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Filter Size Finder
Quick answer: Enter the measured length, width and depth of your old furnace or AC filter, or of the empty slot. The finder rounds an old filter up to the next whole inch, or a slot down, and maps depth to 1, 2, 4 or 5 inches. You get the nominal size to buy, the typical actual size and the face area.
How it works
Filters are sold by nominal size but built about ½ inch smaller on length and width so they slide in. So a measured filter is rounded up: 15½ × 24½ becomes 16 × 25. An empty slot is a little larger than nominal, so it is rounded down: 16⅛ × 25¼ becomes 16 × 25. Depth around ¾ inch maps to 1 inch, around 1¾ to 2 inches, and roughly 3½–4½ inches to 4-inch media; deeper maps to 5-inch. The smaller face dimension is listed first.
Worked example
A filter measures 19⅝ × 24½ × ¾ inches. Entering 24.5, 19.625 and 0.75 with mode 1 gives a nominal 20x25x1, typical actual about 19½ × 24½ × ¾ inches, and about 3.47 ft² of face area.
Illustrative note (2026-10-29): Example measurements were chosen on 2026-10-29 for illustration.
Limits
Some brands are closer to nominal, and deep media cabinets need a cabinet-specific depth. Confirm against the old filter's label or your equipment manual. HVAC air filters only.