Measure twice
Nominal size and slot-depth chart
The number on the box is a name. The number that fits is the actual cardboard. Depth is a third measurement, not a model suffix you can ignore.
How to measure so the pad actually seats
- Read the old filter for the nominal (16×25×1). Then measure the cardboard anyway. Brands vary by a quarter inch.
- Measure the slot — width, height, and the depth the frame can occupy without bowing the door.
- Prefer the smaller actual if you are between sizes. A filter that rattles leaks. A filter you hammer in collapses.
- Count the filters. Some systems use one at the furnace and one at a return grille. They are not always the same size.
Typical actual sizes
Actuals below are the usual North American cardboard sizes, not a guarantee for every mill. Always match the spec line on the SKU. If you live on 16×25, continue to the 16×25×1 MERV comparison. The 16×25×4 row is that face in a media cabinet — 16×25×4 comparison. The same rule covers 20×25×4, 16×20×4, 20×20×4, and 20×30×4. If the rack is 20×25, use the 20×25×1 guide. If it is 16×20 — often a furnace rack paired with a return-grille filter — use the 16×20×1 comparison. If it is 14×25 — a common upflow closet face — use the 14×25×1 comparison. If it is 20×20 — a square return that is easy to rotate the wrong way — use the 20×20×1 comparison. If it is 12×24 — a less-common face that hardware stores often skip — use the 12×24×1 comparison. If it is 14×20 — a smaller return and some older air handlers — use the 14×20×1 comparison. If it is 20×30 — a long return where cheap 1-inch frames sag — use the 20×30×1 comparison. If it is 24×30 — a large return grille — use the 24×30×1 comparison. If it is 24×24 — a less-common square return — use the 24×24×1 comparison.
| Nominal | Typical actual (in) | Common depths | Notes |
|---|---|---|---|
| 12×24×1 | 11½ × 23½ × ¾ | 1 | Less-common face. Measure the slot; do not substitute 16×24 or 12×20. |
| 14×20×1 | 13½ × 19½ × ¾ | 1 | Smaller returns and some older air handlers. |
| 14×25×1 | 13½ × 24½ × ¾ | 1 | Common upflow closet systems. |
| 16×20×1 | 15½ × 19½ × ¾ | 1, 4 | Often paired with a second filter on a return grille. |
| 16×20×4 | Varies by SKU; depth examples ~3⅝–4⅜ inches, not a compatibility range | 4 (nominal) | Match all three actual dimensions and the cabinet-approved replacement part. Do not substitute a 1-inch pad. See the linked guide. 16×20×4 comparison. |
| 16×25×1 | 15½ × 24½ × ¾ | 1, 2, 4, 5 | The default hardware-store face. Also sold as 16×25×4. |
| 16×25×4 | Varies by SKU; depth examples ~3⅝–4⅜ inches, not a compatibility range | 4 (nominal; some replacements are sold as 5) | Match all three actual dimensions and the cabinet-approved replacement part. Do not substitute a 1-inch pad without a manufacturer-approved adapter. See the linked guide for manufacturer examples. 16×25×4 comparison. |
| 20×20×1 | 19½ × 19½ × ¾ | 1, 4 | Square returns; easy to rotate the wrong way. Arrow still faces the furnace. |
| 20×20×4 | Varies by SKU; not a compatibility range | 4 (nominal) | Match all three actual dimensions and the cabinet-approved replacement part. Do not substitute a 1-inch pad. See the linked guide. 20×20×4 comparison. |
| 20×25×1 | 19½ × 24½ × ¾ | 1, 2, 4, 5 | Second-most common retail face after 16×25. |
| 20×25×4 | Varies by SKU; depth examples ~3⅝–4⅜ inches, not a compatibility range | 4 (nominal) | Match all three actual dimensions and the cabinet-approved replacement part. Do not substitute a 1-inch pad. See the linked guide. 20×25×4 comparison. |
| 20×30×1 | 19½ × 29½ × ¾ | 1, 4 | Larger returns. Watch sag on cheap 1-inch frames. |
| 20×30×4 | Varies by SKU; depth examples ~3⅝–4⅜ inches, not a compatibility range | 4 (nominal) | Match all three actual dimensions and the cabinet-approved replacement part. Do not substitute a 1-inch pad. See the linked guide. 20×30×4 comparison. |
| 24×24×1 | 23½ × 23½ × ¾ | 1 | Less common; measure before assuming 20×25 will ‘almost’ fit. |
| 24×30×1 | 23½ × 29½ × ¾ | 1, 4 | Large filter grilles. A 4-inch version here is a lot of media. |
Depth classes
- 1 inch. Retail slide-in. Least media. MERV 13 is the risky upgrade — read MERV 11 vs 13.
- 2 inch. Extra pleat area, still a throwaway rack. Confirm the door actually closes on 2 inches.
- 4 inch. The usual aftermarket media cabinet. This is how most homes should run MERV 13. See depth vs MERV.
- 5 inch. Honeywell-style / deep media. Do not assume a 4-inch pad fills it without the specified spacer.
Things that look like a size and are not
- A door sticker copied from the last wrong filter someone jammed in.
- A grille that takes 20×25×1 while the furnace cabinet takes 16×25×1.
- Oddball “exact” sizes from a custom shop — good, as long as you measured the slot.
- Washable foam cut from a roll. That is a pad, not a MERV 13 pleated filter.
After size and depth are known, the last question is whether the rating is safe for the blower: Is MERV 13 safe for your furnace?
Shop examples on Amazon
Affiliate links. As an Amazon Associate I earn from qualifying purchases, at no extra cost to you. These are Amazon searches for two common nominal sizes, 16×25×1 and 16×25×4, not a ranked list and not products we tested.