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. If the rack is 20×25, use the 20×25×1 guide.
| Nominal | Typical actual (in) | Common depths | Notes |
|---|---|---|---|
| 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. |
| 16×25×1 | 15½ × 24½ × ¾ | 1, 2, 4, 5 | The default hardware-store face. Also sold as 16×25×4. |
| 16×25×4 | ~15⅜–15⅞ × 24⅜–24⅞ × 3¾ | 4 | Media-cabinet size. Do not force a 1-inch pad into a 4-inch rack without a proper adapter. |
| 20×20×1 | 19½ × 19½ × ¾ | 1, 4 | Square returns; easy to rotate the wrong way. Arrow still faces the furnace. |
| 20×25×1 | 19½ × 24½ × ¾ | 1, 2, 4, 5 | Second-most common retail face after 16×25. |
| 20×25×4 | ~19⅜–19⅞ × 24⅜–24⅞ × 3¾ | 4 | Typical aftermarket media-cabinet upgrade size. |
| 20×30×1 | 19½ × 29½ × ¾ | 1, 4 | Larger returns. Watch sag on cheap 1-inch frames. |
| 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?
Related FilterFit guides
Thin media
MERV 11 vs 13 in a 1-inch slotWhy a 1-inch MERV 13 can raise pressure drop and short-cycle a furnace that a MERV 11 would leave alone.
Common size
16×25×1 furnace filtersMERV 8 / 11 / 13 in the most common retail size, with Filtrete-, Aerostar-, and Filterbuy-style brand classes — no fake scores.
20×25 face
20×25×1 furnace filtersMERV 8 / 11 / 13 in the second-most-common retail face, with Filtrete-, Aerostar-, and Filterbuy-style brand classes — no fake scores.
Depth first
1-inch vs 4-inch MERV 13Media area beats a MERV number. When a 4-inch cabinet is the real upgrade, and when a 1-inch rack is enough.
Airflow first
Is MERV 13 safe for your furnace?When MERV 13 is fine, when it chokes PSC blowers, and the warning signs that the filter is winning against the equipment.