Garage Heater Size Calculator: How Many BTU Do I Need?

Heating a garage is a challenge. Most are poorly insulated. Many have open trusses overhead where heat will rise to, far from where you’re working at a workbench or leaning over an engine.

Garage Heating: How Many BTU Do I Need? A lot. You’ll have an exact answer after using the exclusive Pick HVAC Garage BTU Heating Calculator.

  • How many BTU heater for a 1-car garage?
  • How many BTU do I need for a 2-car garage? 3-car garage?
  • We’ll give you ranges based on common sizes – and if you care to enter the exact square footage, your results will be even more accurate.

    In this garage heating BTU guide, we’ll discuss your options – electric, propane or gas – with their pros and cons. Plus, there are tips for:

    Making your garage more energy efficient.

    Getting it warmed up quickly, which is essential on cold days when you’ve got work in the garage to do.

    In short, there is a lot of garage-specific heating guidance below the calculator, so keep reading in order to make an informed decision on a garage heating solution.

    Garage Heating BTU Calculator

    Our Garage BTU calculator gives you the BTUs needed to heat a garage of any common size.

    And it gives you the conversion to watts if you decide to go electric using the formula 1 Watt = 3.412 BTU. The math is done automatically in the calculator.

    Here is the calculator with an explanation below of the steps needed to successfully answer how many BTU needed in the garage.

    Garage Heater Sizing Calculator

    Garage Heating Btu Calculator

    Locate your climate zone on the map above and provide your square footage. You'll get how many BTUs needed for your heating system.

    See the climate zone map above to find your home's climate zone

    5 Step Garage Heater Sizing Calculator

    These instructions walk you through the process, if you have any questions about how it works. There are also a few tips that might be useful.

    Note: The Calculator calculates with every input you make.

    Step 1 – Choose your Climate Zone

    Find where you live, and select the Climate Zone.

    Near a Zone boundary? Choose the colder Zone. That’s definitely a good idea given the fact that garages are difficult to heat.

    Here’s another pro tip: It won’t hurt to automatically choose the next-coldest Zone, especially if your garage is poorly insulated. We would not recommend this for an insulated garage nor for a house, because having a furnace or heater that’s too large for the space is a waste of money. But again, where insulation is sparse, you might want more-than-calculated heat.

    Step 2 – Enter Garage Size

    Multiply length times width to get square footage. If you have house or garage drawings, the dimensions will probably show on them.

    The square footage should fall close to these ranges:

    • 1-car garage: 180 to 288 square feet
    • 2-car to 2.5-car garage: 440 to 600 square feet
    • 3-car to 3.5-car garage: 650 to 770 square feet

    Step 3 – Enter Ceiling Height

    Measure up to the first solid surface you come to.

    If you have a semi-finished or finished garage with drywall on the ceiling, this is an easy step.

    Some garages have boards laid across the truss bottoms to create storage space above them. If that’s your situation, measure how high those boards are off the ground.

    If your garage is unfinished and you can see to the roof, measure to the highest point – the height of the peak.

    Step 4 – Consider the Level of Insulation

    The default choice for this category is “Poor.”

    However, if you’ve added insulation to the ceiling, then select Poor or Average.

    If your garage ceiling and walls are insulated, then Average or Good is your choice.

    When in doubt, choose the lesser level of insulation. Again, we wouldn’t recommend this for a house, but garages generally don’t have the same levels of insulation a home does unless they are built and insulated with the understanding they will be finished and used as a shop, etc.

    Step 5 – Select Attached or Detached

    The BTUs needed for an attached garage are slightly less since one or two sides adjoin heated space, i.e., the house.

    You’re Done! Once you’ve made all the inputs, double-check them for accuracy.

    Watts and BTUs – One watt equals 3.412 BTUs.

    Also, 1,000 watts = 1 kW or kilowatt. We mention this because some electric heaters are listed in kW instead of watts.

    A 10 kW heater = 10,000 watts.

    Heater Size Based on Garage Size

    We punched in data for common garage sizes in all the Climate Zones. Zones 1 and 2 aren’t represented because garage heaters are not at all common in those regions. However, the Calculator can still be used to determine the right heater size in those Zones.

