How to Measure Your Furnace or AC Filter

These basic tips will get you perfect air filter measurements!

Choosing the correctly sized air filter is important in order to keep the air clean in your home. An improperly sized filter may have limited performance. Determining the proper air filter size can be confusing since filter packages generally display two sets of dimensions. It is important to understand that these dimension sets are the nominal and actual dimensions of the filter, and the differences between the two.

Basic Measuring Tips

• Use a steel measuring tape.
• Measure your old filter to the nearest 1/8" of an inch, if the size is on a 1/16" mark, then round down to the next 1/8".

Nominal Size:

The dimensions listed on the side of air filter frames and printed largest on air filter packaging are normally the nominal size dimensions. These measurements indicate the length, height, and depth (thickness) of the air filter, rounded up to the nearest inch. An example of nominal dimensions would be 16” x 24” x 1”.

Actual Size:

These are the exact dimensions of your air filter when measuring length, width, and depth (thickness). On air filter packaging this number is typically listed under or to the side of the nominal size, and is normally in a smaller font size. Most actual size measurements include fractions. An example of actual size dimensions would be: 15 ½” x 19 ¾” x ¾”.

How to measure your air filter:

To measure your air filter, use a tape measure to determine the width, height, and depth. If a pre-existing filter is not available, inspect the unit for a listed size. If easily accessible, measure the air filter housing slot, or refer to your furnace instructions manual for more information.

Air filters are grouped and sold by their standard air filter size, but it is important to know the actual dimensions in order to get the best fit. Different brands may have the same nominal sized air filters, but the actual size could be slightly different.