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Pleated HVAC filter above a dimension line measuring its width

Air Filter Size Checker

Measure your furnace or AC filter and get the exact nominal size to order — the number stores actually use — plus its true dimensions and the right MERV rating.

Updated 2026-08-05 · Free tool · No sign-up · Runs in your browser

How this tool works

HVAC filters are labeled by a rounded-up nominal size, but they physically measure smaller so they slide into the grille or cabinet without binding. The industry convention:

  • Face (length & width): the actual measurement is about 0.5 in under the nominal number. A “16×25” filter really measures ~15.5 × 24.5 in.
  • Depth: a 1 in filter is ~0.75 in thick, a 2 in ~1.75 in, a 4 in ~3.75 in, a 5 in ~4.375 in.

So if you measure your old filter, we round each face dimension up to the nearest nominal size and map the depth — that's the size to buy. If you already know the nominal size, we show you the true dimensions it will measure (handy for checking it'll actually seal). We then match against a list of commonly stocked sizes so you know whether it's an off-the-shelf buy or a custom order.

MERV rates filtration: MERV 8 for basic dust, 11 for allergies and pets, 13 for fine particles and smoke. Higher isn't always better — a very restrictive filter can strain an older blower, so we flag that.

Note: most brands follow the 0.5 in convention, but a few cut exactly to nominal. When possible, match the nominal size printed on the filter you're replacing.

Common HVAC filter sizes: nominal vs. actual (31 sizes)

The size printed on the frame (nominal) and what it actually measures. Order by the nominal size. Last updated 2026-08-05.

Nominal (order this)Actual size (in)Depth
10 × 20 × 19.5 × 19.5 × 0.751 in
12 × 12 × 111.5 × 11.5 × 0.751 in
12 × 24 × 111.5 × 23.5 × 0.751 in
14 × 14 × 113.5 × 13.5 × 0.751 in
14 × 20 × 113.5 × 19.5 × 0.751 in
14 × 24 × 113.5 × 23.5 × 0.751 in
14 × 25 × 113.5 × 24.5 × 0.751 in
15 × 20 × 114.5 × 19.5 × 0.751 in
16 × 20 × 115.5 × 19.5 × 0.751 in
16 × 24 × 115.5 × 23.5 × 0.751 in
16 × 25 × 115.5 × 24.5 × 0.751 in
18 × 18 × 117.5 × 17.5 × 0.751 in
18 × 20 × 117.5 × 19.5 × 0.751 in
18 × 24 × 117.5 × 23.5 × 0.751 in
18 × 25 × 117.5 × 24.5 × 0.751 in
20 × 20 × 119.5 × 19.5 × 0.751 in
20 × 24 × 119.5 × 23.5 × 0.751 in
20 × 25 × 119.5 × 24.5 × 0.751 in
20 × 30 × 119.5 × 29.5 × 0.751 in
24 × 24 × 123.5 × 23.5 × 0.751 in
24 × 30 × 123.5 × 29.5 × 0.751 in
25 × 25 × 124.5 × 24.5 × 0.751 in
16 × 20 × 415.5 × 19.5 × 3.754 in
16 × 25 × 415.5 × 24.5 × 3.754 in
20 × 20 × 419.5 × 19.5 × 3.754 in
20 × 24 × 419.5 × 23.5 × 3.754 in
20 × 25 × 419.5 × 24.5 × 3.754 in
24 × 24 × 423.5 × 23.5 × 3.754 in
16 × 25 × 515.5 × 24.5 × 4.3755 in
20 × 20 × 519.5 × 19.5 × 4.3755 in
20 × 25 × 519.5 × 24.5 × 4.3755 in

Convention: face dimensions run ~0.5 in under nominal; a 1 in filter is ~0.75 in thick, a 4 in is ~3.75 in. A few brands cut exactly to nominal.

FAQ

Why is my filter smaller than the size printed on it?

That's normal and intentional. The printed size is the “nominal” (rounded) size; the filter is cut about half an inch smaller on the face and slightly thinner so it slides in and out of the slot without jamming. A “16x25x1” filter actually measures about 15.5 × 24.5 × 0.75 in.

Should I order the size I measured, or a bigger one?

Order the nominal size — the rounded number — not the smaller number you measured. This tool rounds your measurement up to the correct nominal size to buy. Ordering the measured (actual) number would get you a filter that's too small and leaks air around the edges.

Does thickness (depth) really matter?

Yes — as much as the face size. A 1 in filter won't seal in a 4 in cabinet, and a 4 in filter won't fit a 1 in slot. Match all three numbers. Thicker media filters (4–5 in) last longer and restrict airflow less at the same MERV, but only if your system has the deep cabinet for them.

What MERV rating should I use?

MERV 8 handles everyday dust and lint. MERV 11 is the common upgrade for allergies and pet dander. MERV 13 captures fine particles and smoke. Going higher than your system is designed for can restrict airflow and strain the blower, so MERV 8–11 is the safe range for most homes with a standard 1 in slot.

My size came back as “not a standard size” — now what?

Some equipment uses odd sizes that stores don't stock. You can order a custom-cut filter to your exact nominal dimensions, or double-check your measurement — being off by a fraction can round you to a non-standard size when a standard one actually fits.

How accurate is the nominal-vs-actual rule here?

The ~0.5 in face reduction is an industry-wide convention used by the major filter makers, and the depth mapping matches standard 1/2/4/5 in filters. A few brands cut exactly to nominal, so if your filter is a tight or loose fit, measure and compare to the actual dimensions we show.

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