Nominal vs Actual Air Filter Size: Why 16x20x1 Is Really 15.5x19.5
Grab a tape measure and check a "16x20x1" filter: it actually measures about 15.5″ x 19.5″ x 0.75″. That is not a defect, every filter maker does it, and understanding why saves you from the most frustrating wrong-size order there is.
Nominal size vs actual size
The nominal size is the rounded label printed on the frame, 16x20x1. The actual size is the true measurement, intentionally undercut by about a quarter to a half inch on each face dimension and about a quarter inch on depth. The undercut is what lets the filter slide into your track or grille without binding, the frame around your opening covers the gap.
| Printed (nominal) | True measurement (actual) |
|---|---|
| 16x20x1 | 15.5″ x 19.5″ x 0.75″ |
| 20x25x1 | 19.5″ x 24.5″ x 0.75″ |
| 16x25x4 | 15.5″ x 24.5″ x 3.625″ |
| 20x25x5 | 19.5″ x 24.5″ x 4.75″ |
The two rules that prevent every mistake
- Replacing a filter? Order the nominal size printed on the old frame. Never order what your tape measure says.
- No filter to read? Measure the opening, then round UP to the nearest standard nominal size. A 15.5 x 19.5 opening takes a 16x20.
The exception: exact-cut OEM sizes
Some sizes have fractions right in the name, like 19.5x21x1 or 19.875x21.5x1. Those are exact-cut sizes: the printed size IS the true size, made for specific OEM return grilles and cabinets. If your old filter has fractions on the frame, order exactly those fractions, we stock the common ones and cut customs for the rest.