     Heating BTU needed by garage size:

    Size of Garage

    Warm Climate 

    (Zone 3)

    Moderate Climate 

    (Zone 4)

    Cold Climate 

    (Zone 5&6)

    Very Cold Climate 

    (Zone 7)

    10 x 20 (small garage)

    8,000 BTU

    9,000 BTU

    10,500 BTU

    12,000 BTU

    12 x 20 (1 car garage)

    9, 600 BTU

    10,800 BTU

    13,000 BTU

    14,400 BTU

    16 x 20 (2 car garage)

    14,400 BTU

    16,200 BTU

    18,800 BTU

    21,600 BTU

    20 x 20 (2 large car garage)

    16,000 BTU

    18,000 BTU

    21,000 BTU

    24,000 BTU

    30 x 20 (3 car garage)

    22,400 BTU

    25,200 BTU

    29,000 BTU

    33,600 BTU

    36 x 20 (4 car garage)

    28,800 BTU

    32,400 BTU

    38,200 BTU

    43,200 BTU

    * The table is based on average insulation and detached garage. You can use this formula (1 Watt = 3.412 BTU) to convert if you use an electric heater.

    How Much Space Will a Heater Heat?

    This table shows common heater or furnace sizes in BTUs. Remember, if you want to know how many watts that is, divide the BTUs by 3.4. That means the 50,000 BTU heater/furnace, for example, is equivalent to about a 15,000 watt, or 15 kW, electric heater.

    Garage sizes that can be heated by heater BTU size:

    Garage Heater Size

    Warm Climate 

    (Zone 3)

    Moderate Climate 

    (Zone 4)

    Cold Climate 

    (Zone 5&6)

    Very Cold Climate 

    (Zone 7)

    20,000 BTU

    500 sq ft

    450 sq ft

    370 sq ft

    330 sq ft

    25,000 BTU

    620 sq ft

    550 sq ft

    470 sq ft

    420 sq ft

    30,000 BTU

    750 sq ft

    660 sq ft

    560 sq ft

    500 sq ft

    45,000 BTU

    1125 sq ft

    1000 sq ft

    850 sq ft

    750 sq ft

    50,000 BTU

    1250 sq ft

    1100 sq ft

    950 sq ft

    820 sq ft

    60,000 BTU

    1500 sq ft

    1300 sq ft

    1120 sq ft

    1000 sq ft

    75,000 BTU

    1875 sq ft

    1650 sq ft

    1420 sq ft

    1250 sq ft

    80,000 BTU

    2000 sq ft

    1750 sq ft

    1500 sq ft

    1300 sq ft

    * The table is based on average insulation and detached garage.

    Tips for Making Your Garage More Efficient

    There are two ways that garages lose heat:

    • Lack of insulation
    • Lack of an air barrier / house wrap

    Most homeowners focus on insulation only, but even a well-insulated space can lose heat quickly if air is leaking in and out of drafty windows, an entry door and that loose-fitting garage door.

    Garage Insulation

    Try these tips to lower energy use in the garage. Yes, they’ll make the garage more comfortable too, but the emphasis here is on efficiency.

    1 – Add insulation overhead

    In a finished garage, this will mean putting insulation in the open space above the ceiling. We recommend blown-in insulation rather than batts; because it better covers the entire “floor” of the space above the garage. In the event your garage features vented soffits around the perimeter, use cardboard to keep insulation off the soffits. This will maintain proper ventilation.

    Storage space? If the space above the ceiling is dedicated to storage, and you don’t want to insulate it, you have options.

    Rigid Foam – Foam boards could be glued to the ceiling. No, not very attractive. But functional. They could also be placed beneath the platform you’re using to store things on above the ceiling. Some are tough enough that you could even place them on top of the plywood or OSB platform, and store things on top of the foam boards.

    Spray Foam – While pricier, you get better insulation, a higher R-value, per inch of foam. This is especially true if you choose closed cell foam.

    The foam can be applied to the “floor” of the space above the garage or to the underside of the roof. We definitely recommend you consult an insulation specialist about the best application for spray foam in the garage ceiling/roof.

    2 – Add Wall Insulation

    You’ve got the same basic options as overhead – blown-in insulation, fiberglass batts and spray foam insulation.

    Batts are DIY friendly, and DIY spray foam kits are available too.

    If the walls are covered in drywall but not insulated, then blowing in insulation by cutting small holes in the top of the drywall between each stud is the best way to insulate the wall cavities.

    3 – Insulate the Garage Door

    DIY garage door insulation kits are available that make the job pretty easy. Foam boards can also be cut and glued to the inside of the garage door panels.

    4 – Seal Doors and Windows

    Use spray foam and weatherstripping around all garage windows and an entry door. If you feel any air coming in around the framing of the garage door, insulate that too.

