Nominal vs actual air filter size
The size printed on a filter frame is a name, not a measurement. Makers cut the filter slightly smaller so it slides into the slot.
What the two terms mean
- Nominal size is the rounded size you order by, such as 16x25x1. Nordic Pure calls it “a simplified, rounded figure.”
- Actual size is the real cut. Nordic Pure says “an air filter labeled 16x25x1 might have actual dimensions closer to 15.5×24.5×0.75.”
Actual sizes the makers publish
| Nominal | Filterbuy actual (in) | Glasfloss ZL exact (in) |
|---|---|---|
| 16x25x1 | 15.5x24.5x0.75 | 15-1/2 x 24-1/2 x 7/8 |
| 20x25x1 | 19.5x24.5x0.75 | 19-1/2 x 24-1/2 x 7/8 |
| 16x20x1 | 15.5x19.5x0.75 | 15-1/2 x 19-1/2 x 7/8 |
In these three sizes, both makers cut the length and width about half an inch under nominal. The depth differs: 0.75 in for Filterbuy, 7/8 in for the Glasfloss ZL. That is why a filter from a new brand can feel tighter or looser in the same slot.
When nominal size misleads
- Media cabinets. A 4 in or 5 in cabinet often takes a maker-specific filter. AprilAire's 213 has a nominal size of 20H x 25W x 4D but an actual size of 20.6H x 26.63W x 4D, larger than nominal. See the AprilAire 2210 page.
- Custom slots. If your measurement does not land near any standard nominal size, you may have a custom slot. Measure the slot and the old filter, and buy by measurement.
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