Air filter nominal vs. actual size
The size printed big on a filter (for example 16x25x1) is the nominal size: a rounded shopping label in whole inches. The actual size is the real frame, usually a little smaller so the filter slides into the slot. There is no single industry standard for how much smaller, so the same label can measure differently from brand to brand.[1 2nd-hand 2 · Oct 21, 2026]
Published actual sizes for common labels
| Nominal label | Published actual size (in) | Brand / product family |
|---|---|---|
| 16x25x1 | 15.75 × 24.75 × 0.75 | Honeywell 1-inch (current MERV 11/13), Amazon Basics[1 2nd-hand · Oct 21, 2026] |
| 16x25x1 | 15.69 × 24.69 × 0.81 | Filtrete 1-inch (some SKUs 0.78 thick)[1 2nd-hand 2 · Oct 21, 2026] |
| 16x25x1 | 15.5 × 24.5 × 0.75 | Nordic Pure; AIRx (Discount Filters)[1 2nd-hand 3 · Oct 21, 2026] |
| 20x20x1 | 19.69 × 19.69 × 0.81 | Filtrete[2 · Oct 21, 2026] |
| 20x20x1 | 19.5 × 19.5 × 0.75 | FilterBuy, Nordic Pure[2 · Oct 21, 2026] |
| 20x25x1 | 19.69 × 24.69 × 0.81 | Filtrete[2 · Oct 21, 2026] |
Quick answers
- How much smaller? A 1-inch filter is typically 0.25 to 0.50 in smaller than its label on each face. Deep 4- to 5-inch media filters can be up to about 0.62 in smaller.[2 · Oct 21, 2026]
- A "1-inch" filter is not 1 inch thick. The 1-inch class runs about 0.75 to 0.81 in thick.[1 2nd-hand · Oct 21, 2026]
- Reading a measured filter: round the face measurements up to whole inches (15.5, 15.69 and 15.75 all shop as 16). A thickness near ¾ in is a 1-inch filter, near 1¾ in is a 2-inch, and near 3¾ in is a 4-inch.[1 2nd-hand · Oct 21, 2026]
- Don't mix thickness classes. A 16x25x1 does not replace a 16x25x4 media-cabinet filter.[1 2nd-hand · Oct 21, 2026]
- Best first step: read the frame of your current filter. Many makers print both nominal and actual sizes on the edge.[2 · Oct 21, 2026]
What the MERV number means
MERV (Minimum Efficiency Reporting Value) reports how well a filter captures particles between 0.3 and 10 microns. It comes from an ASHRAE test method, and a higher number means the filter traps more of the tested particle sizes.[4 · Oct 21, 2026] Minimum average capture, per the EPA's summary table:
| MERV | 0.3–1.0 µm | 1.0–3.0 µm | 3.0–10.0 µm |
|---|---|---|---|
| 1–4 | not rated | not rated | under 20% |
| 8 | not rated | ≥ 20% | ≥ 70% |
| 11 | ≥ 20% | ≥ 65% | ≥ 85% |
| 13 | ≥ 50% | ≥ 85% | ≥ 90% |
| 16 | ≥ 95% | ≥ 95% | ≥ 95% |
Higher-MERV filters are not right for every system. The EPA says to choose as high a rating as your system fan and filter slot can accommodate, and to ask an HVAC technician if unsure.[4 · Oct 21, 2026]
Sources: 1. FilterBySize, How to measure a furnace filter (compiles makers' published actual sizes; second-hand). 2. ClariSelect, HVAC filter sizing: nominal vs. actual. 3. Discount Filters, 16x25x1 product specifications. 4. US EPA, What is a MERV rating? Checked Oct 21, 2026.