How to Measure Air Filter Size - Simple Easy Steps

How to Properly Measure Air Filter Size

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When the cold weather starts to hit, it is important to take steps to ensure that both your vehicle and home are ready for the change in seasons.

For your vehicle, you need an emergency kit containing sand, a blanket, some matches, a first aid kit, spare gloves, and similar items in your trunk to make sure it is winter-ready.

You should also change your tires to winter-approved tires and get a maintenance check at your local mechanic to confirm your car is in good working condition.

To get your house ready for the winter, it is important to check the windows and doors for any possible leaks that might have formed over the summer months, double-check the shingles on your roof for loose ones or leaks, and have your furnace serviced to make sure it is in top condition. That includes the filter!

A Closer Look

3d rendering illustration of pollen or virus

If you do not want to pay for service on your furnace, it is important that you at least change out the filtering unit to maintain the quality of air in the residence; especially during these months as seasonal viruses run rampant and air circulation is lower than usual because of closed doors and windows.

But how can you tell the correct air filter size to use as a replacement? What if there isn’t any information on what air filter size is the best?

That’s what we are going to discuss today! How to properly measure air filter size so that you don’t have to worry about losing your source of heat during winter months.

Why Change Your Air Filter?

Many people might ask “Why is it important to change the filtering device on your furnace (or central air system)?”.

The reason is simple. It improves the quality of air that you breathe. Clean air filters supply fresher air throughout the entire residence.

This isn’t the only reason, however, as it can also:

  • Remove all debris, allergens, bacteria, and irritants from the air before it is warmed and circulated throughout the house.
  • Help maintain the operating system of your furnace, as a clogged filter can cause internal operating parts to work harder to circulate the air and in turn, cause the parts to overheat.
  • Save on your monthly electricity bill, as a clogged filter will cause the furnace to work harder (as mentioned above), which uses more electricity by allowing the furnace to stay in operation longer to reach the desired temperatures – increasing energy usage.

There are a few good reasons why it’s important to change your filtering device but to complete this task on your own; you need to understand how to properly measure it before buying a new one to replace it.

What You Will Need

Luckily, when measuring your air filter, fancy tools or gadgets aren’t needed nor is it a complicated task. Instead, simple basic items are needed, and a knowledge of how filtering devices work.
ng items:

  • Measuring tape
  • Pencil
  • Paper

You will need to remember the following:

The manufacturers of air filters typically round up the numbers that make up your measurements, so it is important to do the same when shopping for your new replacement filter.

For example, if your dimensions have been measured to read 19.25” x 19.25” x 2.0”, this is considered the actual size of the filter; you will need to look for an air filter that has the measurements of 20” x 20” x 1”, as this is the nominal size of the air filter.

These smaller numbers are simply the way the manufacturer of the air filter ensures a snug fit with easy removal for replacement reasons.

The measurements are close enough to the actual size that the fit is still secure and won’t damage the furnace. We will discuss how to arrive at these three numbers below.

It doesn’t matter if you measure the length or the width first (and what order the numbers are in) because if you turn the filter on its side – it doesn’t change shape!


Now that you are armed with your measuring tape, a pencil to record your measurements and a pad of paper to write them down on – let’s begin on how to properly measure your air filter.

Step One

Remove all sources of power to your furnace (or central air system) to reduce the risk of electrical injury or appliance damage during the measuring and filtering device removal process.

Step Two

Remove the panel that covers the filtering device and place it to one side – this is usually found near the cold intake duct on the furnace (or central air system) as its closeness to the air being drawn in from the residence allows for a higher removal of allergens rate.

Step Three

Mature man taking out a dirty air filter from a home ceiling air return vent. Male removing a dirty air filter with both hands in a house from a HVAC ceiling air vent.

Grasp the current air filter in both hands and slide it up straight for easy removal.

Step Four

Use your measuring tape to measure the length of the air filter in inches – this is your first number. Use inches to measure air filters as that is the same cubic unit that all manufacturers will use in their advertising and marketing.

Step Five

After measuring the length, use the measuring tape to determine the width of the air filters (also in inches) – this is your second number.

Step Six

The final measurement is to determine the depth of the filtering device – this is your last number.

Step Seven

Mark these measurements down to ensure accuracy and if possible, confirm the rounded measurements against the current filter, as some brands have the measurements written on the cardboard bracket around the filter.

Step Eight

Male pushing a clean air filter into place in the ceiling with both hands. One fresh furnace air filter being secured in the intake grid of the white home ceiling.

Reversing the removal process, insert your device back into your furnace (or central air system) without buckling or damaging the filter in any way.

Step Nine

Replace the panel or cover that houses the filter.

Step Ten

Restore all power sources back to the furnace (or central air system) and turn on the appliance to ensure everything is back in working order.

Step Eleven

Head to your local home hardware store or online merchant to purchase new air filters in the proper size. Congratulations – you just properly measured your air filter!


Man holding furnace filter doing thumbs up

Properly measuring your filtering device shouldn’t require expensive service technicians or special equipment and tools – it simply takes a few easy steps and you can then go to your local home hardware store to buy your replacement filters.

These steps can also be used to properly measure the air filters that come in your residence’s central air system if you also have one of those installed.

All you need is a measuring tape and a method of recording the measurements once they have been established.

If for any reason you are unable to properly measure your air filter, simply call the manufacturer of your furnace or central air system to request information on the appropriate air filters to suit your device.

When it comes to keeping our families (including ourselves) healthy, one of the first steps should be to ensure that the air that is breathed is clean and free of allergens, dirt, and bacteria.

Having a clean filtering device does just this and will leave your home smelling fresher during the winter months when doors are tightly closed and windows are not opened to let in a fresh breeze.

To properly measure your filtering device, head to your local home hardware store, and make sure the air you breathe is clean, fresh, and healthy!

How to Measure Air Filter Size - Simple Easy Steps

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