How to Find the Right Furnace Filter

1. Find the existing filter

Common locations include a return-air grille, a slot beside the furnace or air handler, or a dedicated filter cabinet.

2. Read the printed size

The cardboard frame usually shows a nominal size such as 20x25x1. Use all three numbers. The last number is the nominal thickness.

3. Check actual dimensions

Nominal and actual dimensions can differ slightly. When fit is uncertain, compare the actual measurements printed on the frame or measure the old filter.

4. Measure when the old filter is missing

Measure the inside width, height, and depth of the slot. Do not force an oversized or thicker filter into the opening.

5. Confirm airflow direction

Install the new filter with its arrow pointing toward the blower or air handler.

Next step

Use the FilterMatch Finder after you know your size.