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Nominal vs Actual Filter Size Chart

Why a 16x25x1 filter does not measure 16 by 25 by 1 inches, and how to order the right filter from the actual size. Actual sizes vary by maker, so always check the frame.

Ref HV-0048AHRI 680

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

HVAC filters are sold by nominal size, a rounded name such as 20x25x1. The actual size is a little smaller, so the filter slides into a rack or grille built to the nominal opening. Most makers make the face about 1/4 to 1/2 in smaller than nominal and the depth a little thinner. There is no single fixed rule for every brand. AHRI 680 covers performance rating of residential air filters, and makers print both sizes on the frame.

How to identify

Look at the frame edge. Most filters print the nominal size in large type and the actual size in smaller type.
If there is no label, measure the width, height and depth of the old filter, or measure the rack or grille opening.
Order by nominal size for standard filters. Order by actual size for custom or odd-size filters.
Dimensions are usually listed as width x height x depth. The airflow arrow points toward the blower.

Typical size chart

Nominal size (in)	Typical actual size (in)
14 x 20 x 1	13.5 x 19.5 x 0.75
16 x 20 x 1	15.5 x 19.5 x 0.75
16 x 25 x 1	15.5 x 24.5 x 0.75
20 x 20 x 1	19.5 x 19.5 x 0.75
20 x 25 x 1	19.5 x 24.5 x 0.75
20 x 25 x 2	19.5 x 24.5 x 1.75
16 x 25 x 4	15.5 x 24.5 x 3.75
20 x 25 x 4	19.5 x 24.5 x 3.75
20 x 25 x 5	Varies widely by media cabinet

Typical actual sizes are common values, not a standard. Check the brand's listed actual size.

Key points

4 and 5 in media filters are often made for one cabinet brand. A 5 in filter from one maker may not fit another maker's cabinet. Order by the cabinet model.
Custom or exact-size filters are ordered by the actual size. Tell the supplier which kind of size you are giving.
A filter that is too small lets air bypass. A filter that is too big gets bent to fit and also leaks.
Depth matters as much as face size. A 2 in filter will not seal in a 1 in rack.

Common mistakes

Measuring the old filter and ordering that measurement as the nominal size. The new filter comes out undersized.
Swapping 4 in and 5 in media between cabinet brands.
Installing the filter with the airflow arrow backward.

Governing standards: AHRI 680 (residential air filter performance rating); filter and cabinet manufacturer size data.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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