. This would then reduce the overall amount of energy needed to heat it.

Garage BTU Requirements

Typically, the estimated BTU requirements for a garage can range from 30 BTUs to 60 BTUs, depending on the climate zone of the location.

Garage SizeSquare FootageEstimated BTUs Needed (cold climate)Estimated BTUs Needed (moderate climate)Estimated BTUs Needed (warm climate)Ceiling Height
1-car garage150-250 sq. ft.15,000-20,000 BTUs9,000-15,000 BTUs5,000-10,000 BTUs8 ft.
2-car garage300-500 sq. ft.30,000-40,000 BTUs18,000-30,000 BTUs10,000-20,000 BTUs8-10 ft.
3-car garage450-750 sq. ft.45,000-60,000 BTUs27,000-45,000 BTUs15,000-30,000 BTUs10-12 ft.
4-car garage600-1000 sq. ft.60,000-80,000 BTUs36,000-60,000 BTUs20,000-40,000 BTUs12 ft. or higher

Mr.Heater's propane heater can produce 70,000 BTU running on a paking garage that has 4 cars.

Gas Heater Size Based on Garage Size and Climate Zone

Size of GarageWarm Climate (Zone 3)Moderate Climate (Zone 4)Cold Climate(Zone 5&6)Very Cold Climate(Zone 7)
10 x 20 (small garage)8,000 BTU9,000 BTU10,500 BTU12,000 BTU
12 x 20 (1 car garage)9,600 BTU10,800 BTU13,000 BTU14,400 BTU
16 x 20 (2 car garage)14,400 BTU16,200 BTU18,800 BTU21,600 BTU
20 x 20 (2 large car garage)16,000 BTU18,000 BTU21,000 BTU24,000 BTU
30 x 20 (3 car garage)22,400 BTU25,200 BTU29,000 BTU33,600 BTU
36 x 20 (4 car garage)28,800 BTU32,400 BTU38,200 BTU43,200 BTU

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