Common Furnace Filter Sizes and How to Read Them
Furnace filters are sold by a three-number size: width x height x depth, in inches. Example: 16x25x1.
How to read the size on the frame
- Turn off the system and pull out the current filter.
- Look on the cardboard edge for the printed size, e.g. "16x25x1".
- You may also see a smaller "actual size" such as 15.5 x 24.5 x 0.75. That is normal.
- Buy by the nominal (rounded) size; that is how stores list filters.
- Note the airflow arrow; it points toward the furnace/blower.
Nominal sizes are rounded up. Actual dimensions are usually about 1/4"–1/2" smaller so the filter slides in. If no size is printed, measure the slot or old filter and round up to the nearest inch.
Popular residential sizes
| Nominal size | Typical actual size | Notes | Buy |
|---|---|---|---|
| 16x20x1 | 15.5 x 19.5 x 0.75 | Common in smaller returns | Buy on Amazon |
| 16x25x1 | 15.5 x 24.5 x 0.75 | One of the most common sizes | Buy on Amazon |
| 20x20x1 | 19.5 x 19.5 x 0.75 | Square return grilles | Buy on Amazon |
| 20x25x1 | 19.5 x 24.5 x 0.75 | Very common in larger homes | Buy on Amazon |
| 14x20x1 | 13.5 x 19.5 x 0.75 | Wall/ceiling returns | Buy on Amazon |
| 14x25x1 | 13.5 x 24.5 x 0.75 | Buy on Amazon | |
| 12x24x1 | 11.5 x 23.5 x 0.75 | Buy on Amazon | |
| 20x30x1 | 19.5 x 29.5 x 0.75 | Buy on Amazon | |
| 16x25x4 | ~15.5 x 24.5 x 3.75 | Media cabinet; check brand, actual depth varies | Buy on Amazon |
| 20x25x4 | ~19.5 x 24.5 x 3.75 | Media cabinet; check brand, actual depth varies | Buy on Amazon |
| 16x25x5 / 20x25x5 | varies by cabinet | Match to your cabinet model | Buy on Amazon / Buy on Amazon |
Actual sizes are typical; exact dimensions vary by manufacturer.
Depth: 1" vs 4–5"
1" filters slide into a slot or return grille and are usually changed every 1–3 months. 4–5" media filters sit in a cabinet near the furnace and often last 6–12 months. Do not swap depths; use what your slot or cabinet is built for.
A note on MERV
MERV (Minimum Efficiency Reporting Value, from ASHRAE Standard 52.2) runs 1–16; higher numbers capture smaller particles. Many homes use MERV 8–13. Higher MERV can restrict airflow on some systems, so check your equipment's guidance.
Quick checklist
- Write the size on the furnace or in your phone.
- Check for more than one return/filter location.
- Install with the arrow toward the blower.