The size printed on an air filter is often not the exact measured frame size.
Quick Answer
Nominal size is the rounded retail size, such as 16x25x1. Actual size is the real measured frame size, often slightly smaller so the filter slides into the slot.
Examples
| Printed nominal size | Possible actual size clue | Buying route |
|---|---|---|
| 16x20x1 | About 15.5-15.75 by 19.5-19.75 by 0.75 | Usually buy 16x20x1 if the old label agrees. |
| 16x25x1 | About 15.5-15.75 by 24.5-24.75 by 0.75 | Usually buy 16x25x1. |
| 20x25x4 | Actual depth may vary by brand | Match the old filter or media cabinet label closely. |
Brand Depth Warning
Deeper filters can vary more by brand. A nominal 4-inch filter may not have the same actual depth as another nominal 4-inch filter, so compare the old filter and cabinet label before ordering.
When The Actual Size Matters Most
Actual size matters most when the old filter is missing, the frame was crushed, the slot is tight, or you are buying a deeper media filter. If the old filter fit correctly, the printed nominal size is usually the safer buying target; if it never fit correctly, measure the slot and treat the old label as suspicious.
Use this with the air filter measurement guide and the main air filter size chart.
Nominal vs Actual FAQs
Should I buy the exact measured size?
Usually no. If the old filter label is readable, buy the printed nominal size rather than the slightly smaller actual frame size.
Why are actual sizes smaller?
Manufacturers leave clearance so the filter can slide into the slot without crushing or scraping.
When should I worry about actual size?
Worry when the old filter is missing, the slot is tight, the depth is 4 or 5 inches, or the replacement brand lists unusual actual dimensions.