    Many studies have shown that the ROI for adding insulation is outstanding. If you heat your garage a significant part of the year, you will recoup the cost of insulation through lower energy bills in 2-5 years based on your climate and the level of insulation you choose.

    House Wrap

    If your garage is more than 20 years old, it might not have been wrapped with Tyvek or similar material when built.

    The next time you replace the garage siding, plan to add house wrap / vapor barrier to make the garage’s envelope tighter and less prone to air leaks that waste heat energy.

    The Most Popular Garage Heating Types

    Vented gas heaters like large Mr. Heater units, vent-free propane heaters and electric heaters are all popular. We recommend gas and propane in very cold climates. Electric is acceptable for moderate and mild climates. 

    Natural gas heaters: These are plumbed in with a gas line, so you’ll need to have municipal gas running to your home. 

    Propane gas heaters: In rural areas or where you don’t want the expense of plumbing a gas line, a propane heater is ideal. Both vented and vent-free, clean-burning units are available. Just be sure you know which type you have, so that you can vent it if needed. Manufacturers recommend that vent-free models be used with some ventilation. In a garage, this might mean the entry door or a window slightly open. 

    Electric garage heaters: There are several styles. But all use some form of electric resistance heat, usually with multiple heating coils and a fan to distribute warm air. Most are ceiling-mounted.  

    Tips for Making Your Garage Warmer

    These aren’t efficiency tips, but tips to make your garage more comfortable whatever the insulation value is – though we really do recommend adding insulation.

    Use a Bigger Heater!

    If you turn the garage’s heater or furnace off when you’re not using the garage, then chances are it’s pretty cold sometimes when you want to get to work in there.

    This is less the case if you keep it at a minimum heat level like 50 or 60 degrees F.

    When you want the space heated up quickly, a bigger heater or furnace is the answer. Some experts recommend choosing a heater up to twice the size for fast warming.

    Our recommendations for a fast warm-up of the garage are:

    • Zones 3 & 4: 1.5 times the Calculator result
    • Zones 5 & 6: 1.75 times the Calculator result.
    • Zone 7: 2 times the Calculator result.

    Take the insulation level into consideration too. A well-insulated garage will heat up more quickly due to less heat loss.

    Again, this is for fast warm up. This is not energy efficient.

    Use a Timer or WiFi Thermostat

    If you know you want the garage warm at a certain time of day, the heat source should start 30-60 minutes before that time. This saves energy compared with leaving the heater on. 

    • 60+ minutes for a poorly insulated garage
    • About 30 minutes for a well-insulated garage

    Electric heaters can be plugged into a timer that is plugged into an outlet. Some come with built-in timers. If it is an electric furnace controlled by a thermostat, you can program the thermostat for when you want the furnace to start. 

    Any furnace can be controlled by a thermostat with WiFi capability. Use the included app on your smart device to start the furnace ahead of when you want to use it.

    Gas vs Electric vs Propane Pros and Cons

    Here are the relative advantages of electric furnaces and gas furnaces (propane or natural gas). An oil furnace has the same pros and cons as a gas furnace.

    Electric Furnaces: They cost less for the equipment, and they don’t have to be vented. That reduces installation cost, though you’ll probably need to run a 220-volt line to the garage.

    There’s no danger of carbon monoxide poisoning either.

    The bad news about electric furnaces is that they cost much more to operate than gas and oil furnaces. The price difference in electricity and other fuels varies by region, but using an electric furnace is going to cost up to twice as much to operate.

    Gas Furnaces: It’s the opposite with gas. The upfront cost is higher for both equipment and installation.

    But your energy costs will be significantly lower. Gas furnaces often heat the space more rapidly too.

    What about ventless propane heaters? These are a popular option for many garage owners. You get the quick heat gas provides without the cost of installing a vent - the best of both worlds. The cost of propane is higher than the cost of natural gas, but in most parts of the country, it is cheaper than the cost of electricity. The most cost-effective way to use propane is to have a “pig” installed, a tank outside the garage with a line running directly to the heater. Tank sizes vary from about 100 or 150 gallons to 1000, with 300 gallons a good size for a garage heater that gets plenty of use. 

    Talk with several propane providers because local prices vary from one to another. Some providers charge rent on the tank unless you buy a minimum amount of propane each year - often in the range of 150 to 300 gallons.

    There is more information on natural gas vs propane vs electric in this Pick HVAC Guide.

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