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Furnace air filter — measure slot depth if the size stamp is gone

Furnace filter

If the furnace air filter size stamp is missing, measure the slot depth and face size before you buy a new MERV filter — one beat, no guessing.

measure furnace filter slot depth if stamp gone — related household tip photo
Photo: Mcbdixon / Wikimedia Commons (CC0)

When the furnace air filter cardboard stamp is torn off, guessing furnace filter size leads to a loose or crushed fit. Measure the slot depth and the face width/height with a tape before you shop so the next MERV filter matches the opening, not a faded memory of the old carton.

Measure furnace filter slot depth when the stamp is gone

  1. Pull the old filter straight out. If the size stamp is unreadable, do not toss the filter until you finish measuring the slot.
  2. Measure the slot opening: width, height, and depth (thickness). Note depth carefully — a 1-inch slot will not hold a 4-inch media cabinet filter.
  3. Write the three numbers on tape near the slot or in your phone before you leave for the store or order online.
  4. Buy the matching furnace air filter size, check the airflow arrow, and slide it in so the seal sits flat with no big edge gaps.

Honest stop: if the slot has a warped frame or missing rails, a correct size alone will not seal — repair the track before chasing higher MERV.

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