Furnace Filter Sizes: What Size Air Filter Do You Need?

When was the last time you changed the air filter in your furnace? 

Choosing the right furnace filter size for your home is very important. It will help maintain your furnace’s efficiency and keep your home’s air clean.

In this article, I’ll go over how to find out what size furnace filter you need. I’ll also give you a few tips and tricks when it comes to changing your furnace’s air filter.

How to find your furnace filter size – 3 different ways

When you change furnace filters, the first thing you want to do is know what size furnace filter you have. The furnace filter needs to be able to fit snugly into its slot.

If the filter is too big, then it won’t fit into the filter slot and you won’t be able to put the cover back on. This can cause dirty air to infiltrate the inside of your furnace (and home).

If the filter is too small, then there will be a gap around the air filter. This allows dirty air to get around the filter. It’s almost like having no filter at all.

Worse yet, a small filter won’t completely fit into the filter slot. The filter might implode and get stuck in your blower wheel, causing a bunch of damage.

Below I’ll go over 3 different methods of finding your furnace filter’s size. So no matter what situation you’re in, you’ll be able to figure what size filter goes in your furnace.

Check the old air filter for its size

The first (and easiest) way to figure out your furnace filter’s size is to check your old air filter for its size.

Most air filters have their size printed on the side of the frame. The length, width, and depth of the filter are measured in inches. For example, it will say something like “16x25x1”, where 16” is the length, 25” is the width, and 1” is the depth.

Once you know the old air filter’s size, you can easily go and purchase a new filter with the same dimensions.

Most air filters have their size printed on the side of the frame

Measure the old air filter

If your old air filter doesn’t have the measurements printed on its side, then you can measure the filter.

Use a measuring tape to measure the length, width, and depth of the air filter. Keep in mind that the actual size of the air filter will be smaller than the nominal size of the air filter.

For example, an air filter with a nominal size of 16”x25”x1” will have an actual size of 15-½”x24-½”x3/4”.

Basically, when you take measurements of an air filter, you just need to round up to the nearest inch to find the dimensions of the filter that you need to purchase.

Instructions on how to measure an air filter

Measure air register or air filter slot

If all else fails, then you can always measure the slot that the air filter goes in. You’ll need to do this if your furnace is missing its air filter and you don’t know what size the air filter is supposed to be.

Keep in mind that the air filter slot will be the actual air filter size. So the filter slot will measure a little bit smaller than the size of the air filter that you need to purchase.

Air filter MERV rating

Knowing the size of your air filter is only one piece of the puzzle. The other thing that you need to know is the MERV rating that you should use.

The air filter’s MERV rating is a measurement of how effective an air filter is at removing particles from the air. A higher MERV rating means that an air filter is more effective at removing smaller particles from the air.

To learn more about air filter MERV ratings, check out my article below:

Where is the furnace filter located?

The air filter on a furnace is usually located in the return air intake at the furnace. Look for where the return duct connects to your furnace, and there should be a plate that covers the slot that the air filter goes in.

Some homes have HVAC systems that do not have air filters at the air handling unit. Instead, the air filter is located at the return vent inside the home.

If your furnace has an air filter at the return vent, then changing the filter is easy since it can be done from the inside of your home. You just need to make sure that you can reach the return vent safely to swap out the filter.

For more information about return vent filters, check out my article below:

Can you stack air filters to make them fit?

No, you should not stack air filters to make them fit in a larger slot.

If you stack air filters, you will greatly reduce airflow through your furnace. This will reduce airflow and make your furnace work much harder to heat up your home.

In the long term, this can actually burn out your blower motor since it will struggle to blow air through your system.

Instead of stacking filters, you should get the correct filter size to begin with. This ensures that you won’t restrict too much airflow through your furnace and keep everything running smoothly.

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