1-inch furnace filters
A 1-inch filter slides into a slot on the furnace (or on the indoor fan unit some homes have instead), or sits behind a large vent cover on a wall or ceiling. Change it every month.
Find the right fit
- Model number. The label on the furnace, or its manual, lists the filter size it takes.
- Size or part number. The size printed on the edge of the old frame, like 16x25x1, is what you match. It is a rounded size; the real frame runs about a half inch smaller, so go by the print, not a tape measure. No print? Measure the old frame and round up to the next whole inch.
- Two or more retailers. Look the model or part number up at two or more retailers. Buy the one listed as a fit for your model, and check the printed size or part number on the box matches.
What to look for
- Pleated filters catch more dust than flat fiberglass ones.
- MERV, MPR or FPR is the rating printed on the package: a score for how fine a dust the filter catches. A higher number catches smaller bits but can slow the air. When in doubt, match what came out.
- The airflow arrow on the frame points toward the furnace.
- Buy a few at once so the next change is ready.
When to swap
Every month, or sooner if it looks gray.
Where to buy
Set a reminder for the next one in the Reminder Tool or the home hunt.