Furnace Filter Size Chart: Nominal vs Actual Size for Common Sizes

Last checked 2026-10-24 27 sources

Quick answer

The size printed on a furnace filter is a rounded "nominal" size. The filter itself is usually a little smaller: Filterbuy cuts its filters about half an inch under on width and length, and its 1-inch filters are 0.75 in deep. [S1] [S2] Order by the nominal size on your old filter, and use the actual size to check the fit.

  • Filterbuy says it undercuts on purpose so the filter slides in and seats without binding. [S1]
  • Media-cabinet filters are the exception. Honeywell, Carrier and Lennox cabinet filters are about 4⅜ to 4½ in deep, and Aprilaire's are larger than their nominal face size. [S17] [S18] [S19] [S20]
  • If your old filter fit, buy the same nominal size. If it didn't, measure the slot. [S21]

Standard filters: nominal vs actual (Filterbuy)

Actual sizes from Filterbuy's own size pages, in inches (width × length × depth). Other brands cut a little differently; see the linked guides for brand comparisons.

Nominal sizeActual size (in)GuideSource
10x20x19.5 × 19.5 × 0.75[S3]
12x24x111.5 × 23.5 × 0.75[S4]
14x20x113.5 × 19.5 × 0.75[S5]
14x24x113.38 × 23.38 × 0.7514x25x1 vs 14x24x1[S6]
14x25x113.5 × 24.5 × 0.7514x25x1 vs 14x24x1[S7]
16x20x115.5 × 19.5 × 0.7516x20x1 vs 16x20x4[S8]
16x24x115.38 × 23.38 × 0.75[S9]
16x25x115.5 × 24.5 × 0.7516x25x1 vs 16x25x4[S10]
18x24x117.38 × 23.38 × 0.75[S11]
20x20x119.5 × 19.5 × 0.7520x20x1 actual size by brand[S12]
20x24x119.38 × 23.38 × 0.75[S13]
20x25x119.5 × 24.5 × 0.7520x25x1 vs 20x25x4[S14]
20x30x119.63 × 29.63 × 0.75[S15]
24x24x123.38 × 23.38 × 0.75[S16]
25x25x124.5 × 24.5 × 0.75[S22]
16x25x215.5 × 24.5 × 1.75[S23]
20x25x219.5 × 24.5 × 1.75[S24]
16x20x415.5 × 19.5 × 3.6316x20x1 vs 16x20x4[S25]
16x25x415.5 × 24.5 × 3.6316x25x1 vs 16x25x4[S26]
20x20x419.5 × 19.5 × 3.63[S27]
20x25x419.5 × 24.5 × 3.6320x25x4 actual size by brand[S1]

Depths: Filterbuy's 1-inch filters are 0.75 in, 2-inch are 1.75 in, and 4-inch are 3.63 in deep.

Media-cabinet filters: the ones that break the rule

These come from the cabinet maker's own documents. Don't swap in a standard 4-inch filter without checking.

FilterCabinetSold asActual size (in)GuideSource
Honeywell FC100A1029 / FC200E1029Honeywell F100/F200, 16 x 2516 x 25 media filter16 × 24⅞ × 4⅜Honeywell FC100A1029 size[S17]
Honeywell FC100A1037 / FC200E1037Honeywell F100/F200, 20 x 2520 x 25 media filter20 × 24⅞ × 4⅜20x25x4 actual size by brand[S17]
Honeywell FC100A1011 / FC200E1011Honeywell F100/F200, 20 x 2020 x 20 media filter20 × 19¾ × 4⅜20x20x1 actual size by brand[S17]
Lennox X6670 (HCF16-11)Lennox HCC16-2816x25x515¾ × 24¾ × 4⅜Lennox X6670 / X6673[S18]
Lennox X6673 (HCF20-11)Lennox HCC20-2820x25x519¾ × 24¾ × 4⅜Lennox X6670 / X6673[S18]
Carrier EZ Flex EXPXXFIL0016 / 0316Carrier EZXCAB / FILCAB, 16 x 2516 x 2524.6875 × 15.9 × 4.5 assembledCarrier EZ Flex[S19]
Carrier EZ Flex EXPXXFIL0020 / 0320Carrier EZXCAB / FILCAB, 20 x 2520 x 2524.6875 × 19.9 × 4.5 assembled20x25x4 actual size by brand[S19]
Aprilaire 210 / 213Aprilaire 2210 and others20 x 25 x 420.6 × 26.63 × 4 (H × W × D)Aprilaire 201 vs 213 vs 413[S20]

Carrier lists its dimensions as length × width × depth with a tolerance of ± 0.125 in. [S19]

How to read your filter

  1. Find the nominal size on the frame. DOE's Building America guide says the dimensions are usually printed on the cardboard frame. If that filter fit, buy the same size. [S21]
  2. Check the depth. A 1-inch filter that measures about ¾ in is normal. A "4-inch" or "5-inch" filter from a boxed cabinet needs the cabinet maker's size, not a standard one. [S1] [S17] [S18] [S19]
  3. No label? Measure the slot. The right filter covers the whole opening and lies flush with the frame. It should not sit diagonally. [S21]

Choosing a MERV rating for a 1-inch slot? See MERV 8 vs 11 vs 13 for a 1-inch filter.

