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Furnace filter size

20x20x1 Furnace Filters

The widest spread between stated actual sizes anywhere in this catalog, at a single nominal size.

NOMINAL — PRINTED ON THE BOX20 x 20 x 1 inACTUAL — AS STATED19.50–19.75 x 19.50–19.75 x 0.75 in0.25 to 0.50 in under nominal on the faces
Depth — 0.75 in stated on all three listings, against a nominal 1 in

Three filters in this catalog carry the name 20x20x1. One states an actual size of 19.50 in per face; the other two state 19.75 in. A quarter of an inch, at the same printed size, from different manufacturers.

That gap is the clearest illustration on this site of why nominal sizing is a naming convention rather than a measurement. Neither filter is mislabeled. Both are built to slide into a slot cut for twenty inches, and each manufacturer decided for itself how much clearance to leave.

All three listings state the same depth: 0.75 in, against a nominal one inch. If your slot has a retaining rail sized closer to a full inch, a 0.75 in frame will have play in it, and air takes the easy route around the media rather than through it.

What this catalog carries at 20x20x1

3 filters, at MERV 5 and MERV 8.

Stated figures at 20x20x1

  • Filterbuy 20x20x1 MERV 8, 4-pack

    Actual size stated
    19.50 x 19.50 x 0.75 in
    MERV
    MERV 8
    MPR
    Not stated
    Pack
    4 per box

  • Simply 20x20x1 MERV 8, 6-pack

    Actual size stated
    19.75 x 19.75 x 0.75 in
    MERV
    MERV 8
    MPR
    MPR 600
    Pack
    6 per box

  • Amazon Basics 20x20x1 MERV 5, 6-pack

    Actual size stated
    19.75 x 19.75 x 0.75 in
    MERV
    MERV 5
    MPR
    Not stated
    Pack
    6 per box

Check the one you have first

  1. Take the filter out of the slot and read the nominal size printed on the frame.
  2. Measure the actual frame with a tape — all three dimensions, on the cardboard rather than the pleats.
  3. Look at how the old filter sat: rattling in the slot points to a frame at the small end of the range, binding points to the large end.
  4. Match your measurement to the stated actual size in the listing before ordering a multi-pack of anything.

The arrow, and the schedule

A square filter has no long edge to orient by, which is exactly why the arrow matters more here than anywhere else. Nothing about the shape tells you which way round it goes. Read the arrow, point it toward the blower, and note where the old one faced before it leaves the slot.

Follow the interval the filter manufacturer states, and whatever the system documentation says. Two of the three options at this size come six to a box, which is eighteen months of quarterly changes — worth storing somewhere clean and dry rather than in a damp basement corner.

A higher MERV rating is not an upgrade by default

This size is stocked here at MERV 5 and MERV 8, both in the band most residential systems are specified around. That is not a reason to assume any rating is fine. Finer media raises resistance to airflow, and the system manufacturer documentation states the rating and pressure drop the equipment is designed for. If that document is unavailable, ask a licensed HVAC professional rather than guessing upward.

The 20x20x1 filters in this catalog

Links to Amazon on this page are affiliate links. If you buy through one, this site may earn a commission at no additional cost to you. Nothing on the page was tested here and no product placement was paid for.

Actual size stated
Filterbuy 20x20x1 MERV 8, 4-pack

Filterbuy 20x20x1 MERV 8, 4-pack

  • Nominal 20x20x1
  • Actual 19.50 x 19.50 x 0.75 in
  • MERV 8
  • 4 per box

Nominal 20x20x1, and the listing states an actual size of 19.50 x 19.50 x 0.75 in — a full half inch under the printed number on both faces. That is the widest gap of any 20x20x1 listing in this catalog, which is precisely why the actual figure being published matters. MERV 8, electrostatic pleated, four to a box, made in USA per the listing.

What the listing supports, and what it does not
  • The actual size is printed in the listing rather than left to the nominal number
  • MERV 8 sits in the range most residential systems are specified around
  • Four filters per box covers a year at a quarterly interval
  • Electrostatic pleated construction and US manufacture are both stated
  • At 19.50 in the frame is noticeably smaller than a 19.75 in filter in the same nominal size
  • Depth is stated at 0.75 in, so a slot expecting a fuller one-inch frame may leave play
  • MERV 8 is not the finest option in this catalog, which some owners will want

Best forA 20x20x1 slot where the difference between nominal and actual has already caused a problem once.

Check it on Amazon
Simply 20x20x1 MERV 8, 6-pack

Simply 20x20x1 MERV 8, 6-pack

  • Nominal 20x20x1
  • Actual 19.75 x 19.75 x 0.75 in
  • MERV 8
  • MPR 600
  • 6 per box

Also nominal 20x20x1, also MERV 8, and a stated actual size of 19.75 x 19.75 x 0.75 in — a quarter inch larger on each face than the Filterbuy in the same nominal size. That difference is the reason this catalog publishes actual sizes at all. The listing also states MPR 600.

What the listing supports, and what it does not
  • Stated actual size is 19.75 in per face, the larger of the two 20x20x1 options here
  • Six filters per order, which covers eighteen months at a quarterly interval
  • MPR 600 and MERV 8 are both stated, giving two reference points
  • Moderate rating keeps airflow restriction lower than the MERV 11 and 12 options
  • A larger actual frame can be a tight push into a slot sized for a 19.50 in filter
  • Stated depth is 0.75 in despite the nominal one-inch size
  • Six at a time is a lot of storage for a filter that should not sit around indefinitely

Best forA 20x20x1 slot that has proven loose with a smaller-framed filter.

Check it on Amazon
Amazon Basics 20x20x1 MERV 5, 6-pack

Amazon Basics 20x20x1 MERV 5, 6-pack

  • Nominal 20x20x1
  • Actual 19.75 x 19.75 x 0.75 in
  • MERV 5
  • 6 per box

The plainest listing in the group: nominal 20x20x1, MERV 5, stated actual size 19.75 x 19.75 x 0.75 in, six to a box. No MPR figure is given, which is arguably clearer than carrying two scales at once. The frame matches the Simply filter in the same nominal size exactly as stated.

What the listing supports, and what it does not
  • Actual size is stated despite the listing being otherwise minimal
  • MERV 5 keeps resistance at the low end of this catalog
  • Six per box at the most common small nominal size
  • No second rating scale to confuse the comparison
  • No MPR figure is stated, so it cannot be lined up against MPR-labeled filters directly
  • MERV 5 is the coarsest rating stocked here
  • The listing states less about construction than the other filters in this group

Best forA 20x20x1 slot being kept stocked without paying for a finer rating than the system calls for.

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