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 15 1/2″ x 19 1/2″ x 3/4″. 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. It is cut a little smaller on purpose so the filter slides into your track or grille without binding. The frame around your opening covers the gap.
On most 1 and 2 inch filters, each face is 1/2 inch smaller than the label and the depth is 1/4 inch smaller. On 4 inch filters, each face is 5/8 inch smaller and the depth is 3 3/4 inches. A few sizes run a little different, like 24x24, which measures 23 3/8 inches square. For AAF's standard sizes we show AAF's published measurements. For other sizes the actual size shown is our best estimate, so check it against your old filter.
| Printed (nominal) | True measurement (actual) |
|---|---|
| 16x20x1 | 15 1/2″ x 19 1/2″ x 3/4″ |
| 20x25x1 | 19 1/2″ x 24 1/2″ x 3/4″ |
| 24x24x1 | 23 3/8″ x 23 3/8″ x 3/4″ |
| 16x25x2 | 15 1/2″ x 24 1/2″ x 1 3/4″ |
| 16x25x4 | 15 3/8″ x 24 3/8″ x 3 3/4″ |
| 20x25x4 | 19 3/8″ x 24 3/8″ x 3 3/4″ |
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.
Whole-home media cabinets are different
Media cabinets from Honeywell, AprilAire, Lennox and Carrier take their own filters, cut to fit the cabinet instead of the standard 4-inch size. Two filters can both say 16x25 and still not swap. For example, a Honeywell 16x25 media filter measures 16″ x 24 7/8″ x 4 3/8″. Our standard 16x25x4 measures 15 3/8″ x 24 3/8″ x 3 3/4″. If you have a media cabinet, use our custom size page: enter the actual size printed on your old filter and put any part number in the notes.
The exception: exact-cut 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 return grilles. If your old filter has fractions on the frame, order exactly those fractions. The common ones have their own pages here, and you can request others on our custom size page.