Find your furnace model number
The model number is on a label inside the furnace's front door.
Turn the power switch off before removing a door.
Where the label is
- Upflow furnace (basement or closet, air comes out the top): inside the lower door, usually on the left or right wall of the cabinet.
- Horizontal furnace (attic or crawlspace): on the door facing the access side.
- Downflow furnace (air comes out the bottom): inside the upper door.
Reading the label
Look for "Model" or "M/N". Ignore the serial number ("S/N"). Type the model number exactly; extra letters at the end are fine, we match the start.
No label? Measure the old filter
- Slide the old filter out of its rack or grille.
- Read the size printed on its edge, e.g. "16x25x1". That is the size to buy.
- If nothing is printed, measure width and height to the nearest inch and the thickness. Filters are sold a bit under their stated size (a 16x25x1 really measures about 15.5 x 24.5 x 0.75), so round up.
If your home has a filter grille in a wall or ceiling, that grille sets the filter size, not the furnace.