How to Size A Furnace Properly

We want to feel cozy inside our house when it’s freezing outside. Nothing is more crucial to ensuring comfort in the winter than a furnace. However, the furnace must be the correct size to operate well. Knowing how to size a furnace is vital in deciding when it’s time for installation or a replacement.

Follow our thorough instructions on precisely determining the best furnace size for your home to avoid these hassles.

Measure Your Home’s Square Footage

The first step is to measure the square footage of your home. It is crucial to note that you are under no obligation to measure for spaces without vents or for spaces you do not intend to heat.

To do this, you’ll need to measure the length and width of each room and multiply those numbers. For example, if a room is 10 feet wide by 12 feet long, its square footage would be 120. Then, add up all the square footage for each room in your house to determine how much space you need to heat.

For example, if your home has the following dimensions:

– 10 x 12 living room

– 15 x 20 family room

– 12 x 12 kitchen

– 30 x 30 garage

The total square footage of your home would be 1,080 square feet (120 + 300 + 144 + 900).

Determine the BTUs Per Square Foot 

The next step is determining how many BTUs (British Thermal Units) of heat you need per square foot. BTU is a measurement unit that quantifies your furnace’s heat output.

For newer homes, 30 to 35 BTUs per square foot is typical. Living in an older home may need as many as 45 BTUs per square foot. And, if any of your rooms have high ceilings (over 8 feet), you’ll need to add 600 BTUs for each room.

We’ll need 30 to 35 BTUs per square foot for our example home. That means we’ll need between 324 and 378 BTUs per square foot (1,080 x 0.3 = 324; 1,080 x 0.35 = 378).

Consider The Climate

When sizing your furnace, you’ll also need to consider the climate. In a milder climate, you can get away with fewer BTUs. However, you’ll need more BTUs to heat your home in a colder climate.

For instance, if you live in Toronto, you’ll only need between 35 and 45 BTUs per square foot. But, if you live in Winnipeg, you’ll need between 40 and 50 BTUs per square foot.

In our example home, we’ll need between 35 and 45 BTUs per square foot because we live in a milder climate.

Consider Other Factors

You’ll need to consider a few other factors when sizing your furnace.

  • Ceiling height: If your ceilings are above 8 feet, you’ll need to add 600 BTUs per square foot.
  • The number of windows: More windows means more heat loss. For every window in a room, add 1,000 BTUs.
  • Doors: Just like windows, doors can also lead to heat loss. Add 400 BTUs for each door in a room.
  • Ceiling fans: Ceiling fans can help circulate heat, so you may be able to reduce the size of your furnace if you have them.
  • Desired temperature: If you like your home to be warmer than most, you’ll need a bigger furnace.
  • Ductwork: The size of your ductwork and furnace may not be compatible, which might lead to issues. For example, if your furnace’s ductwork is too small, the furnace’s lifetime will be shortened since the air that the furnace generates won’t have enough room to pass through. As a result, the airflow will be restricted, leading to an overheated furnace. Ensure that the furnace you choose fits your ducting properly.
  • The number of people in the house: Because the human body radiates heat into the air around it, it will need fewer BTUs to heat if it often accommodates many people.


When it comes to sizing a furnace, there are several factors you’ll need to consider, including the square footage of your home, the climate, and other features like windows and doors. By considering all these things, you can be sure to choose a furnace that’s the right size for your home. If you need assistance, contact a furnace specialist. They can help you determine the perfect size for your needs.

Contact Furnace Repair Toronto for all your furnace repair and replacement needs in the GTA. We are a team of certified technicians serving the Greater Toronto Area for many years. Our team is available 24/7 for emergency services. Call us today!