Ratings decoder
Nominal Size Is a Name, Actual Size Is a Measurement
Two filters, the same printed size, and a quarter of an inch between them on every face.
The same thing, read twice — Filterbuy 20x20x1 MERV 8 and Simply 20x20x1 MERV 8
What the printed size says
20x20x1. That is the nominal size, and it is what both of these filters have on the frame and in the product name.
What the listings state as actual
One states 19.50 x 19.50 x 0.75 in. The other states 19.75 x 19.75 x 0.75 in. Same nominal name, a quarter inch apart on both faces, from two manufacturers.
Both readings come from the listing for Filterbuy 20x20x1 MERV 8, 4-pack, which states: listing states an actual size of 19.50 x 19.50 x 0.75 in, electrostatic pleated, made in USA.
What is actually going on
Nominal size is a naming convention. It describes the slot a filter is meant for, rounded to whole inches, and it has been used that way long enough that everyone treats it as a measurement. It is not one.
Actual size is what a tape measure gets. Filters are built under the nominal figure so they slide into a slot rather than jam in it, and the amount they are built under is a decision each manufacturer makes for itself. In this catalog the gap on a nominal 20-inch face runs from 0.12 in to a full 0.50 in depending on who made the filter.
The depth is where it catches people hardest. Every one-inch filter listed here states an actual depth between 0.75 and 0.81 in. Not one of them is an inch. A slot with a retaining rail sized around 0.81 in leaves visible play around a 0.75 in frame, and air takes the path around the media rather than through it.
Four-inch filters follow the same convention and the same trap. The four-inch filter in this catalog states an actual depth of 3.63 in. That is a cabinet-mounted filter, and a cabinet built for a deep filter cannot be served by a one-inch frame regardless of how the faces line up.
The practical move is the boring one. Before ordering, measure the filter that is in the slot right now — all three dimensions, with a tape, on the frame itself. Then read the actual size in the listing rather than the nominal name in the title. That is the whole of it, and it takes about a minute.
Lined up against each other
| Nominal size on the frame | Actual sizes stated in this catalog | Spread between the listings |
|---|---|---|
| 20x20x1 | 19.50 x 19.50 x 0.75 in and 19.75 x 19.75 x 0.75 in. | A quarter inch apart on both faces, from three listings at this nominal size. |
| 20x25x1 | 19.68 x 24.68 x 0.81 in and 19.62 x 24.62 x 0.78 in. | Same manufacturer, two ratings, and the frames still differ. |
| 16x25x1 | 15.50 x 24.50 x 0.75 in and 15.68 x 24.68 x 0.81 in. | Roughly 0.18 in apart on each face between two manufacturers. |
| 20x30x1 | 19.81 x 29.81 x 0.81 in and 19.88 x 29.88 x 0.81 in. | The tightest pair here, within 0.12 in of the nominal name. |
| 20x25x4 | 19.50 x 24.50 x 3.63 in. | A nominal four-inch filter measuring well under four inches deep. |
What this scale does not tell you
- Whether a particular frame will seat in your slot, which depends on rails and retainers you have to look at.
- What another manufacturer will state for the same nominal size, since the figure varies by maker.
- Anything about rating or resistance — size and MERV are independent decisions.
- Whether the slot itself was cut to the nominal dimension in the first place.