Furnace filter size: nominal vs actual.

Short answer

A filter measuring 15.75 x 24.75 x 0.75 inches is a nominal 16x25x1: round each dimension to the nearest whole inch. In the other direction a nominal 16x25x1 usually measures about 15.5 to 15.75 by 24.5 to 24.75 by 0.75 inches. Always measure your old filter, because brands and deep cabinet filters vary.

How it works, with a worked example

A filter is sold by its nominal size, a rounded size such as 20x25x1, and is made slightly smaller so it slides into the slot. Filterbuy shows a nominal 20 x 25 x 1 measuring 19.5 x 24.5 x 0.75 and says nominal and actual typically differ by 0.25 to 0.5 inch. For depth it lists nominal 1 inch as 0.75, 2 as 1.75 and 5 as 4.75 inches; its 4-inch figure appears as 3.75 in one table and 3.63 in another, so measure a 4-inch filter. To find the nominal size from a measurement, round each dimension to the nearest whole inch.

Worked example. You measure 15.75 x 24.75 x 0.75 inches. Rounded: 16 x 25 x 1, so you order a 16x25x1 filter. From 16x25x1 going the other way, length and width usually run 0.25 to 0.5 inch under, so 15.5 to 15.75 by 24.5 to 24.75, and depth 0.75 inch. In millimetres, 400 x 635 x 19 mm is 15.75 x 25.0 x 0.75 inches, also a nominal 16x25x1.

Where it breaks. Atomic Filters notes that dimensions can round down or up independently and some brands run larger than nominal, and that brand-specific deep filters have their own actual sizes. For example a Lennox X6670 labelled 16x25x5 measures 15 3/4 x 24 3/4 x 4 3/8 inch, so a 4.375 inch depth is a nominal 5, not 4. Use the printed size on your old filter when it is there.