furnacefiltersize.com

Furnace filter sizes: nominal vs actual.

The label is the size you order. The cardboard is smaller so it seats. Furnace filter and AC filter are the same slot in a forced-air house (Filterbuy size chart; Trane Filters 101).

Filterbuy production actuals for 1-inch (their cut, not an industry standard — they say this): length/width 0.5 in under nominal; 1-inch depth actual 0.75 in; 2-inch actual 1.75 in; 4-inch actual 3.63 in; 5-inch actual 4.75 in. Retrieved 31 Aug 2026 from Filterbuy’s size chart. This is a short table, not their full 29-row production list copied as if it were ours.

Common rows for day one. Source: Filterbuy size chart, retrieved 31 Aug 2026.
Nominal	Filterbuy actual (their cut)	Typical slot
14x20x1	13.5 × 19.5 × 0.75	Furnace slot
14x25x1	13.5 × 24.5 × 0.75	Furnace slot
16x20x1	15.5 × 19.5 × 0.75	Return grille
16x25x1	15.5 × 24.5 × 0.75	Common furnace slot
20x20x1	19.5 × 19.5 × 0.75	Filterbuy: “most common US size”
20x25x1	19.5 × 24.5 × 0.75	Air handler / large slot
16x25x2	15.5 × 24.5 × 1.75	Thicker furnace slot (Filterbuy 2-inch rule)
20x25x2	19.5 × 24.5 × 1.75	Thicker furnace slot (Filterbuy 2-inch rule)
16x25x4	15.5 × 24.5 × 3.63	Media cabinet
20x25x4	19.5 × 24.5 × 3.63	Media cabinet — not an Aprilaire 213
16x25x5	15.5 × 24.5 × 4.75	Media cabinet — verify by cabinet
20x25x5	19.5 × 24.5 × 4.75	Media cabinet — verify by cabinet

If this is a media cabinet, stop using this table. Look up the sticker: cabinet model → part number. A 20x25x4 Filterbuy will not substitute for an Aprilaire 213.

Brand drift on the same nominal (July 2026)

Filterbuy’s comparison, retrieved 31 Aug 2026. We have not re-measured these SKUs yet. Amazon Basics 20x25x4 was marked “[VERIFY]” on that chart — we do not repeat unverified numbers.

Brand	20x25x1 actual	20x25x4 actual
Filterbuy	19.50 × 24.50 × 0.75	19.50 × 24.50 × 3.63
Filtrete	19.68 × 24.68 × 0.81	19.88 × 24.63 × 4.31
Amazon Basics	19.75 × 24.75 × 0.75	not published here (unverified on the source chart)

BTU Size notes Filtrete 16x25x1 at 15.69 × 24.69 vs house brands at 15.5 × 24.5. Same point: labels match, filters do not.

Photo needed: Tape on Filterbuy vs Filtrete 20x25x1, same ruler.

Our own 20x25x1 actuals are still outstanding. This page cites Filterbuy’s July 2026 comparison until we photograph SKUs.

Aprilaire 213 is the counter-example

Official shop lists nominal 20 × 25 × 4 and actual 20.6 × 26.63 × 4 — larger than nominal because of Self-Seal. Retrieved 31 Aug 2026 from shop.aprilaire.com. A 20x25x4 Filterbuy (19.5 × 24.5 × 3.63) will not substitute. See Aprilaire cabinets.

Finder

Odd rows: Filterbuy (custom plant). The four hardware-store sizes (16x20x1, 16x25x1, 20x20x1, 20x25x1): Amazon as in-stock backup. Links are placeholder searches, rel=nofollow sponsored.

Full finder · odd and custom · cabinets · MERV.