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Nominal vs Actual Size
Quick answer: Nominal size is the rounded label on an air filter, such as 16x25x1. Actual size is the real measurement, usually about ½ inch smaller on length and width and about ¾ inch thick for "1-inch" filters. Measure the old filter, round each side up to the next whole inch, and buy that nominal size. Some brands differ slightly.
Two numbers on one filter
Look at the frame of almost any HVAC air filter and you will see two sets of dimensions. The large print says something like "16x25x1" — the nominal size. Smaller print, often on the side, says something like "15.5 x 24.5 x 0.75" — the actual size. The nominal size is how filters are sold, searched and stocked. The actual size is what physically fits. Mixing them up is the most common filter-buying mistake.
Why filters are undersized
Filter slots and return grille frames are usually built to the nominal dimension. If a filter were exactly 16 × 25 inches, it would jam, buckle or tear when pushed into a 16 × 25 slot, especially once the frame swells a little in humid air. Making the filter roughly a quarter inch smaller on each edge — half an inch overall on length and width — gives clearance for easy installation and removal. Thickness follows a similar logic: a "1-inch" filter is commonly around ¾ inch thick, and "4-inch" media filters are often somewhere between about 3½ and 4⅜ inches depending on the cabinet design they are made for.
Typical nominal to actual conversions
| Nominal | Typical actual (in) | Approx. nominal face area (ft²) |
|---|---|---|
| 14x20x1 | 13.5 × 19.5 × 0.75 | 1.94 |
| 16x20x1 | 15.5 × 19.5 × 0.75 | 2.22 |
| 16x25x1 | 15.5 × 24.5 × 0.75 | 2.78 |
| 20x20x1 | 19.5 × 19.5 × 0.75 | 2.78 |
| 20x25x1 | 19.5 × 24.5 × 0.75 | 3.47 |
| 16x25x4 | 15.5 × 24.5 × ~3.6–3.75 | 2.78 |
| 20x25x4 | 19.5 × 24.5 × ~3.6–3.75 | 3.47 |
Actual sizes vary by manufacturer. A few brands make filters closer to nominal; some media cabinets need a brand-specific depth. Always compare with the actual size on your old filter or the cabinet manual.
How to round a measurement
- Measure the old filter's length, width and depth with a tape measure.
- Round length and width up to the next whole inch. 15¾ becomes 16; 24½ becomes 25.
- For depth, ¾ inch is sold as 1 inch, about 1¾ inches as 2 inches, and roughly 3½ to 4⅜ inches as 4 or 5 inches depending on the cabinet.
- If a measurement is already a whole number, such as exactly 20 inches, the slot may be 20½ or 21; measure the slot too.
- Write length × width × depth, conventionally with the smaller face dimension first.
The filter size finder applies these rules for you.
Worked example
A tenant measures a filter at 17¾ × 23¾ × ¾ inches. Rounding up gives 18 × 24 × 1. They check the store and find 18x24x1 filters listed with an actual size of 17.75 × 23.75 × 0.75, a match. Had they rounded to the nearest inch instead of up, they would still get 18 × 24 — but a filter measuring 15.4 inches would round to 15 rather than the correct 16, which shows why "round up" is the safer rule.
Illustrative note (2026-10-29): This example was written on 2026-10-29 to illustrate rounding. The measurements are hypothetical and the listed actual size is a typical pattern, not a specific product's specification.
When there is no old filter
If the slot is empty, measure the slot or grille opening. The nominal size is usually the slot size rounded down to a whole inch; a slot measuring 16⅛ × 25⅛ inches takes a 16x25 filter. Read how to measure an air filter for grilles, cabinets and unusual housings.
Custom and odd sizes
Some older furnaces, mobile homes and specialty equipment use non-standard sizes such as 15x20x1 or 17x21x1, and some sizes use fractional nominal numbers like 11¼ × 21¼. If the measurement does not round cleanly to a common size, check the equipment manual or label, or look for custom-size filters. Do not fold or trim a larger filter; it will leak and can collapse.
Why fit matters
A filter that is too small leaves gaps around the edges. Air takes the path of least resistance, so unfiltered air bypasses the filter and carries dust onto the blower and the cooling coil. Over time, that can reduce efficiency and require cleaning by a technician. A filter that is too large will not seat properly and may bow or block airflow. The correct nominal size, installed with the airflow arrow pointing toward the equipment, gives the best seal.
The airflow arrow
Most filters print an arrow on the frame. It points in the direction air moves — away from the return duct and toward the furnace or air handler. In a ceiling return grille, that usually means pointing up into the duct. Installing backwards can reduce performance and, with some media, cause the filter to deform.
Reading labels from different makers
Packaging layouts vary. Some brands print only the nominal size in large type and put the actual size on the end of the frame; others list both on the front. Online listings sometimes show the actual size in the product title, which can make a 16x25x1 look like a different item. When comparing listings, convert everything back to nominal before deciding, and check the depth carefully — a 4-inch and a 5-inch media filter share face sizes but are not interchangeable.
Frequently asked questions
Why does my 20x20x1 filter measure 19.5 inches?
That is its actual size. The half-inch reduction lets it slide into a 20-inch slot.
Should I buy the actual size or the nominal size?
Buy by nominal size, then confirm the listed actual size matches your old filter.
Is the thickness also nominal?
Yes. "1-inch" filters are typically about ¾ inch thick.