furnacefiltersize.com
Furnace filter size finder.
If a size is printed on the old filter, buy that exact printed size. Ignore the tape. (BTU Size, July 2026.)
Why two rounding rules
Measuring the old filter: measure to the nearest 1/8 in, then round each face up to the whole inch. 19.5 × 24.5 × 0.75 → 20x25x1. Round up, never to nearest, or you buy a gap. (BTU Size.)
Measuring an empty slot or return grille: measure frame-edge to frame-edge; round to the nearest whole inch. The slot is built near nominal; the filter is cut small. (BTU Size. Filterbuy’s size-chart page instead says measure the slot and round up unless you land on a true fraction — we follow the two-rule version and say so.)
True fractions (17.5, 22.25, 23.75) are custom slots. Rounding either way costs the seal. Order the measured inches. Custom is usually non-returnable. (Filterbuy size chart, “What if my slot measures between sizes?”; BTU Size custom section.)
Do not cut a rigid pleated filter to fit. (BTU Size.)
Thresholds this finder uses (IA §4.3 rule 6)
After snapping to 1/8 in:
- Empty slot: remainder 0.00–0.12 or 0.88–0.99 → nearest whole inch. Remainder 0.13–0.87 → custom.
- Old filter: round up unless remainder 0.13–0.37 or 0.63–0.87 (true-fraction custom). So 19.5 rounds up; 19.25 and 19.75 stay custom.
We can change those bands after we measure real slots. They are in js/finder.js and the test file, not buried in prose.
Related: nominal vs actual chart · when not to round · MERV · 1-inch MERV 13 (draft) · cabinets · how often.