Furnace filter size

20x25x1 Furnace Filters

The most common residential slot size, and two filters here that do not measure the same.

Depth — 0.78–0.81 in stated, against a nominal 1 in

If a residential furnace has a one-inch filter slot, there is a good chance this is the size in it. That ubiquity is why 20x25x1 is stocked everywhere and why it is the size most often bought on autopilot without measuring anything.

The two filters in this catalog at this nominal size come from the same manufacturer, and their stated actual dimensions still differ — 19.68 x 24.68 x 0.81 in for one, 19.62 x 24.62 x 0.78 in for the other. Six hundredths of an inch is not much. It is also not zero, and it is between two products from one company.

Both options here sit at MERV 11 or above, which is the part of this size worth pausing over. This catalog has no MERV 5 or MERV 8 filter at 20x25x1, so if the system documentation points to a lower rating, that filter exists but is not on this page.

What this catalog carries at 20x25x1

2 filters, at MERV 11 and MERV 12.

Stated figures at 20x25x1

Listing	Actual size stated	MERV	MPR
Filtrete 20x25x1 MERV 11, 2-pack	19.68 x 24.68 x 0.81 in	MERV 11	MPR 1000
Filtrete 20x25x1 MERV 12, 2-pack	19.62 x 24.62 x 0.78 in	MERV 12	MPR 1500

Filtrete 20x25x1 MERV 11, 2-pack
Actual size stated
19.68 x 24.68 x 0.81 in
MERV
MERV 11
MPR
MPR 1000
Filtrete 20x25x1 MERV 12, 2-pack
Actual size stated
19.62 x 24.62 x 0.78 in
MERV
MERV 12
MPR
MPR 1500

Check the one you have first

Pull the filter currently in the slot and read the printed size on the cardboard frame — that is the nominal size.
Measure the frame itself with a tape: width, height, and depth, on the frame rather than across the pleats.
Note which way the airflow arrow points before the old filter leaves the slot.
Compare your measured figures with the actual size stated in the listing, not with the nominal size in the product title.

The arrow, and the schedule

Because this is the size most often bought without looking, it is also the size most often pushed in facing the wrong way. The airflow arrow on the cardboard points toward the blower. Check where the old one pointed before you pull it, and if the old one was in backwards, do not copy it.

Follow the replacement interval the filter manufacturer states and whatever the system documentation says about it. Pets, nearby construction, and heavy system runtime all shorten it. A loaded filter restricts airflow more than a coarse clean one does.

A higher MERV rating is not an upgrade by default

Both filters listed at this size are MERV 11 or MERV 12. Finer media resists airflow more, and the system manufacturer publishes the filter rating and pressure drop its equipment is designed around. That document decides what belongs in this slot — not a preference for a higher number. If it cannot be found, a licensed HVAC professional can read the equipment and advise.

The 20x25x1 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.

Most common slot

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 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.