25x25x1🌬 1-inch HVAC filter

25x25x1 Air Filter: Actual Size, MERV Guide & Replacement

Reviewed 2026-10-22 · 8 sources

Nominal vs actual size

Nominal (order this)	25 x 25 x 1 in
Actual	24.5 x 24.5 x 0.75 in (Filterbuy product spec)

A large square 1-inch filter, mostly found in big return-air grilles in ceilings or walls. Filterbuy's size chart lists it as a large-return size at 24.5 x 24.5 x 0.75. It is easy to confuse with 24x24x1 (actual about 23.38 x 23.38 at Filterbuy), which is roughly an inch smaller each way and would leave a gap in a 25x25 slot, so read the actual size printed on your old frame before you order. Filters are deliberately cut smaller than their nominal label so they slide into the slot; length and width typically run about 1/4 to 1/2 inch under nominal. Actual sizes vary slightly by brand, so compare against the small print on your old frame.

How to measure

Turn the system off at the thermostat (and the service switch) before pulling the filter.
Read the frame first: the large print is the nominal size (what you order); small print, if present, is the actual size.
If there is no label, measure length, width and depth edge to edge with a tape measure.
Round each measurement up to the nearest whole inch to get the nominal size (e.g. 19.5 x 24.5 x 0.75 → 20x25x1).
No old filter? Measure the filter slot or cabinet opening the same way.
Note the airflow arrow: it should point toward the furnace or blower.

MERV 8 vs 11 vs 13

MERV	Minimum capture (EPA / ASHRAE 52.2)	Good for
8	≥20% of 1–3 µm, ≥70% of 3–10 µm	Basic dust, pollen, lint; the minimum EPA Indoor airPLUS requires
11	≥20% of 0.3–1 µm, ≥65% of 1–3 µm, ≥85% of 3–10 µm	A step up for pets and allergies
13	≥50% of 0.3–1 µm, ≥85% of 1–3 µm, ≥90% of 3–10 µm	EPA's recommended level for fine particles (PM2.5), if your system can accommodate it

⚠ When higher MERV can restrict airflow

EPA notes that higher-MERV filters tend to restrict airflow, and recommends using the highest rating your system fan and filter slot can accommodate. Most systems handle MERV 13 if the filter is replaced frequently. Thin 1-inch slots are the most restrictive; deeper 4-inch media filters generally create less static pressure. Weak airflow from vents, a system that runs longer, or a filter that bows are signs to step down a grade or ask an HVAC technician.

How often to replace

ENERGY STAR: check your filter every month and change it when it looks dirty, at least every three months. Pets, dust, renovations and heavy heating/cooling seasons can mean more frequent changes.

Equivalent & nearby sizes

There is no exact substitute for a different size: length, width and depth must all match. These nearby sizes are commonly confused with 25x25x1:

FAQs

What is the actual size of a 25x25x1 air filter?

25x25x1 is the nominal (ordering) size. Filterbuy's product page and size chart list its 25x25x1 filters at 24.5 x 24.5 x 0.75 inches (24 1/2 x 24 1/2 x 3/4). Other brands can differ slightly, so check the actual dimensions printed on your old frame if you're switching brands.

My old filter says 24 1/2 x 24 1/2 x 3/4. Is that a 25x25x1?

Yes. Round each measurement up to the nearest whole inch: 24 1/2 becomes 25 and 3/4 becomes 1, which gives the nominal size 25x25x1.

Can I use a 24x24x1 instead?

No. A 24x24x1 is about an inch smaller in both directions, which leaves a gap around the edges where air bypasses the filter. Length, width and depth must all match the slot.

How often should I replace a 25x25x1 filter?

ENERGY STAR says to check your filter monthly and change it when it is dirty, at least every three months. Homes with pets, dust or construction may need more frequent changes.

Is MERV 13 safe for my system?

EPA says most furnaces and HVAC systems can accommodate a MERV 13 filter without equipment problems if it is replaced frequently, but higher-MERV filters do tend to restrict airflow. If you notice weak airflow or are unsure, ask an HVAC technician.

Which way does the arrow go?

Per ENERGY STAR, the airflow arrow on the filter frame should point toward the furnace or blower unit.

Sources

← All filter sizes