SizingUpdated June 2, 2026
How to measure your air filter (the right way)
Most filter returns are sized to a nominal dimension that's not what the filter actually measures. Here's how to avoid the trap.
First, check the existing filter label — printed dimensions are the nominal size (e.g. 20x25x4).
If there's no label, measure the slot, not the filter — width and height to the nearest 1/4 inch, then depth.
Round up to the closest standard size. Filters are slightly undersized to slide in.
If you're between sizes (e.g. 19.5x24.5), match to the nominal 20x25 — that's what's on Filtermap.
Stay ahead of your filter
Get a reminder when it's time to change.
Filtermap sends a friendly email when your specific filter is due — and the best Amazon price the day you need it.