The Ultimate Furnace Size Calculator: 4 Easy Steps

Most people who need a new furnace are probably intimidated by the prospect. It’s understandable: furnaces are a major investment, and it’s hard to know how to get started. 

Given the intimidation factor, most homeowners will hire an HVAC company to size their home and do all of the furnace selection and installation for them. While that’s certainly what we would recommend you do as well, it’s also a good idea to go through the sizing process on your own to make sure your results align with the professionals. 

Fortunately, this process isn’t nearly as complicated as it might seem

As professional furnace installers with over 100 years in business, we’ve helped thousands of homeowners like yourself find the right size furnace. Below, you’ll find the easy four-step guide we use to get the job done so you can accurately size up your home yourself.

Step #1: Determine the BTUs your furnace must output

Generally speaking, the more heating capacity your furnace needs to provide, the larger it needs to be. The size of a furnace is typically measured in BTUs (British Thermal Unit, a unit of heat) per square foot. One BTU is the amount of heat required to raise the temperature of a pound of water by one degree Fahrenheit.

Getting the BTUs your furnace needs to put out is one of the most important parts of furnace sizing. If you buy a furnace with too little heating capacity, your system won’t be able to keep up. It will run constantly, driving up your heating bill, while struggling to even bring your home up to the correct temperature. If you buy a furnace that produces too many BTUs, on the other hand, it will heat your space too quickly, cycle on and off too quickly, and wear out faster than it should.

The number of BTUs your furnace should put out depends on several factors, including the age of your home, the status of your insulation, and your home’s overall heat loss. The number one determinant is the climate you live in. 

The colder your winters, the more heat output you’ll need. If you live in Texas, for example, you may only need to turn your furnace on occasionally throughout the winter months. Accurately accounting for your regional climate in your BTU calculation is the first step to sizing your furnace correctly.

Use the table and map below to determine how many BTUs are necessary for your climate zone in the winter. Then, keep it handy for the calculations we’ll need to do in Step 3.

Image credit: University of Hawaii Hilo

Step #2: Estimate Your Home’s Square Footage

In addition to weather and climate, the size of your home also has a direct impact on the heating capacity you’ll need. The more space you need to heat, the more heat output you’ll need to heat the entire area effectively.

For some homeowners, this is a relatively simple step that just requires you to look up your home’s square footage from the original appraisal, listing, or lease. However, if you don’t know your home’s square footage and can’t find it listed anywhere, you’ll need to break out the tape measure to get each room’s length and width. Avoid measuring rooms that don’t have a duct (like a garage), because they’re not connected to your home’s heating system and you don’t want it to inflate your measurement.

To measure the square footage of your rooms, measure the length and width of the room in feet using your tape measure. Then, multiply the length by the width. 

Square footage = Length of room X width.

If a room in your home is an unusual shape, try dividing it into several rectangular-shaped sectors and then add them together.

After you’ve calculated the square feet of each of the room’s in your home, add all of them together to determine the total square footage of your home.  To give you an idea of how you might make the assessment on your own, take a look at our sample calculations below.

Family Room:    24 ft. X 15 ft. = 360 square feet

Dining Room:    10 ft. X 15 ft.= 150 square feet

Kitchen:    25 ft. X 12 ft.= 300 square feet

Bedroom 1:     20 ft. X 15 ft.= 300 square feet

Bedroom 2:    20 ft. X 13 ft. = 260 square feet

Bathroom 1:   10 ft. X 16 ft. = 160 square feet

Bathroom 2:   12 ft. X 8 ft. = 96 square feet

Basement:     25 ft. X 22 ft. = 550 square feet

Laundry Room:    7 ft. X 10 ft. = 70 square feet

Total Square Footage:  2,246 square feet

Step #3: Calculate the Necessary BTU Output

The next step involves a bit more math, but if you’ve followed the above steps to a T, it’s a lot easier.

Take your home’s square footage and multiply it against the BTUs your climate requires. We included an example for you below:

2,000 square feet X 40 BTUs = 80,000 BTU output required

Step #4: Be Sure to Check Furnace Efficiency

Thanks to the equation, we know we need 80,000 BTUs of heat. But before we buy an 80,000 BTU furnace, there’s furnace efficiency to consider as well. For instance, a furnace with 80,000 BTU input and an 85% efficiency rating will only produce 68,000 BTUs. We need something stronger. A 100,000 BTU furnace with an 80% efficiency rating or higher will produce the 80,000 BTUs necessary and then some.

A furnace’s efficiency rating is called its AFUE, or Annual Fuel Utilization Efficiency. This number represents how efficiently the furnace converts its fuel source into BTUs of heat. For example, a gas-burning furnace converts (on average) 100 units of natural gas into 80 BTUs of heat.

You can find a furnace’s AFUE rating either on its cabinet or in its user manual. If you’re interested in buying a very high-efficiency furnace, you could also see if it has earned the official ENERGY STAR® label from the U.S. Department of Energy. This means it has achieved an AFUE of at least 90 percent.

Keep in mind that a furnace’s AFUE rating may not remain accurate, especially as it gets older. To keep your furnace as efficient as it should be, have it regularly maintained by a professional every year. You can even ask your technician to doublecheck your AFUE rating during installation or maintenance.   

A Furnace That’s “Just Right”

When it comes to your home’s heating, the last thing you want to do is take Goldilocks’ approach to finding a solution. Playing around with the BTU output of your home’s furnace can result in major energy costs, not to mention a majorly cold winter. Instead, use the three steps above to find the furnace that’s “just right” for your home.

Still not sure which furnace you should buy? Learn more about the different types of furnaces.