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Furnace filter size

20x25x4 Furnace Filters

A different piece of equipment from a one-inch return slot, not a thicker version of one.

NOMINAL — PRINTED ON THE BOX20 x 25 x 4 inACTUAL — AS STATED19.50 x 24.50 x 3.63 in0.50 in under nominal on both faces
Depth — 3.63 in stated, against a nominal 4 in

A four-inch filter goes into a purpose-built cabinet, usually bolted to the furnace with its own door and its own sealing arrangement. It is not a one-inch return slot that happens to be deeper, and the two are not interchangeable in either direction.

The single filter in this catalog at this size states an actual dimension of 19.50 x 24.50 x 3.63 in. All three numbers are under nominal, and the depth is under by more than a third of an inch — worth knowing before deciding whether a cabinet gasket will close on it.

Only one option is stocked here at this size, so there is no rating comparison to make on this page. What there is instead is the reason people end up on it: a one-inch filter was tried in a four-inch cabinet, and air went around it.

What this catalog carries at 20x25x4

One filter, so there is nothing here to compare it against.

Check the one you have first

  1. Confirm you have a cabinet rather than a slot: a four-inch filter sits in a housing with a door, not a gap in the ductwork.
  2. Measure the filter that came out, all three dimensions, and measure the cabinet opening as well.
  3. Note the arrow direction on the old frame before it leaves the cabinet.
  4. Check the stated actual depth in the listing against the cabinet, since a deep filter that is a third of an inch short changes how the door and gasket meet.

The arrow, and the schedule

Cabinet filters sit in rails with a door closing on them, so a reversed one is hidden until the next service. The arrow points toward the blower here as it does on a one-inch filter, and it is worth photographing the old one in place before it comes out.

Deep filters hold more media, and manufacturers generally state longer intervals for them than for one-inch filters. Follow the interval stated for the specific filter and whatever the system documentation says, rather than assuming depth alone buys extra months.

A higher MERV rating is not an upgrade by default

A deeper media bed can present less resistance than a one-inch filter at the same rating, because the air passes through more surface area. That is a property of the cabinet design, not a license to raise the rating. The system manufacturer documentation states the filter rating and pressure drop the equipment is designed around, and that decides. A licensed HVAC professional can advise where the documentation is unavailable.

The 20x25x4 filters in this catalog

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Four-inch depth
Filterbuy 20x25x4 MERV 8, 2-pack

Filterbuy 20x25x4 MERV 8, 2-pack

  • Nominal 20x25x4
  • Actual 19.50 x 24.50 x 3.63 in
  • MERV 8
  • 2 per box

The only four-inch filter in this catalog. The listing states an actual size of 19.50 x 24.50 x 3.63 in, which is well under four inches on the depth as well as half an inch under on both faces. A four-inch cabinet is a different piece of equipment from a one-inch slot, and a one-inch filter will not sit correctly in it.

What the listing supports, and what it does not
  • Actual size is stated on all three dimensions including the depth
  • A deeper media bed holds more before it loads up
  • MERV 8 in a deep frame keeps the rating moderate where cabinets allow depth
  • Electrostatic pleated construction and US manufacture are stated
  • Only fits systems built with a four-inch filter cabinet, which most homes do not have
  • Stated depth is 3.63 in, so gasketing and rail fit should be checked before ordering more
  • Two per box, and deep filters cost more per unit than one-inch ones

Best forA furnace with a purpose-built four-inch filter cabinet rather than a one-inch return slot.

Check it on Amazon

To repeat what is said at the top of the page: the Amazon links here are affiliate links and may earn this site a commission. Ordering, badges, and every claim on the page were decided from what the listings state, not from any commercial arrangement.

Other sizes and the scales behind them

Sources