1. Pull out your current filter. It's usually behind the return-air grille or in a slot at the air handler. Slide it out like a drawer.
2. Read the size printed on the cardboard frame. It looks like this:
   16 x 25 x 1
   That's length × width × thickness, in inches.
3. No printing left? Measure it with a tape: length, width, and how thick it is. Round to the nearest whole inch.

Every size in the Draynix plan is a 1-inch-thick filter. If yours is thicker (4–5 inch media filters), your size isn't in the plan yet — sized right or we make it right, so reach out from the store and we'll help.