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Furnace & HVAC Filters

Ten filters, five nominal sizes, and the actual dimensions each listing states.

The job of this categoryfitting the slot and not choking the blower. Every listing below is read against that, and the badges go to the ones that answer it most directly.

A furnace filter has two jobs and they pull against each other. It has to sit in the slot without gaps around the frame, and it has to let the blower move air through it. Every rating decision on this page is a trade between those two.

The number printed on the frame is the nominal size. The number in the listing is the actual size. They are never the same, they differ by different amounts between manufacturers, and the gap is the reason a filter that says 20x20x1 can rattle in a slot that just held another filter saying 20x20x1.

Everything below is grouped by rating band first, because that is the decision with consequences for the equipment, and by size second, because that is the decision with consequences for whether the thing goes in at all.

Higher MERV means more resistance to airflow

Finer filter media makes it harder for the blower to pull air through. That is not a marketing detail; it is the mechanism. A system moving less air than it was designed to move runs differently, and the consequences show up at the equipment rather than in the room.

The system manufacturer publishes the filter rating and the pressure drop its equipment is designed around. That document decides what belongs in the slot, not a preference for a bigger number. If you cannot find it, a licensed HVAC professional can read the equipment and tell you.

A clean filter at a moderate rating restricts airflow less than a loaded filter at a fine rating. Replacing on schedule does more for the system than upgrading the rating does.

The arrow on the frame is not decoration

Every pleated filter has an airflow direction arrow printed on the cardboard frame. It points toward the blower, which is the direction the air travels. Installed backwards, the pleat structure is loaded from the wrong side and the filter does not hold up the way it was built to.

Note which way the arrow pointed on the old filter before it comes out. If the old one was in backwards, the arrow on the new one still goes toward the blower, not toward wherever the last one happened to face.

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.

How this page is orderedOrder runs from the listings that state the most about physical fit down to the ones that state the least. Nothing here is ranked on performance, because no performance testing was done for this site.

MERV 5 to 8

The band most residential systems are specified around. Less resistance, coarser capture.

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
Filterbuy 16x25x1 MERV 8, 6-pack

Filterbuy 16x25x1 MERV 8, 6-pack

  • Nominal 16x25x1
  • Actual 15.50 x 24.50 x 0.75 in
  • MERV 8
  • 6 per box

The MERV 8 answer in 16x25x1, with a stated actual size of 15.50 x 24.50 x 0.75 in. Set beside the MERV 11 filter in the same nominal size, the frames differ by roughly 0.18 in on each face — small on paper, and enough to change how a filter seats in a slot with rails. Six to a box, electrostatic pleated, US made per the listing.

What the listing supports, and what it does not
  • Actual size is stated, which allows a direct comparison with other 16x25x1 listings
  • MERV 8 keeps resistance lower than the MERV 11 filter in the same slot size
  • Six per box is the largest quantity offered in this nominal size here
  • Electrostatic pleated construction and US manufacture are stated
  • A 15.50 in face is the smallest stated 16-inch dimension in this catalog
  • Stated depth is 0.75 in against a nominal one inch
  • MERV 8 will not satisfy anyone specifically shopping for a finer rating

Best forA 16x25x1 slot where the priority is not restricting the blower.

Check it on Amazon
Filtrete 20x30x1 MERV 8, 4-pack

Filtrete 20x30x1 MERV 8, 4-pack

  • Nominal 20x30x1
  • Actual 19.88 x 29.88 x 0.81 in
  • MERV 8
  • MPR 700
  • 4 per box

A 20x30x1 filter with a stated actual size of 19.88 x 29.88 x 0.81 in, which is the closest any listing in this catalog comes to its nominal number. MERV 8 with a stated MPR 700. Compared with the MERV 5 filter in the same nominal size, the frame here is stated a fraction larger on both faces.

What the listing supports, and what it does not
  • Stated actual size sits within 0.12 in of nominal on both faces, the tightest gap here
  • MERV 8 with MPR 700 stated, so both scales are on the label
  • Four per box covers a year at a quarterly schedule
  • The larger 30-inch face gives more media area than the 25-inch sizes
  • A frame this close to nominal can bind in a slot that was cut generously
  • MERV 8 is a step above MERV 5 in resistance as well as in rating
  • 20x30x1 is less commonly stocked than 20x25x1 if a replacement is needed urgently

Best forA 20x30x1 return where the frame has previously been too loose.