Where to buy

We don't link to specific sellers here yet. Search for the exact part number or size on this page at a retailer you trust, compare the listed actual dimensions, and check the current price. We don't list prices because they change daily.

FAQ

What is the difference between nominal and actual filter size?

The nominal size is the rounded number printed on the frame and used for ordering. The actual size is what the filter measures. Filterbuy cuts its filters about half an inch under nominal so they slide in without binding. [S1]

How thick is a 1-inch furnace filter really?

Filterbuy's 1-inch filters are 0.75 inches deep. [S10]

How thick is a 4-inch furnace filter really?

Filterbuy's are 3.63 inches deep. [S26] Media-cabinet filters from Honeywell, Carrier and Lennox are about 4⅜ to 4½ inches deep. [S17] [S18] [S19]

My filter measures 15½ x 24½. What size is it?

That's a 16x25 by Filterbuy's cut. Its 16x25x1 is 15.5 × 24.5 × 0.75 inches. [S10]

What if my slot doesn't match any size?

Measure the slot. DOE's Building America guide says a properly sized filter covers the whole opening and lies flush. [S21]

What we could not verify

  • Which sizes are the most common. We picked sizes Filterbuy sells as standard pages and the sizes our guides already cover, not a sales ranking.
  • Other brands' actual sizes for the sizes without a guide. Brands cut differently, so we only list Filterbuy's numbers here.

Sources

  1. [S1] 20x25x4 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 20x25x4 actual size; half-inch undercut on purpose.
  2. [S2] 16x25x4 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: Half-inch undercut so the filter seats without binding.
  3. [S3] 10x20x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 10x20x1 actual size.
  4. [S4] 12x24x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 12x24x1 actual size.
  5. [S5] 14x20x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 14x20x1 actual size.
  6. [S6] 14x24x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 14x24x1 actual size.
  7. [S7] 14x25x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 14x25x1 actual size.
  8. [S8] 16x20x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 16x20x1 actual size.
  9. [S9] 16x24x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 16x24x1 actual size.
  10. [S10] 16x25x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 16x25x1 actual size.
  11. [S11] 18x24x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 18x24x1 actual size.
  12. [S12] 20x20x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 20x20x1 actual size.
  13. [S13] 20x24x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 20x24x1 actual size.
  14. [S14] 20x25x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 20x25x1 actual size.
  15. [S15] 20x30x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 20x30x1 actual size.
  16. [S16] 24x24x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 24x24x1 actual size.
  17. [S17] Honeywell Media Air Cleaners Professional Installation Guide (68-0239EF-12, Rev. 10-17), Honeywell International (PDF hosted by Build.com). Accessed 2026-10-24. Used for: Honeywell FC100/FC200 filter sizes.
  18. [S18] Healthy Climate HCC Media Air Cleaners product specifications (Form 210450, February 2026), Lennox Industries. Accessed 2026-10-24. Used for: Lennox X6670/X6673 sizes and cabinets.
  19. [S19] EXPX EZ Flex Expandable Air Filter, Sizes 016, 020, 024: Product Data, Carrier (PDF hosted by a Carrier distributor). Accessed 2026-10-24. Used for: EZ Flex dimensions and tolerance.
  20. [S20] AprilAire 213 MERV 13 Air Filter, AprilAire. Accessed 2026-10-24. Used for: Aprilaire 213 nominal and actual size.
  21. [S21] Replace Air Filter Located in Furnace or Central Heat Pump Air Filter Slot, Building America Solution Center (PNNL, U.S. Department of Energy). Accessed 2026-10-24. Used for: Reuse the old size if it fit; measure the slot; filter covers the opening and lies flush.
  22. [S22] 25x25x1 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 25x25x1 actual size.
  23. [S23] 16x25x2 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 16x25x2 actual size.
  24. [S24] 20x25x2 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 20x25x2 actual size.
  25. [S25] 16x20x4 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 16x20x4 actual size.
  26. [S26] 16x25x4 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 16x25x4 actual size.
  27. [S27] 20x20x4 Air Filters, Filterbuy. Accessed 2026-10-24. Used for: 20x20x4 actual size.

Last checked 2026-10-24. Spot an error? Tell us and we'll recheck it against the source.