Furnace Filter Sizing Guide

Why Is It So Important to Get the Right Size Furnace Filter?

The most important reason is that if you select the wrong size filter, and want to return it, you'll be responsible for the cost of shipping it back to us. 

Additionally, the filter(s) must be in the exact same condition as when received, i.e. like new - no damage, or dirt from trying to install it in your furnace, etc... We will not accept damaged filters.   

This type of return policy is common with online filter retailers.  In fact, most of our competitors also charge restocking fees and make you pay the original shipping cost if your order qualified for a free shipping promotion.  

Check out other Canadian online furnace filter retailers return policies - after reading these, we're sure you'll find that our return policy is actually one of the more reasonable ones out there.

Read our full return policy.

Aside from return policies, getting the right size and type of filter for your furnace is essential for other reasons. First, if it's too big, it's not even going to fit in your furnace, and if it's too small, it could permit air to leak out and reduce the efficiency of your furnace, costing you money in the form of higher energy bills.

In addition, the gaps around a filter that’s too small can also allow dust and other airborne particles to pass around it and not be effectively captured, reducing the air quality in your home. This can result in allergies flaring up, faster dust accumulation throughout your home, and other air quality problems that could easily be avoided by installing the right size filter.

Ensuring that you know exactly what size and type of filter you need for your exact make and model of furnace before you buy can save you money and time, and a lot of headache.

How Are Furnace Filters Measured?

Furnace filters are rectangularly shaped, and are sized according to all three measurements of length, width, and depth (thickness). The vast majority of furnace filters utilize a standardized measuring format in inches, with the width and length being the first two values, and thickness or depth being the last value. For example, a filter that measures 16” wide by 20” long, with a thickness of 1” will be shown as 16x20x1.

Image courtesy of HVAC Training Shop.com.

Now, it’s important to know that there will be a difference between the standardized filter size and the precise physical measurement of the filter itself. Being aware of this is essential to ensuring that you select the correct size when you purchase replacements. If you were to take a tape measure and jot down the exact dimensions of a furnace filter, you’ll notice they won’t come in at perfect 1” increments. Using the example above of a 16x20x1 filter, the precise physical dimensions will likely be closer to 15 1/2” x 19 1/2” x 3/4”. This is normal, as filter brands have to account for tolerance variances between furnace manufacturers.

The actual precise dimensions of the filter are referred to as the ‘actual size’, whereas the simplified ‘rounded up’ dimensions of filters are known as the ‘nominal size’. In most cases, both values will be listed on the filter packaging and in the product details on filter websites. In general, the nominal size will be much more prominently visible and will be what you’ll primarily look for when you shop for replacement furnace filters.

How Can I Tell What Size Furnace Filter I Need?

Identifying exactly which size of filter your furnace needs is actually quite easy. There’s several different places you can quickly look to find this information. Here’s how you can do it!

Method #1 – Check the outer frame and casing of the existing furnace filter for size information. Pretty much every furnace filter will have markings on it that specify the make, model, and size, so this should be easy to find. Simply look for a sequence of numbers that follows the same format as the 16x20x1 example we discussed earlier. The actual numbers might be different, but they’ll always appear in that sequence. Jot this size down for future reference, as you’ll want to have it handy when you go shopping for replacement filters.

Method #2 – Refer to the furnace owner’s manual for specifications on the size and style of filter recommended for that particular model. Inside the manual, there should be a list of parts and components, including the filter, which will specify the dimensions of the size for reference when purchasing a replacement. Alternatively, if the owner’s manual is not available, you can always visit the furnace manufacturer website. By searching for the model number of your furnace, you should be able to download a digital copy of the manual from the manufacturer to review and find the precise furnace filter size you need.

Method #3 – Measure the existing filter with a tape measure. Switch your furnace off temporarily to ensure it is not operating, and then remove the existing filter. Using your tape measure, jot down each linear measurement (width, length, and depth). Remember that the nominal size of a furnace filter will be rounded up to the nearest inch from the actual measured size. For example, this means a width of 15 3/4” should be rounded up to 16”. With the nominal size determined, you are now ready to shop for replacement furnace filters of the correct size.

Method #4 – If none of the previous methods work, perhaps because the make and model of furnace is unknown or the old filter is missing and can’t be measured, you can attempt to measure the physical opening inside the furnace/ductwork where the filter is to be installed. Using a tape measure, start by measuring the tight inside dimensions of the space where the filter sits. Take note of any raised tabs or edges where the filter will contact the metal of the furnace/duct, as these will sometimes cause the actual filter size to be smaller than the total opening.

Similar to how the actual size of a filter is incrementally different from the nominal size, the opening in the furnace/duct itself will likely be slightly larger than the filter size dimensions. For example, a filter opening in a furnace that measures 16 ½” x 20 ¼” x 1 ¾” will most likely require a filter size of 16x20x1 to account for the small gaps needed to fit the filter in the space without being too tight.

What Are the Most Common Home Furnace Filter Sizes?

Furnace filters come in a huge array of different sizes, but most residential furnaces in Canada will use a furnace filter that is one of a smaller set of fairly standard options. Chances are pretty good that your home furnace will require replacement filters in one of the following sizes:

• 16x20x1
• 16x25x1
• 20x20x1
• 20x25x1
• 20x25x5

If you see the same size listed here as what’s shown on your existing furnace filter, the good news is that replacements will be very easy to find, particularly here at Dan the Filter Man! We’ve got filters in all those standard sizes in stock and ready to ship direct to your home.

What If I Have Other Questions About Furnace Filters?

We’d be glad to help! Simply reach out to us with your question and we’ll get back to you soon with all the details you need. You can also check out our other helpful references here at Dan the Filter Man, such as our Guide to MERV Ratings, our Frequently Asked Questions, and our blog. We’re always publishing new articles filled with handy tips and advice you can use to lower your energy bills, improve the air quality in your home, and get more value for your money in each replacement furnace filter you buy.

Thanks for stopping by, and we hope you’ve found this information helpful!