Custom and Odd Furnace Filter Sizes: How to Measure and Order
Quick answer
Order by the nominal size printed on a filter that already fits. If nothing stock matches, measure the slot and order a custom size a little under that opening. [S1] [S2]
- The size on the cardboard frame is a rounded "nominal" size. Filterbuy cuts its filters about half an inch under on width and length so they slide in without binding. [S3]
- DOE's Building America guide says a properly sized filter covers the whole opening and lies flush. It should not sit diagonally. [S1]
- US Home Filter's measurement guide says that for a standard filter slot by the unit, aim about 1/4 in under the opening (up to 5/8 in under is still usable). For vents without clips, stay closer: about 1/8 in under. [S2]
- Media-cabinet filters (Honeywell, Carrier, Lennox, Aprilaire) do not follow the usual undercut rule. Use the cabinet maker's size. See the size chart.
How to decide what to order
| Situation | What to do | Source |
|---|---|---|
| Old filter fit and has a size on the frame | Buy that same nominal size | [S1] |
| Old filter didn't fit, or there is no filter | Measure the slot (width × length × depth) | [S1] [S2] |
| Measured size matches a common stock size | Order that nominal size; check the brand's actual size | [S3] |
| No stock size matches the opening | Order a custom filter to the measured size, usually a little under the opening | [S2] |
| Filter is from a boxed media cabinet | Use the cabinet maker's part and size, not a standard 4-inch filter | See size chart |
Custom undercut tips (US Home Filter)
These are US Home Filter's own tips for ordering a custom size. Other makers may cut differently.
| Where the filter sits | Suggested undercut vs the opening | Source |
|---|---|---|
| Wall or ceiling vent without clips or brackets | About 1/8 in smaller than the opening | [S2] |
| Wall or ceiling vent with clips or brackets | 1/8 in under is fine; up to 1/2 in under is okay | [S2] |
| Standard filter slot by the furnace / air handler | About 1/4 in under (up to 5/8 in under) | [S2] |
US Home Filter also says some manufacturers allow up to 1/4 in under on a labeled custom filter, and that it will cut the exact size you request. [S2]
Start with the size chart
Most homes take a common stock size. Check the furnace filter size chart first, then the size guides for 16x25, 20x25, 20x20, 16x20, and 14x25 vs 14x24.
How to measure
- Read the frame. DOE says the length and width are usually printed on the cardboard. If that filter fit, buy the same size. [S1]
- Confirm with a tape. US Home Filter says to measure the filter to confirm the printed size, and to measure the slot if you do not have a filter that fits. [S2]
- Depth matters. A labeled 1-inch filter is often about ¾ in deep. A media-cabinet filter sold as 4 or 5 inches is usually about 4⅜ to 4½ in deep. [S1] [S3]
- Airflow arrows. Install so the arrows point toward the furnace / into the duct. [S1]
When you need a custom size
If your opening does not match any stock size on the chart, order a custom filter to the measured opening, sized a little under so it seats. [S2] Do not trim a stock filter with scissors and hope. A gap around the frame lets air bypass the media.
Cabinet filters from Honeywell, Carrier, Lennox and Aprilaire are a different problem. Those sizes come from the cabinet maker. See the media-cabinet section of the size chart, plus Honeywell FC100A1029, Lennox X6670 / X6673, Carrier EZ Flex, and Aprilaire 201 / 213 / 413.
Disclosure: We earn a commission if you buy through FiltersFast links. It doesn't change which filters we say fit. How we handle affiliate links.
Where to buy
We don't list prices because they change daily. Check the current price at the seller.
- Buy at FiltersFast: furnace filters Affiliate: FiltersFast furnace filter category. Match a stock size when you can. Check current price.
- Buy at FiltersFast: 16x25x1 (example stock size) Affiliate: Example size landing page. Swap the size in the URL once you know yours. Check current price.
FAQ
What is the difference between nominal and actual filter size?
Nominal is the rounded size printed on the frame and used for ordering. Actual is what the filter measures. Filterbuy cuts about half an inch under so the filter seats without binding. [S3]
My filter measures 15½ x 24½ x ¾. What do I buy?
That matches Filterbuy's 16x25x1 cut (15.5 × 24.5 × 0.75). [S4] See the size chart and 16x25x1 vs 16x25x4.
What if no stock size matches my slot?
Measure the opening and order a custom filter. US Home Filter recommends sizing a little under the opening so the filter seats cleanly. [S2]
Should I force a filter that is too big?
No. DOE's guide shows a filter that is too big sitting diagonally, which is wrong. The filter should cover the opening and lie flush. [S1]
Can I put a thicker filter in my slot?
DOE says to use the thickest filter the slot can take when the cabinet allows it, because deeper pleats mean more surface area. Check that the slot is actually deep enough before you switch. [S1]
What we could not verify
- Which custom-cut shops match US Home Filter's undercut tips. We only cite US Home Filter's own guide for those numbers.
- A complete list of odd stock sizes every retailer carries. Use the size chart for the common ones we have sourced.
Sources
- [S1] Replace Air Filter Located in Furnace or Central Heat Pump Air Filter Slot, Building America Solution Center (PNNL, U.S. Department of Energy). Accessed 2026-10-28. Used for: Reuse the printed size if it fit; measure the slot; flush fit; no diagonal install; thicker filter when the slot allows; MERV overview.
- [S2] Measurement Guide, US Home Filter. Accessed 2026-10-28. Used for: Nominal vs actual; measure the slot; custom undercut tips for vents and furnace slots; custom makers may allow 1/4 in under.
- [S3] 20x25x4 Air Filters, Filterbuy. Accessed 2026-10-28. Used for: Half-inch undercut on purpose so the filter seats without binding.
- [S4] 16x25x1 Air Filters, Filterbuy. Accessed 2026-10-28. Used for: 16x25x1 actual size 15.5 × 24.5 × 0.75.
Last checked 2026-10-28. Spot an error? Tell us and we'll recheck it against the source.