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Air Filter Actual Size Calculator

A “20 × 25 × 4” filter is not 20 × 25 × 4 inches. Convert a nominal size to typical actual dimensions, work out the nominal size from a filter you measured, or check the clearance between a filter and its slot.

I want to
Size
in
Nominal mode: the size on the label. Measured mode: the old filter’s frame. Slot mode: the opening the filter slides into.
in
in
Filter you plan to buy

Results

Typical actual size≈ 19.75 × 24.75 × 3.5 inNominal 20 × 25 × 4 in — actual sizes vary by brand
Width
19.5 in – 19.75 in
Height
24.5 in – 24.75 in
Depth
3.5 in – 3.9 in

Nominal sizes are rounded labels. Face dimensions are usually ¼–½ in. smaller than nominal, and 4 and 5 in. media filters vary most between brands.

This does not tell you whether a particular filter fits your system. Compare the actual dimensions printed on your current filter, or the filter cabinet’s model number, before buying.

How it works

  1. Choose what you want to do: look up a nominal size, identify a filter you measured, or check a slot.
  2. Enter width, height and depth in inches (fractions as decimals: ¾ = 0.75, ⅞ = 0.875).
  3. For a slot check, also enter the actual dimensions printed on the filter you plan to buy.
  4. Read the result and confirm against the label on your current filter before ordering.

Formula

Actual face ≈ nominal − ¼ to ½ in (width and height)
Actual depth ≈ 1 in → ¾ in · 2 in → 1¾ in · 4 in → 3½–3⅞ in · 5 in → 4⅜–5¼ in
Measured → nominal: round width and height up to the next whole inch
Clearance = slot − filter (gaps over ½ in let air bypass)

Filters are named by rounded “nominal” sizes so they fit standard openings with a little clearance. Most 1 in. and 2 in. filters follow the ¼–½ in. rule closely. Deep media filters (4 in. and 5 in.) vary by brand and cabinet, so their ranges are wide and the model number matters more than the nominal size.

Example

An old filter with a worn-off label measures 15¾ × 24¾ × ¾ inches. What nominal size should be ordered?

Inputs

I want to
Measured → nominal
Width
15.75 in
Height
24.75 in
Depth
0.75 in

Result

Likely nominal size16 × 25 × 1 inFrom a measured 15.75 × 24.75 × 0.75 in
Width rounds up to16 in
Height rounds up to25 in
Depth class1 in
Load this example into the calculator →

Why filter sizes are confusing

The size printed in large type on a filter is nominal. The real dimensions — often in small print on the frame — are usually a fraction of an inch smaller, so a 20 × 25 × 1 filter commonly measures about 19½ × 24½ × ¾ in. That small difference lets the filter slide into a 20 × 25 opening.

Deep media filters are where mistakes happen. Several brands sell “4 in.” and “5 in.” filters for specific cabinets, and their depths overlap. For those, order by the replacement part number on the old filter or the cabinet, not by nominal size alone.

Frequently asked questions

What is the actual size of a 20×25×4 air filter?

Commonly about 19½–19¾ × 24½–24¾ × 3½–3⅞ in., but some brand-specific 4 in. filters are deeper. Check the actual dimensions printed on your current filter.

What is the actual size of a 16×25×1 filter?

Typically about 15½–15¾ × 24½–24¾ × ¾ in.

I measured 19.5 × 24.5 × 0.75 — what size do I buy?

That is a nominal 20 × 25 × 1 filter. Round width and height up to the next whole inch; a ¾ in. depth is a 1 in. filter.

Can I use a filter that is slightly smaller than the slot?

A small gap is normal. Gaps larger than about ½ in. let air bypass the filter; foam gasket tape can seal them. Never force a filter that is too large.