What Size Air Filter Do I Need? Find Your Size in 30 Seconds
Finding your filter size takes 30 seconds if you know where to look, and buying the wrong size is the single most common filter mistake we see. Here is the foolproof way.
Step 1: Read the frame of your old filter
Pull your current filter out of the return grille or the slot next to your furnace. The size is printed on the cardboard frame edge, something like 16x20x1 or 20x25x4. That printed number is the nominal size, and it is the size you order. Done. That is the whole method for almost everyone.
Step 2 (only if there is no filter): measure the opening
- Measure the width and height of the slot or grille opening in inches.
- Measure the depth of the track (usually 1″, sometimes 2″ or 4″).
- Round each measurement UP to the nearest standard size. A 15.6 x 19.6 opening takes a 16x20 filter.
Which way does the airflow arrow point?
While you have the old filter out: the arrow on the frame points in the direction air flows, toward the furnace/blower and away from the return grille. If you are replacing a filter in a wall or ceiling return, the arrow points into the duct, away from you.
The most common sizes in American homes
These twelve cover the majority of US systems, and ATC sells every one:
| Size | Typical spot |
|---|---|
| 14x20x1 | Furnace slot or return grille |
| 12x24x1 | Furnace slot or return grille |
| 16x20x1 | Furnace slot or return grille |
| 14x25x1 | Furnace slot or return grille |
| 18x20x1 | Furnace slot or return grille |
| 16x25x1 | Furnace slot or return grille |
| 20x20x1 | Furnace slot or return grille |
| 16x25x4 | 4-inch filter rack (change every 6 to 12 months) |
| 20x25x1 | Large return grille |
| 20x25x4 | 4-inch filter rack (change every 6 to 12 months) |
| 24x24x1 | Large return grille |
| 20x30x1 | Large return grille |
Odd size? Ask us.
Older homes and specific OEM cabinets sometimes take non-standard dimensions, including exact-cut fractional sizes like 19.875x21.5x1. We list 136 sizes. If yours isn't one of them, send us your measurements on our custom size page and we'll check what our supplier can make, then reply with price and lead time. If you are staring at a weird measurement, call (270) 534-5492 and a tech will sort it in two minutes.