Check it on Amazon
Filtrete 20x30x1 MERV 5, 6-pack

Filtrete 20x30x1 MERV 5, 6-pack

  • Nominal 20x30x1
  • Actual 19.81 x 29.81 x 0.81 in
  • MERV 5
  • MPR 300
  • 6 per box

The lowest MERV rating in this catalog at MERV 5, with a stated MPR 300 and an actual size of 19.81 x 29.81 x 0.81 in. Low rating is not a defect — it is the least restrictive media stocked here, and a clean MERV 5 does more for a system than a loaded MERV 12. Six per box, pleated electrostatic per the listing.

What the listing supports, and what it does not
  • The least restrictive media in this catalog by stated rating
  • Actual dimensions are stated to hundredths on all three axes
  • Six per box, the largest quantity in this nominal size
  • Pleated electrostatic construction is stated rather than a flat panel
  • MERV 5 captures a coarser range than the MERV 11 and 12 options here
  • MPR 300 is the lowest figure on that scale in this catalog
  • 20x30x1 is a less common slot size to restock in a hurry

Best forA 20x30x1 return where keeping resistance low is the priority the system documentation points to.

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

What each listing states — MERV 5 to 8

  • 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

  • Filterbuy 16x25x1 MERV 8, 6-pack

    Actual size stated
    15.50 x 24.50 x 0.75 in
    MERV
    MERV 8
    MPR
    Not stated
    Pack
    6 per box

  • Filtrete 20x30x1 MERV 8, 4-pack

    Actual size stated
    19.88 x 29.88 x 0.81 in
    MERV
    MERV 8
    MPR
    MPR 700
    Pack
    4 per box

  • Filtrete 20x30x1 MERV 5, 6-pack

    Actual size stated
    19.81 x 29.81 x 0.81 in
    MERV
    MERV 5
    MPR
    MPR 300
    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

MERV 11 to 12

Finer media, more resistance. Whether it belongs in your slot is a question for your system documentation.

Most common slot
Filtrete 20x25x1 MERV 11, 2-pack

Filtrete 20x25x1 MERV 11, 2-pack

  • Nominal 20x25x1
  • Actual 19.68 x 24.68 x 0.81 in
  • MERV 11
  • MPR 1000
  • 2 per box

20x25x1 is the size most residential furnace slots take, and this listing states both scales at once: MPR 1000 and MERV 11. Actual size is stated as 19.68 x 24.68 x 0.81 in. Carrying two rating numbers on one product is the everyday reason the MPR versus MERV page exists — they are not the same scale and they are not convertible.

What the listing supports, and what it does not
  • Actual dimensions are stated to hundredths rather than rounded to the nominal size
  • Both MERV 11 and MPR 1000 appear on the same listing for direct comparison
  • The most widely stocked nominal size, so replacements are easy to find
  • Stated depth of 0.81 in is closer to a full inch than most one-inch filters here
  • MERV 11 media restricts airflow more than the MERV 5 to 8 options in this catalog
  • Two per box means reordering twice a year at a quarterly schedule
  • Two rating numbers on one label invites comparing across incompatible scales

Best forThe standard 20x25x1 slot in a system whose documentation allows a MERV 11 filter.

Check it on Amazon
Filtrete 16x25x1 MERV 11, 4-pack

Filtrete 16x25x1 MERV 11, 4-pack

  • Nominal 16x25x1
  • Actual 15.68 x 24.68 x 0.81 in
  • MERV 11
  • MPR 1000
  • 4 per box

16x25x1 with a stated actual size of 15.68 x 24.68 x 0.81 in, and MPR 1000 alongside MERV 11. The 0.32 in shortfall on the 16-inch face is the same proportional gap this manufacturer states across its one-inch line, which makes the numbers at least internally consistent. Four per box.

