Nominal vs Actual Filter Size: Why Your Filter Measures Smaller
Nominal size is the slot dimension (e.g., 16x25x1). Actual size is the physical filter measurement (e.g., 15.75x24.75x0.75). Manufacturers intentionally make filters slightly smaller so they slide into the slot without binding. Always buy by nominal size — it's printed on your old filter's frame.
What is Nominal Size?
Nominal size is the slot dimension — the opening in your HVAC system where the filter goes. It's rounded to the nearest inch for easy naming. Example: 16x25x1 means a slot approximately 16 inches wide, 25 inches tall, and 1 inch deep.
Nominal size is what you see printed on the cardboard frame of your old filter. It's also what you type into Filterbuy's size selector.
What is Actual Size?
Actual size is the physical filter measurement — width, height, and depth in decimal inches. Example: 15.75 x 24.75 x 0.75 inches.
For standard filters (like the Filters Fast sizes in the table below), actual size is 0.25 to 0.625 inches (¼ to ⅝ inch) smaller than nominal in each dimension. Some OEM filters, like Aprilaire, don't follow this rule (see below). This clearance lets you slide the filter in and out of the slot without force.
Common Nominal → Actual Conversions
| Nominal Size | Actual Size (Width x Height x Depth) |
|---|---|
| 16x25x1 | 15.75 x 24.75 x 0.75 |
| 20x25x1 | 19.75 x 24.75 x 0.75 |
| 16x20x1 | 15.75 x 19.75 x 0.75 |
| 20x20x1 | 19.75 x 19.75 x 0.75 |
| 16x25x4 | 15.50 x 24.50 x 3.50 |
| 20x25x4 | 19.375 x 24.375 x 3.50 |
Actual sizes in this table are Filters Fast's published values for its own MERV 8/11/13 filters (the "What is the actual size" FAQ on each Filters Fast size listing). Aprilaire 413 (nominal 16x25x4) is 15.81 x 29.38 x 4.05 per Filters Fast's product specs, a non-standard size for proprietary cabinets.
Why the Difference Matters
If you measure your old filter with a tape measure, you'll get a number like 15.75 x 24.75 x 0.75. That's the actual size. If you search for "15.75x24.75x1," you'll usually find nothing, because retailers (Filters Fast included) list filters by nominal size.
Instead, round up to the nearest inch: 15.75 → 16, 24.75 → 25, 0.75 → 1. Search for 16x25x1.
How to Find Your Filter Size
- Check the old filter's label — it's printed on the cardboard frame (e.g., 16x25x1). Use that nominal size.
- No label? Measure the filter with a tape measure (width x height x depth).
- Round up to the nearest inch for each dimension. Example: 15.75 x 24.75 x 0.75 → 16x25x1.
- Search by nominal at Filters Fast, Nordic Pure, or your retailer of choice (pick your nominal size).
Exception: Aprilaire Filters
Aprilaire uses non-standard actual dimensions for some models. Example: Aprilaire 413 is nominal 16x25x4, but actual 15.81 x 29.38 x 4.05 inches (per Filters Fast's product specs). The width is wider than standard 16x25x4 filters (the Filters Fast 16x25x4 is 15.5 x 24.5 x 3.5).
If you have an Aprilaire whole-home air cleaner (models 2410, 3410, 4400), use Aprilaire OEM filters by model number, not by guessing nominal size:
- Aprilaire 410: MERV 11, for models 1410/1610/2410
- Aprilaire 413: MERV 13, for models 2410/2416/3410/4400
- Aprilaire 416: MERV 13, wider cabinet (for models with 20x25 slot)
Can I Use a Filter That's Too Small?
No. If the filter is smaller than the slot, unfiltered air bypasses around the edges. Example: using a 15x25x1 nominal filter in a 16x25x1 slot leaves a 1-inch gap on one side.
Always use the exact nominal size for your slot. Filters are designed to fit snugly with minimal clearance.
Can I Use a Filter That's Too Large?
No. A filter larger than the slot won't fit. Example: forcing a 20x25x1 filter into a 16x25x1 slot will bend the frame and damage the filter.
Shop by nominal size
Once you know the nominal size on the frame, check current listings. These are sponsored affiliate links.
- Filters Fast 16x25x1 · Nordic Pure 16x25x1 — most common 1-inch
- Filters Fast 20x25x1 · Nordic Pure 20x25x1 — wider 1-inch
- Filters Fast 16x20x1 · Nordic Pure 16x20x1 — compact 1-inch
- Filters Fast 20x20x1 · Nordic Pure 20x20x1 — square 1-inch
- Filters Fast 16x25x4 · Nordic Pure 16x25x4 — standard 4-inch
- Filters Fast 20x25x4 · Nordic Pure 20x25x4 — wider 4-inch
Slot doesn't match a standard nominal size? Filter King sells custom-size filters, and its custom-filter page says to use the measured actual size when ordering a custom size (unlike standard sizes, where you round up to nominal). See Filter King custom air filters
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