What does nominal size mean on an air filter?

The rounded, printed size used for labeling and shopping. Actual dimensions run about 3/8 to 1/2 inch smaller on the face and about 1/4 inch shallower, so the filter slides into its slot. A nominal 16x25x1 typically measures about 15.5 x 24.5 x 0.75 inches.

How do I find my filter size without the old filter?

Measure the slot or return grille opening (length, width, depth) and round each face dimension up to the nearest standard nominal size. A slot measuring about 15.6 x 24.6 x 0.8 inches takes a nominal 16x25x1. The filter should slide in with light friction, not force and not rattle.

Can I use a slightly smaller filter than the slot?

A loose filter leaks — air bypasses around the edges unfiltered, which defeats the rating you paid for. If no standard size fits well, several manufacturers cut custom sizes for a small premium, or a properly sized filter can be sealed with a gasket. Never downsize and leave a gap.

Sizing · The nominal/actual trap

Air filter sizes: why the box lies (politely)

Every filter wears two sizes: the tidy nominal size printed in big type, and the actual dimensions it really measures — a fraction of an inch smaller so it slides into the slot. Most sizing frustration is these two numbers being confused. Two minutes here ends it permanently.

Verified: June 10, 2026

Example: the most common sizeNominal vs actual

16x25x1 ≈ 15½ × 24½ × ¾″ actual

Faces run ⅜–½″ under the printed size; depth about ¼″ under. Exact undercut varies by brand.

Size decoder

Runs locally in your browser. Many filters also print actual dimensions in small type on the frame edge — worth checking before measuring.

Measuring when no label survives

Three reliable paths, in order of preference. The old filter: measure its actual length, width and depth, then round each face dimension up to the nearest whole inch for the nominal size (24.5″ → order 25). The slot: measure the opening's height, width and depth; the filter you want has actual dimensions just under those numbers, so order the nominal size the slot implies. The unit's manual or sticker: furnace and air-handler cabinets often list the filter size near the door. Whichever path: a correct fit slides in with light friction. Force means too big; rattle means too small — and a rattling filter is leaking unfiltered air around its edges, which quietly cancels whatever MERV rating you paid for.

The standard sizes

Common nominal sizes and typical actual dimensions (inches)
Nominal	Typical actual	Nominal	Typical actual
10x20x1	9½ × 19½ × ¾	18x20x1	17½ × 19½ × ¾
12x12x1	11½ × 11½ × ¾	18x24x1	17½ × 23½ × ¾
12x24x1	11½ × 23½ × ¾	18x25x1	17½ × 24½ × ¾
14x14x1	13½ × 13½ × ¾	20x20x1	19½ × 19½ × ¾
14x20x1	13½ × 19½ × ¾	20x24x1	19½ × 23½ × ¾
14x25x1	13½ × 24½ × ¾	20x25x1	19½ × 24½ × ¾
15x20x1	14½ × 19½ × ¾	20x30x1	19½ × 29½ × ¾
16x16x1	15½ × 15½ × ¾	24x24x1	23½ × 23½ × ¾
16x20x1	15½ × 19½ × ¾	25x25x1	24½ × 24½ × ¾
16x25x1	15½ × 24½ × ¾	16x25x4 / 20x25x4	15⅜ × 24⅜ × 3¾ / 19⅜ × 24⅜ × 3¾
16x24x1	15½ × 23½ × ¾	16x25x5 / 20x25x5	≈ 15¾ × 24¾ × 4⅜ (brand-specific)

Undercuts are conventions, not standards — one brand's 20x25x1 can differ from another's by an eighth of an inch, and 4–5″ media cabinet filters are the most brand-specific of all (Honeywell, Aprilaire and Lennox cabinets each expect their own actual dimensions). Buying a case? Match the actual dimensions of the filter that fit well, not just the nominal size.

When nothing standard fits

Older homes and some air handlers use slots no chart covers. You're not stuck: several filter manufacturers cut custom sizes to the sixteenth of an inch for a modest premium, usually with a minimum order of a few filters. That beats the two common improvisations — cutting down a bigger filter (destroys the frame seal) or leaving a gap (unfiltered bypass). If the slot is within ¼″ of a standard size, a standard filter plus foam gasket tape on the slack edge is a legitimate fix.

Common questions

Length or width first — does order matter?

Convention is smaller dimension first (16x25, not 25x16), but retailers index both. Depth always comes last. The decoder above accepts either orientation.

Does a deeper filter fit my 1″ slot if I push?

No — depth is the one dimension with zero forgiveness. A 1″ slot takes a 1″ nominal (¾″ actual) filter, full stop. If you want the real benefits of 4–5″ media (more surface area, lower resistance, longer life), that's an HVAC tech installing a media cabinet, typically a few hundred dollars and worth it for many systems.

My filter size is fine but airflow seems weak. Size problem?

More likely a loading or rating problem: a clogged filter (see the replacement guide) or a high-resistance filter the blower dislikes (see the pressure-drop section of the MERV guide). Size shows up as rattles and bypass dust streaks, not weak flow.

Sources

Manufacturer sizing specifications (printed actual dimensions on filter frames; media-cabinet documentation from Honeywell, Aprilaire, Lennox)
ASHRAE 52.2 terminology for filter classification context