furnacefiltersize.com
Odd furnace filter sizes, and when to order custom.
Hardware stores stock the four common 1-inch sizes (16x20x1, 16x25x1, 20x20x1, 20x25x1). Everything else is an online order. (BTU Size.)
Decision tree
Printed size exists → buy it. Do not “improve” it to 20x25x1.
Measurement within ~0.25 in of a whole inch → that nominal. This finder treats empty-slot remainders 0.00–0.12 and 0.88–0.99 as near-whole (19.9 → 20). See thresholds.
True fraction (17.5, 22.25, 23.75, 19.25, 19.75) → custom. Rounding either way costs the seal.
Media cabinet sticker → cabinets, not custom. An Aprilaire 213 is not a custom 20x25x4.
How to write the custom order
Custom is ordered by exact measured dimensions, not rounded nominal. Measure to 1/8 in. Usually non-returnable. (BTU Size; Filterbuy custom: 3/8 in to 30 in any dimension, cut to the number you enter.)
Write: L × W × D to 1/8 in, airflow arrow, MERV, quantity.
Filterbuy custom example: a 20x22.25x1 they cut measures 20.00 × 22.25 × 0.75 — the 0.5-in-under rule stops at custom. (Filterbuy size chart.)
- Do not trim a rigid pleated filter. Cut-to-fit rolls are for washable frames and window units, not a furnace slot. (BTU Size.)
- Usual suspects for odd slots: older homes, site-built returns, some Rheem / American Standard air handlers. (BTU Size.) We will not claim a brand “always” takes an odd size without an IOM cite.
- A too-small filter bypasses: air takes the gap. EPA: the filter must fit snugly, without leaking around it or being bent to fit. (EPA Guide, cited on Filterbuy’s size page.)
Doesn’t match the chart
Affiliate on this page: Filterbuy custom primary; Nordic Pure secondary (made-to-order). Amazon only if a close standard size actually exists — never as “close enough.”
Finder · size chart · cabinets.