Free size finder
What size furnace filter do I need?
Read it off the edge of the filter you have now. If the edge is blank, measure the slot and pick the closest size here.
Width and height can be either way round: 25x16 is the same filter as 16x25.
Your size
Pick a common size, or a width and height, to see your size.
Nominal and actual size
The size printed in big type, like 16x25x1, is the nominal size. It is rounded up, and it is what stores sell by.
The filter itself is usually about half an inch smaller each way and three quarters of an inch thick. If you measured 15-1/2 x 24-1/2, buy 16x25.