How to Read Your Air Filter Size: Nominal vs Actual Size Explained

Air filter sizing can be confusing at first. You might see a filter listed as 20 x 25 x 1, then measure it and realize it is not exactly 20 inches by 25 inches by 1 inch.

Good news: that is normal.

Most air filters have two different size ideas: nominal size and actual size. Once you understand the difference, ordering the right replacement filter gets much easier.

What is nominal air filter size?

Nominal size is the rounded size used to label and order an air filter.

For example, a filter listed as 20 x 25 x 1 is the nominal size. That is the size most people use when shopping for replacement filters.

Nominal size keeps things simple. Instead of listing every filter by its exact measurement, manufacturers use rounded sizes that are easier to read, remember, and shop for.

What is actual air filter size?

Actual size is the real measured size of the filter.

In many cases, the actual size is slightly smaller than the nominal size. That allows the filter to slide into the filter slot, cabinet, or return grille without being too tight.

For example, a filter labeled 20 x 25 x 1 may actually measure closer to 19.5 x 24.5 x 0.75. That does not mean the filter is wrong. It usually means the filter is made to fit a standard 20 x 25 x 1 opening.

Why are air filters smaller than the size on the label?

Filters are often made slightly smaller than the listed size so they can install properly.

If a filter were exactly the same size as the opening, it could be too tight, hard to install, or easy to damage. A slightly smaller actual size gives the filter enough room to slide into place.

This is why the size printed on the filter is usually the best place to start when ordering a replacement.

Where do you find your air filter size?

The easiest place to find your filter size is on the frame of your current filter.

Look around the cardboard frame for printed numbers. You may see something like:

• 16 x 20 x 1
• 20 x 25 x 1
• 16 x 25 x 4
• 20 x 20 x 2

If the printed size is still readable and the filter has been fitting correctly, use that size when ordering your replacement.

How to measure your air filter

If the size is missing, faded, or unreadable, you can measure the filter yourself.

Measure the length, width, and depth of the filter. The depth is the thickness of the filter.

Once you have the actual measurements, round up to the nearest common nominal size when shopping for a standard replacement filter.

For example, if your filter measures about 19.5 x 24.5 x 0.75, you likely need a 20 x 25 x 1 filter.

When should you order a custom size?

Most homes use standard nominal filter sizes. But some filter openings do not match standard sizes.

You may need a custom size if:

• Your current filter has an unusual measurement
• The standard size is too loose or too tight
• The filter slot was custom built
• Your filter is being cut to fit an older or odd sized opening
• You cannot find a standard size that matches closely enough

If your current filter fits well, matching that size is usually the safest move. If it has never fit correctly, measuring the opening may be helpful.

Why filter depth matters

Do not ignore the last number in the filter size.

In a size like 20 x 25 x 1, the last number is the depth. That tells you how thick the filter is.

A 1 inch filter, 2 inch filter, and 4 inch filter are not interchangeable unless the system is designed to accept those depths. A filter that is too deep may not fit. A filter that is too thin may not sit properly in the cabinet or grille.

You can shop common replacement sizes by filter depth, including 1 inch air filters, 2 inch air filters, and 4 inch air filters.

What if your filter is in a return grille?

Some homes use a filter behind a wall or ceiling return grille. These filters may have different depths than a standard filter installed at the air handler or furnace.

If your filter sits behind a return grille, make sure the replacement has the correct width, height, and depth. The filter also needs enough room behind the grille so the door can close without crushing it.

If your home uses this setup, you can shop our return grille filters.

Quick filter size checklist

Before ordering, check these three things:

• Look for the size printed on your current filter
• Measure length, width, and depth if the size is missing
• Make sure the replacement filter fits without bending, forcing, or leaving large gaps

If the filter has been fitting well, the printed size is usually the easiest ordering guide. If the filter has never fit correctly, measure carefully before replacing it.

Final answer: nominal vs actual air filter size

Nominal size is the rounded size used for ordering. Actual size is the real measured size of the filter.

Most air filters are slightly smaller than the nominal size printed on the frame. That gives the filter enough room to slide into place properly.

When ordering a replacement, start with the size printed on your current filter. If the size is missing or the filter does not fit well, measure the filter or opening carefully before buying.

Ready to find your replacement? Shop air filters online from Filters Shipped and choose the right size for your home.