Find the furnace filter that actually fits

Order by the printed size on the frame. Your tape measure will read smaller — that gap is normal. Pick your size below for nominal vs actual dimensions, MERV guidance, and where to buy.

Start with the size printed on your current filter’s cardboard frame (not what the tape says).

20×25×1 16×20×1 soon 16×25×1 soon 20×25×4 soon 20×20×1 soon 14×25×1 soon 14×20×1 soon 16×16×1 soon

Browse the full size index →

How to measure (60 seconds)

  1. Turn the system off. Pull the filter from the slot or return grille.
  2. Read the printed size on the frame edge (e.g. 20×25×1). That is the nominal size — order that.
  3. Optional check: measure the frame’s outside width, height, and thickness. Expect roughly ¼–½″ under on each face, and about ¾″ thick for a “1-inch” filter.
  4. Note the airflow arrow direction before you slide the new one in.
Do not order the tape-measure reading. Retailers sell by the printed (nominal) size. Brands cut the same nominal slightly differently — if the slot is tight, compare the brand’s published “actual size” to what is in your rack.

Quick MERV primer

MERV (Minimum Efficiency Reporting Value) rates how well a filter catches particles. Higher is finer — and usually more airflow resistance. Per the U.S. EPA:

  • MERV 8 — everyday dust and pollen; least restrictive for most 1″ slots.
  • MERV 11 — better for pets and mild allergies; a common residential step-up.
  • MERV 13 — finer particles and smoke; only if your blower and slot can handle the pressure drop (EPA: use as high a rating as your system can accommodate).