How to Measure Your Air Filter Size
To measure an air filter, read the nominal size printed on the side of the old filter (length x width x depth). If it's missing, measure the actual length, width, and thickness with a tape measure and round up to the nearest standard nominal size — or order a custom filter built to the exact actual dimensions.
Step-by-step
- Find the printed nominal size on the filter frame (e.g. 16x25x1).
- If unreadable, remove the filter and measure length, width, and depth.
- Record measurements to the nearest 1/8 inch (this is the actual size).
- Round actual size up to the nearest standard nominal size for off-the-shelf filters.
- For a perfect fit on odd sizes, order a custom filter using the actual dimensions.
Measuring tips
Always note the orientation and airflow arrow when removing the old filter. Measure the filter itself, not the duct opening, unless you're sizing a custom filter to a slot — in which case allow a small clearance so the new filter slides in without binding while still sealing.
Frequently Asked Questions
Should I order nominal or actual size?
Order the nominal size for standard off-the-shelf filters. Provide the actual measured size only when ordering a custom filter built to exact dimensions.
Which number is length and which is width?
By convention filters are listed length x width x depth, but for a rectangular filter the first two numbers are interchangeable as long as the filter fits the slot. The third number is always the thickness/depth.
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Need help choosing the right filter?
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