Keep scrolling to use our easy & free BTU calculator

When a garage is ice-cold in the wintertime, it can be hard on your vehicles and anything else that you store in there. Furthermore, you may want to use that area for a home workshop or laundry room, as an extension of your living space. To choose a heater that will turn your garage from an oversized refrigerator into a comfortable, welcoming place, you need to pay attention to BTUs. Here’s how to use these units to calculate the right size garage heater for your unique situation:

What are BTUs?

The initials stand for British Thermal Units. Each BTU is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. Even though your garage heater is heating air, and not water, this unit of measurement is the standard way of discussing heating capacity. BTUs are a handy way to compare the effectiveness of different heater types, so that you can easily decide on the energy source that’s best for you: a natural gas or propane heater, electric heater, and so on. Air conditioners also use BTUs, as a measure of how much heat they can remove from the air.

Steps to Choosing the Right Sized Heater

With an easy calculator option just a little further down the page.

1. Square Footage

Time to get out your tape measure! Your first step is to figure out the area, in square feet, of the garage you want to heat. You do that by measuring the length and width of the space, and multiplying them together. So, for instance, if you have a garage that is 20 feet long and 15 feet wide, your square footage is 20 X 15, or 300 sq ft. Just as a rule of thumb, the standard area required for safely parking one car is 10 by 20 feet, or 200 square feet, and a 2-car garage is at least double that area.

2. Ceiling Height

To get an even more accurate measurement of your garage, you’ll want to calculate how many total cubic feet are in it. This is important, because heat rises, so the area by the ceiling will get warm first. Understanding the volume of a room (its total three-dimensional size) is more useful than simply knowing its floor area. To calculate cubic feet, you measure the ceiling height, and then multiply that by the number of square feet in the space. If you have a 300 square foot area, with a 10 foot high ceiling, then you multiply 300 X 10, to get 3,000 cubic feet.

Calculating what size garage you have is only the beginning. When you think about making a room comfortable in cold weather, obviously many elements must be considered besides size.

3. Insulation

Indoor spaces vary enormously in their amount of heat loss, depending on how they are constructed. A new garage will probably meet strict energy efficiency standards, whereas older homes often have little insulation in the garage and may even have actual gaps and holes where cold air can enter. You probably know your own garage well enough to estimate how well it’s insulated on a 3-point scale: Is it new energy-efficient construction, or does it have average insulation, or is it older with lots of air leaks and single-pane windows? You can also get by with fewer btus if you give your garage an energy upgrade. You can buy garage door insulation kits at home improvement stores.

4. Climate Zone

The next logical factor in planning which heater size you need is to consider the winter temperatures where you live. Obviously, a garage heater in Louisiana or Southern California doesn’t have to work as hard as one in Minnesota. Many people choose a garage heater based on an average winter temperature. Or, you may want the safety margin of a heater large enough to handle freak cold spells. In any case, decide on a winter temperature that you want your heater to be able to fully offset. If you’re new to the area, a climate zone map can show your average low temperature in winter.

Climate Zone Map


5. Temperature Zone

Are you simply looking for a way to keep your garage above freezing? Or do you want it to be a comfortable working temperature for a shop or studio space? Are you someone who prefers to wear a sweater and work in 55 to 60 degree temperatures, or do you want the air temperature to be a cozy 70 or 75? To calculate the temperature rise you need your heater to supply, subtract your cold outdoor winter temperature from your preferred indoor garage temperature. For instance, if you want to prepare for winter days that reach 15 degrees Fahrenheit, and you want your garage to be 65 degrees, you need a temperature rise of 50 degrees.

6. Calculate the BTUs You Need

Once you have completed steps 1 through 5 above, you’re ready to calculate the BTUs you need. As a basic rule, the figure will be between 30 and 60 BTUs per square foot. This part is simple, because you can just plug your information into your free and easy BTU calculator just below, and let it do the math for you. As you can see, it’s not going to be an exact result, because every building is a bit different. Even the color of your roof can have an effect on how much heat you need. However, plugging in your information will give you a result that is sufficient to help you select the right size heater. You can add an extra safety margin, just in case you overestimated the quality of your insulation.

BTU Calculator

Fill in the inputs section to find out how many BTUs you need. If you don't know what climate you live in, see the section 4, the climate zone section above.

How Many Watts Are Needed to Heat Your Garage?

Some garage heaters only list their heating capacity in watts, so it’s useful to be able to convert watts to BTUs and vice versa. One watt is equal to about 3.41 BTUs per hour. More importantly, 0.29 BTUs per Watt. Just multiple the BTUs by 0.92 to find the number of Watts needed. Since watts are a measurement of the amount of electricity used, you have to add “per hour” into the formula when you convert from BTU units to watts. Heating your garage to 70 degrees for ten hours obviously takes more energy than heating it for only one hour. If you don’t want to do the math yourself, you can convert from watts to BTUs per hour and back again very simply on an online conversion page.

You Have a Wide Selection of Heater Options

Now that you know the BTU output that you need for your garage heater it’s time to look at your choices.

Gas Heaters

Do you have a gas line that goes out to your garage? If so, you may want to install a natural gas heater or propane heater. Many gas heaters, such as the Modine Hot Dawg, require venting to the outdoors, so you have to consider where you would cut a hole in your garage to safely install that vent. Small “buddy heaters” can be connected to a propane bottle, and can provide heat in a natural disaster without the need for any electricity. There are also some kerosene heaters that can be used indoors, although they have to be filled and lit carefully according to the instructions.

Ceiling Mount Electric Heaters

Hardwired into your home’s electrical system, a ceiling mounted heater allows your floor space to be used to the greatest degree. These reliable heaters don’t have to be constantly plugged and unplugged. Furthermore, they can be installed on a different circuit than your power tools, so you’ll have no worries about blowing a fuse. A ceiling fan will help circulate the warm air down to floor level instead of letting it all pool up above your head. Because there is no danger of cords to trip on, ceiling mounted heaters are considered the safest garage heater options. However, they do need to be installed by a professional electrician or knowledgeable DIY homeowner.

Powerful Garage Heaters Can Also Be Portable

If you want the convenience of moving your heat source around, you don’t have to sacrifice heating power. Portable heaters made to plug into a 240 Volt outlet can generate significant amounts of heat. NewAir’s portable electric garage heater has a heating capacity of more than 19,000 BTUs -- enough for most 2-car garages in a cool climate.

Many Other Heating Options 

Portable electric baseboard heaters can provide an energy-efficient source of warmth for smaller garages, with a convenient low profile. Forced air space heaters are easy to move around between ordinary outlets, and great for bringing comfort to a one-car garage or smaller work space. Combination btu heater / air conditioners provide year-round climate control for your garage, using heat pump technology. If you have a central forced-air furnace, you can talk with a contractor about installing an additional heating duct in your garage.

The great thing about knowing your BTU requirement is that you can choose any type of fuel and shop with confidence, certain of getting a heater that’s exactly the right size for your garage.

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