Nominal vs actual air filter size chart
The number on the box is not what a tape measure shows. Use this chart so you order the right furnace filter without leaving bypass gaps.
Rule of thumb
Shop by nominal size (the bold label). Expect actual length and width about ¼–½″ smaller, and a 1″ filter closer to ¾″ thick. That undercut is intentional so the filter slides in. Brand-to-brand actuals still vary — when switching brands, check the product’s listed actual dimensions.
Common 1″ residential sizes
| Nominal (order this) | Typical actual | Common use |
|---|---|---|
| 14×20×1 | ~13.5 × 19.5 × 0.75 | Smaller wall returns |
| 14×25×1 | ~13.5 × 24.5 × 0.75 | Furnace / return |
| 16×16×1 | ~15.5 × 15.5 × 0.75 | Small return |
| 16×20×1 | ~15.5 × 19.5 × 0.75 | Mid-size return |
| 16×24×1 | ~15.5 × 23.5 × 0.75 | Return grille |
| 16×25×1 | ~15.5 × 24.5 × 0.75 | Very common furnace slot |
| 18×18×1 | ~17.5 × 17.5 × 0.75 | Return grille |
| 18×20×1 | ~17.5 × 19.5 × 0.75 | Return grille |
| 18×24×1 | ~17.5 × 23.5 × 0.75 | Return grille |
| 20×20×1 | ~19.5 × 19.5 × 0.75 | Most common US size |
| 20×22×1 | ~19.5 × 21.5 × 0.75 | Air handler |
| 20×24×1 | ~19.5 × 23.5 × 0.75 | Air handler |
| 20×25×1 | ~19.5 × 24.5 × 0.75 | Large residential |
| 20×30×1 | ~19.5 × 29.5 × 0.75 | Large return |
| 24×24×1 | ~23.5 × 23.5 × 0.75 | Large / light commercial |
| 25×25×1 | ~24.5 × 24.5 × 0.75 | Large return |
Common 4″ media-cabinet sizes
| Nominal | Typical actual | Notes |
|---|---|---|
| 16×20×4 | ~15.5 × 19.5 × 3.75 | Media cabinet |
| 16×25×4 | ~15.5 × 24.5 × 3.75 | Whole-home media |
| 20×20×4 | ~19.5 × 19.5 × 3.75 | Media cabinet |
| 20×25×4 | ~19.5 × 24.5 × 3.75 | Whole-home media |
| 24×24×4 | ~23.4 × 23.4 × 3.75 | Larger systems |
How to use the chart
- Read the bold size on your old filter — that is nominal. Order that.
- If the label is gone, measure the slot and round to the nearest whole inch for length/width.
- Confirm depth: grille slots are often 1″; a bolted media cabinet is usually 4″ or 5″.
- If switching brands, compare listed actual sizes so the new filter seals as tightly as the old one.
- No match within ~½″? Look at custom-cut filters — don’t force a loose fit.
Related
How to measure your HVAC air filter size — step-by-step if you have no old filter.
Next guides
Dedicated pages for 16×25×1, 20×25×1, and MERV 8 vs 11 vs 13.
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