What size furnace filter you need, and how to find it

Last updated: 2026-10-28

Read the size printed on the side of the filter you have now, such as 16 x 25 x 1, and buy that size. If no size is printed, measure it: Trane says a 16 x 25 x 1 filter “might be 15.5 x 24.5 x .7”, about half an inch smaller.

The size is printed on the filter's side

Trane says: “Most filters have the size printed on one or more sides.” It names 16 x 25 x 1, 20 x 20 x 1, 16 x 20 x 1 and 20 x 25 x 1 as common sizes. Makers' actual sizes for three of them: 16x25x1, 20x25x1 and 16x20x1.

Source: Trane, Furnace Filter Guide.

With no size printed, measure the old filter

Trane says the actual size “might be half an inch smaller than the size you need to buy”. Filterbuy cuts each filter “to a tolerance of ±1/8"”. Why the two sizes differ: nominal vs actual size.

Sources: Trane, Furnace Filter Guide; Filterbuy, Shipping, Returns, and Cancellations.

Where the filter is

Trane says the furnace could be in the attic, the basement or a utility closet. The filter sits in a filter cabinet next to the furnace, or behind a large return air vent cover on a wall or ceiling.

Source: Trane, Furnace Filter Guide.

Media cabinets use a filter number

A 4 or 5 in media cabinet takes its maker's filter. AprilAire prints the filter number inside the cabinet door and on the rail of the old filter. Lennox, Carrier and Honeywell part sizes: filter size by brand.

Source: AprilAire, Air Purifiers and Air Filters FAQs.

One filter serves the furnace and the AC

Trane says to change the filter year-round: the furnace “acts as an air handler for your AC unit or heat pump”.

Source: Trane, Furnace Filter Guide.

The arrow points toward the furnace

• Look for the arrow on one side of the filter, usually the top.
• Slide the filter in with the arrow pointing to the furnace.
• Trane suggests marking an arrow on the furnace to remember the direction, and writing the date on the filter edge.

Source: Trane, Furnace Filter Guide.

AprilAire and Trane give these filter-change steps

1. AprilAire says to turn off your HVAC system before removing the old filter.
2. AprilAire says to have a plastic bag ready and to avoid shaking or bumping the old filter.
3. AprilAire says to tape the bag securely and place it in an outdoor trash can.
4. Trane says to slide the new filter in with the arrow pointing to the furnace.
5. AprilAire says to insert the replacement filter and then turn the system back on.

Sources: AprilAire, Air Filter Guide, “Proper air filter disposal”; Trane, Furnace Filter Guide.

See also: how often to change your furnace filter, filter size by brand, nominal vs actual size. All filter sizes.

Sources

1. Trane, Furnace Filter Guide (updated 2025-09-08): https://www.trane.com/residential/en/resources/troubleshooting/gas-furnaces/furnace-filter-guide/, accessed 2026-10-28
2. Filterbuy, Shipping, Returns, and Cancellations (undated page): https://filterbuy.com/customer-service/shipping-returns-cancellations/, accessed 2026-10-28
3. AprilAire, Air Purifiers and Air Filters FAQs (updated 2026-03-13): https://www.aprilaire.com/blog/aprilaire-air-purifiers-and-air-filters-faqs, accessed 2026-10-28
4. AprilAire, Air Filter Guide (updated 2026-10-20): https://www.aprilaire.com/whole-house-products/aprilaire-air-filter-guide, accessed 2026-10-28