Air Filter Size Finder
Nominal size, actual size, the MERV range that fits your home and a change interval by pets, allergies and household size. Add a repeating reminder in one tap.
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Data: ASHRAE Standard 52.2 MERV scale; EPA indoor air quality guidance on home filtration; DOE furnace and air conditioner maintenance guidance · Updated September 2026
How filter sizes and ratings work
Nominal size is not the real size
The size printed on a filter (say 20 x 25 x 1) is rounded up from its real size, usually 19.5 x 24.5 x 0.75. That is what lets it slide into the slot. If you measure a filter, round each side up to the next whole inch to get the size to shop for.
MERV is how fine the filter strains
MERV runs 1 to 16 for home filters. MERV 8 catches dust and pollen; 11 adds pet dander and mold spores; 13 catches smoke and the finest particles. Most homes do well at 8 to 13. Above 13 the filter pushes back hard on the blower, so it needs a pro’s OK.
The change interval depends on the house
A 1-inch filter in a quiet home runs about 90 days. One pet or allergies brings it to 60; two pets, or pets plus allergies, to 30. Thick 4-inch and 5-inch media filters hold far more and run 3 to 12 months. A dirty filter starves the blower of air, which is a top cause of no-heat and no-cool calls.
Common questions
Check the frame of the current filter first; the nominal size is printed on it. If it is missing, measure the width, length and depth of the filter or the slot and round each up to the next whole inch. This tool does that rounding and shows the actual size too.
Every 90 days for a 1-inch filter in a home without pets or allergies, every 60 with one pet or allergies, every 30 with two or more pets. Check it monthly in the heating and cooling seasons. Thick media filters run 3 to 12 months.
MERV 8 to 11 for most homes, 11 to 13 for allergies, asthma or pets. Higher ratings catch more but restrict airflow; only go above 13 if a pro confirms your blower and ducts can handle it.
A filter that is too restrictive, too dirty or the wrong size lets the blower overheat or freezes the indoor coil. Reduced airflow is behind many of the breakdowns pros see in winter and summer.
A 4-inch or 5-inch media filter has more surface area, so it catches more while restricting airflow less and lasts longer between changes. It only fits systems built with a media cabinet; a pro can add one to many furnaces and air handlers.
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