How to find your air filter size

Most people only need about a minute to find their filter size. Here are three ways to do it, from easiest to most hands-on.

First, find the filter

Before you can read a size, you need to know where the filter lives. In most homes it is in one of two places:

At the furnace or air handler. Look for a slot or a removable cover on the side, top, or bottom of the unit, often where the large return duct meets the equipment.
Behind a return air grille. Some homes have the filter in a wall or ceiling vent, usually a large grille with a latch or a couple of screws.

Some homes have more than one return grille, and each one may hold its own filter. If you are not sure, check every large grille in the house as well as the unit itself. Turn the system off at the thermostat before you pull a filter out.

Method 1: Read the size printed on the frame

This is the easiest way. Nearly every disposable filter has its size printed on the cardboard or plastic frame, usually along one edge. It will look something like 16x25x1 or 20 x 20 x 1. The three numbers are length, width, and depth in inches.

That printed number is the nominal size, a rounded number used for shopping. It is the number you want. You can type it straight into our size lookup or use it when you shop. The first two numbers can usually be listed in either order (16x25 and 25x16 are the same filter), but the depth matters.

You may also see a second, smaller set of numbers with decimals or fractions, such as 15.5 x 24.5 x 0.75. That is the actual size. Our guide to nominal vs. actual filter size explains the difference.

Method 2: Measure the old filter

If the label is worn off or missing, measure the filter itself with a tape measure:

Measure the length and width of the frame from edge to edge.
Measure the depth (thickness) of the frame.
Round each number up to the nearest whole inch to get the likely nominal size. For example, 15.5 x 24.5 x 0.75 becomes 16 x 25 x 1.

Rounding works because actual sizes are usually a little smaller than the nominal size. Most of the time that is about 1/4 to 1/2 inch, so a measurement just under a whole number points to that whole number.

Method 3: Measure the filter slot

If there is no old filter, or it is damaged, measure the opening it slides into. Measure the length, width, and depth of the slot or grille opening, and compare it with the nominal sizes sold in stores. The right filter should slide in without bending and without big gaps around the edges. If your slot measures an unusual size, check our list of common filter sizes. If nothing fits well, you may need a custom-size filter.

Check the airflow arrow

Most filters have an arrow printed on the frame. It should point in the direction the air moves, which is toward the furnace or air handler and away from the return duct or grille. A filter installed backwards may not work as well as it should.

Write it down

Once you know your size, write it somewhere easy to find, such as on the furnace cabinet with a marker, in your phone's notes, or on the inside of a utility closet door. That saves you from pulling the filter out every time you shop. While you are at it, note the date you last changed it. Our guide on how often to change your air filter can help you pick a schedule.

When to ask a pro

If the filter slot is hard to reach, the filter seems to be missing entirely, or you find a filter in a cabinet you don't recognize (such as a thick media cabinet or an electronic air cleaner), check your HVAC manual or ask a licensed technician. Those systems sometimes use special sizes or parts.

Find your filter in seconds. Enter your size in the Air Filter Sizes lookup.

This guide is general information, not professional HVAC advice. Your equipment manual and filter packaging take priority. Some links on this site may be affiliate links. See our affiliate disclosure.