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How Do I Measure My Air Filter, Or Furnace Air Filter ?

Most air filters have their dimensions stated in their nominal state which simply means the actual dimensions of the filter are rounded up or down. Below is an example of the nominal dimensions, and the actual size of the air filter compared to each other. Why air filter manufacturers began this practice is beyond me. What I do know is that this practice has made buying an air filter very confusing for some folks.

Take the popular 16x25x5. Without even thinking about it I can come up with three 16x25x5 filters that are all different in their actual sizes depending on the brand of the air filter. As a matter of fact, as I write this, a good customer that has been ordering from us for several years just emailed us that this time he ordered the wrong size filter. Below is his situation:

Always ordered:........Honeywell.... 16X25X5 Real Size 15 7/8 x 24 3/4 x 4 3/8

This Time Ordered:....BDP Brand:... 16X25X5 Real Size 15 3/8 x 25 1/2 x 5 1/4
No way the filter he ordered this time will fit his Honeywell filter housing.

The take-away of this story is that you should be very careful when ordering an air filter because the dimensions you see on the filter frame or filter housing are almost never the true dimensions of the filter. These items are very expensive to ship back and forth, not to mention the inconvenience to you. So, double check, double check. Don't assume that if you see a 16x25 by whatever on our site, that this is the filter to order. You need the model number, or the "ACTUAL" dimensions.

I refer to
the 16x25x5 air filter in this article, but these discrepancies in sizes apply to any filter, including 20x20 - 20x25, etc. The table below shows two 16x25x1 that have different actual dimensions. There are more.

See below for more ideas.

Filter A Filter B
Nominal Dimensions 16 x 25 x 1" 16 x 25 x 1"
Actual Dimensions 15½ x 24½ x ¾" 15¾ x 24¾ x 7/8"

air filter sizes


Method 1: If you are currently using a throw-away fiberglass air filter, take the

size written on your existing air filter and deduct 1/4 inch from both sides. For

example, if your throw away filter is: 20 X 20, then you would order

19 3/4 X 19 3/4 actual dimension.


Method 2: Actually measure your existing air filter, but be sure that your existing

filter fits well.


Method 3: Measure the opening provided for the air filter in your furnace, ceiling

filter grill, or wall filter grill and deduct 1/4 inch from both sides. For example, if

the opening provided for the filter in your furnace measures 18 X 20, then you

would order 17 3/4 X 19 3/4


Method 4: Look up the model number of the filter housing, or the part number of

the existing filter