Furnace and HVAC filters are sold by nominal size — the rounded number printed on the frame, like 16x20x1 or 20x25x1. The actual size is about a quarter to half an inch smaller in each direction, so the filter can slide into its slot. A “16x20x1” filter really measures roughly 15½ x 19½ x ¾ inches.
That gap trips people up in both directions: measuring the old filter with a tape and ordering that exact number gets you a filter that’s too big, and assuming the printed number is exact gets you confused when the new one seems small. Buy by the printed nominal size, always.
A handful of nominal sizes cover most homes: 16x20x1, 16x25x1, 20x20x1, 20x25x1, 14x20x1, and 12x20x1. Depth matters too — most systems take 1-inch filters, but many newer air handlers use a 4- or 5-inch media filter instead. Thicker filters hold more dust, restrict airflow less for the same rating, and only need changing every 6–12 months instead of every 1–3.
Once you know the size, the other decision is the filtration level — the MERV rating. Higher numbers catch finer particles but can strain systems that weren’t designed for them. We break down what the ratings actually mean (and decode 3M’s MPR and Home Depot’s FPR scales) in our guide to MERV, MPR, and FPR ratings.
One-inch filters are a true consumable — you’ll change them every 1–3 months, more often with pets or allergies. Per-filter prices drop sharply in 4- and 6-packs, and the same multipack can be priced very differently from store to store. Compare before stocking up.
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