What the listing supports, and what it does not
  • Actual dimensions are stated on all three axes
  • Four per box covers a full year on a quarterly schedule
  • MERV 11 and MPR 1000 both appear, so either scale can be matched
  • Stated depth of 0.81 in is at the fuller end for a one-inch filter
  • MERV 11 media raises resistance compared with the MERV 5 to 8 group
  • The narrower 16-inch face means less media area than a 20-inch filter of the same depth
  • Two rating scales on one label is still two scales, not one converted into the other

Best forA 16x25x1 return in a system whose manufacturer documentation permits MERV 11.

Check it on Amazon
Filtrete 20x25x1 MERV 12, 2-pack

Filtrete 20x25x1 MERV 12, 2-pack

  • Nominal 20x25x1
  • Actual 19.62 x 24.62 x 0.78 in
  • MERV 12
  • MPR 1500
  • 2 per box

The finest rating in this catalog: MERV 12 with a stated MPR 1500, actual size 19.62 x 24.62 x 0.78 in. The listing uses the wording allergens, bacteria and viruses; that phrasing belongs to the listing and is reproduced here only as the seller wording, with no claim about outcomes in your home. Finer media also means more resistance, so the system documentation decides whether this filter belongs in the slot.

What the listing supports, and what it does not
  • The highest MERV rating stocked here, with actual dimensions stated to hundredths
  • MPR 1500 gives a second reference point for anyone shopping on that scale
  • The same widely available 20x25x1 nominal size as the MERV 11 option
  • Stated depth of 0.78 in is within the usual one-inch slot range
  • Higher MERV means greater airflow resistance, which is a question for the system manufacturer documentation, not a preference
  • The allergens, bacteria and viruses wording is a listing claim, not a measured result in a home
  • Two per box, so it needs reordering more often than the six-packs here

Best forA 20x25x1 slot in a system whose own documentation permits a filter this fine.

Check it on Amazon

What each listing states — MERV 11 to 12

  • Filtrete 20x25x1 MERV 11, 2-pack

    Actual size stated
    19.68 x 24.68 x 0.81 in
    MERV
    MERV 11
    MPR
    MPR 1000
    Pack
    2 per box

  • Filtrete 16x25x1 MERV 11, 4-pack

    Actual size stated
    15.68 x 24.68 x 0.81 in
    MERV
    MERV 11
    MPR
    MPR 1000
    Pack
    4 per box

  • Filtrete 20x25x1 MERV 12, 2-pack

    Actual size stated
    19.62 x 24.62 x 0.78 in
    MERV
    MERV 12
    MPR
    MPR 1500
    Pack
    2 per box

4-inch depth

For furnaces built with a deep filter cabinet. A one-inch filter will not sit correctly in one of these.

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

Only one listing sits under 4-inch depth, so there is nothing here to compare it against. Everything it states is on the card above.

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.

Questions this category gets asked

Why does a 20x20x1 filter measure 19.5 inches?

Nominal sizing is a naming convention, not a measurement. Filters are built slightly under the nominal figure so they slide into a slot cut to that size. How far under varies by manufacturer, which is why every listing on this page has its stated actual size printed next to it.

Should I buy the highest MERV rating that fits?

No. Higher MERV media resists airflow more, and the system manufacturer documentation states what rating and what pressure drop the equipment is designed for. That document is the answer. If it is not available, a licensed HVAC professional can advise on the specific equipment.

MERV or MPR — which one do I compare?

They are two separate scales published by different parties, and one does not convert into the other. Compare MERV against MERV and MPR against MPR. Several filters on this page state both, which makes them useful reference points but does not make the scales equivalent.

Can I put a one-inch filter in a four-inch cabinet?

It will not seat correctly. A four-inch cabinet has rails and a gasket arrangement built around a deep filter, and a one-inch frame in that space leaves paths for air to go around the media rather than through it. Deep cabinets take deep filters.

How often does a furnace filter get replaced?

Follow the interval in the filter manufacturer instructions and the system documentation. Pets, nearby construction, and how much the system runs all shorten it. A loaded filter is harder on the equipment than a coarse clean one.

Where this comes up elsewhere

Sources