20x25x4 AC Filters: Why One Size Can Mean Three Different Filters

You check your old filter, it says 20x25x4, you order a new 20x25x4... and it doesn't fit. You're not doing anything wrong. That size is one of the most confusing in the filter world, because it can mean three different filters.

We get this question all the time at Ned Air Filters, so here's how to tell which one you actually have.

Why one size has three meanings

Filter sizes are "nominal," which means they're rounded for labels and boxes. With 1-inch filters that rounding rarely causes trouble. But thick media filters are designed to slide into a specific cabinet, called a media air cleaner, and each brand built its cabinet a little differently. So several filters ended up being called 20x25x4 even though they're not interchangeable.

1. The standard 20x25x4 (3 5/8 inches thick)

This is what most people mean by a 4-inch filter. The pleated filter is about 3 5/8 inches deep, and it fits the many generic 4-inch filter cabinets and some furnace and air handler slots.

If you measure your old filter and it's closer to 3.5 inches than 4.5, this is the one you need.

2. The Honeywell-style filter (4 3/8 inches thick), often sold as 20x25x5

Honeywell media air cleaners, and the many compatible replacements made for them, use a thicker filter of about 4 3/8 inches. The extra depth means more surface area to catch dust, so they usually last longer than a standard 4-inch.

Here's where the confusion comes in: Honeywell calls it a 20x25x4, while many other brands sell the exact same size as a 20x25x5. Same filter, two names.

3. Aprilaire 210 and 213 filters

Aprilaire media air cleaners take their own replacement filters, such as the 210 and 213. Because they're roughly 20 by 25 inches on the outside, they get called 20x25x4 too, but they're built for Aprilaire's cabinet and aren't a match for the other two.

If your cabinet has the Aprilaire name on it, go by the filter's model number rather than its size.

How to tell which one you have

• Measure the thickness of your old filter. About 3 5/8" means a standard 4-inch. About 4 3/8" means Honeywell-style (20x25x5).
• Look at the cabinet. A thick filter usually slides into a metal box on the side of your air handler or furnace. The brand, often Honeywell or Aprilaire, is usually printed on its door or a sticker.
• Check the old filter's label for a brand name or model number, like Aprilaire 210 or 213.
• Take a photo of the filter label and the cabinet. If you're still not sure, send them to us and we'll tell you exactly what you need.

What happens if you buy the wrong one?

Usually it just won't fit. A filter that's too thick won't slide in, and one that's too thin leaves gaps. Gaps are the bigger problem, because air sneaks around the filter instead of through it, carrying dust straight into your system. Never force a filter in or leave the cabinet door off to make one fit.

How often to change a thick filter

Thick filters last much longer than 1-inch ones. As a rough guide, a standard 4-inch lasts around 4 to 6 months, and a 5-inch or Aprilaire filter around 6 to 12 months, sooner if you have pets, allergies or construction nearby. Our guide on how often to change your AC filter in San Antonio heat goes into more detail, and if you've never swapped one before, see how to find, check and change your AC filter.

Frequently asked questions

Is 20x25x5 the same as 20x25x4?

Sometimes. A 20x25x5 is the same filter as a Honeywell-style 20x25x4 (about 4 3/8" thick), but it's not the same as a standard 20x25x4 (about 3 5/8" thick). Measure to be sure.

Can I put a standard 4-inch filter in a Honeywell cabinet?

It's not a good idea. It's too thin for the cabinet, which leaves room for air to slip around it. Use the filter made for your cabinet's depth.

Are Aprilaire filters the same as other 20x25x4 filters?

No. Aprilaire 210 and 213 filters are made for Aprilaire air cleaners. If you have an Aprilaire cabinet, buy by the model number.