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

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 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
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.

 Heating BTU needed by garage square footage:

Garage Size (sq ft)Garage Heater Size
300 sq ft9,000 – 18,000 BTU
400 sq ft12,000 – 24,000 BTU
500 sq ft15,000 – 30,000 BTU
600 sq ft18,000 – 35,000 BTU
700 sq ft20,000 – 40,000 BTU
800 sq ft23,000 – 46,000 BTU
900 sq ft26,000 – 52,000 BTU
1000 sq ft29,000 – 58,000 BTU
1100 sq ft32,000 – 64,000 BTU
1200 sq ft35,000 – 70,000 BTU
1300 sq ft38,000 – 75,000 BTU
1400 sq ft40,000 – 82,000 BTU
1500 sq ft45,000 – 88,000 BTU
1600 sq ft47,000 – 94,000 BTU
2000 sq ft48,000 – 116,000 BTU
BTU size ranges based on where you live. We highly recommend using our above garage heater calculator to get an accurate btu size.

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 BTU500 sq ft450 sq ft370 sq ft330 sq ft
25,000 BTU620 sq ft550 sq ft470 sq ft420 sq ft
30,000 BTU750 sq ft660 sq ft560 sq ft500 sq ft
45,000 BTU1125 sq ft1000 sq ft850 sq ft750 sq ft
50,000 BTU 1250 sq ft1100 sq ft950 sq ft820 sq ft
60,000 BTU1500 sq ft1300 sq ft1120 sq ft1000 sq ft
75,000 BTU1875 sq ft1650 sq ft1420 sq ft1250 sq ft
80,000 BTU2000 sq ft1750 sq ft1500 sq ft1300 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.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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