MadMax Trade Supply

Home › Guides

What size furnace filter do I need?

The size you need is the nominal size printed on the cardboard frame of your current furnace or air-conditioner filter, for example 16x25x1 (width x height x depth, in inches). That is also the size used to buy a replacement. Do not measure only the slot and guess a size if the old filter is still readable.

Nominal vs actual size

The actual filter is usually a little smaller than the nominal size so it slides into the slot. We list measured sizes only when they already appear in a product name (some 16x25x5 and 20x25x5 grille filters in our catalog do). For other sizes we do not have measured actuals and we do not invent them.

Match the depth

A 4-inch media cabinet needs a 4-inch filter, not a 1-inch one. If you only have a 1-inch slot, stay with a 1-inch filter and change it more often.

Then pick a MERV rating

After you have the size, choose a MERV rating your system can handle. See How to choose an HVAC air filter MERV rating.

Sizes we carry

Each size, and each size plus MERV combination, has its own page.

Shop HVAC air filters Size lookup

Related categories: HVAC Air Filters. General information only; follow the equipment maker's instructions and local codes. Updated 2026-11-14.