No size printed on my air filter: how to find it

Often the print is only worn, and part of it can still be read. Filterbuy says most frames carry two sizes: the nominal size in large type, which is the one you order, and the actual size in smaller type. Turn the old filter over and check all four edges before you reach for the tape.

Source: Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.

• All four edges of the frame, front and back. The small print can survive when the big print has rubbed off.
• The box or bag the last filter came in, or a past order in your email.
• Your home records binder, if you kept one, or a note on the duct next to the slot.
• Every other return grille in the house. If two grilles look the same, their filters may be the same size, but measure to be sure.

1. Switch the system off

   Set the thermostat to off before you pull the filter, so the blower doesn't draw dust into the unit.

2. Photograph the old filter

   Take a photo that shows the airflow arrow, so the new one goes in the same way.

3. Measure the opening

   Measure the short side and the long side edge to edge across the opening, to the tenth of an inch. The grille frame around it is always larger, so leave that out.

4. Measure the depth

   Measure how far the slot runs front to back. This is the number people skip, and it decides whether a filter fits at all.

5. Write it in order

   Write the three numbers as short side x long side x depth, then round each one as shown below.

Source: Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.

Filterbuy's rule: if a measurement is within about a quarter inch of a whole number, that whole number is your nominal size. A slot at 19.9 or 20.1 inches takes a 20. The filter itself is cut half an inch under, which leaves about a quarter inch of clearance on each side.

Source: Filterbuy, How to Measure Your Air Filter, checked October 9, 2026; Filterbuy air filter size chart, checked October 9, 2026.

A flat, unbent old filter can be measured too. Filterbuy cuts length and width half an inch under the nominal size and a 1-inch filter to 0.75 inch deep, so add 0.5 inch to the length and width and round. Other brands cut a little differently: Filterbuy lists a Filtrete 20x25x1 at 19.68 x 24.68 x 0.81 inches. Both round to the same 20x25x1. If the filter is bowed, measure the slot instead.

Source: Filterbuy air filter size chart, checked October 9, 2026; Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.

Filterbuy says larger homes often have more than one filter. Filters sit on the return side: behind a return grille, in a slot on the furnace or air handler, or in a media cabinet in the ductwork. With the system running, a return grille pulls air in and a supply vent blows it out. Measure each return slot you find, because two slots in one house can take different sizes.

Source: Filterbuy, Where Is My HVAC Air Filter?, checked October 9, 2026.

• Tip 01

  Measure to the tenth, not the eighth

  A reading in tenths shows at a glance whether you are near a whole number or sitting on a fraction.

  Source: Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.

• Tip 02

  Depth is the one to double-check

  Filterbuy names depth as the size it sees ordered wrong most often. A 1-inch slot won't take a 4-inch filter.

  Source: Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.

• Tip 03

  Feel for gaps after fitting

  Run a hand round the edges of the new filter with the system on. Moving air at the edge means a gap.

  Source: Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.

• Tip 04

  Buy one before you buy a case

  Test the fit of a single filter first, then order the multipack in the same size.

• Tip 05

  Ignore the furnace brand

  Filterbuy says the slot sets the size, not the brand: two homes with the same model can take different filters.

  Source: Filterbuy air filter size chart, checked October 9, 2026.

This page is one part of a bigger job. The system page puts the parts in order.

• SystemFurnace filter record: every slot, its size and the change datesFind every filter slot, write down its size and arrow, buy a year of spares, then check monthly and log each change on one printable record.8 steps · 4 parts · 1 page to printOpen the system

• Chapter 01HVAC filter size finder and replacement plannerTurn a nominal size like 16x25x1 into the filter's actual dimensions, or measure an old filter to find the size to buy, then plan the next change.Open the tool
• Practical noteWhat to change on move-in day: locks, garage opener and codesNew house, old keys. Here's the order to work in on move-in week: count what you got, change the locks, clear the garage opener, reset codes, then cut copies.Read the guide
• Practical noteFilter change reminders that actually stickA monthly look on a fixed day, the date written on the frame, a 60 to 90 day change window, and the next filter already on the shelf. Set up in ten minutes.Read the guide
• Chapter 01All records toolsPrintable home records with a calculator behind each one: appliance and warranty log, HVAC filter sizes and change dates.Open the chapter

• How to Measure Your Air Filter: Complete Step-by-Step Guide, Filterbuy (US air filter manufacturer). Checked October 9, 2026.
• Air Filter Sizes: Nominal vs. Actual Dimensions, Filterbuy (US air filter manufacturer). Checked October 9, 2026.
• Where Is My HVAC Air Filter? Return Vents, Air Handlers, and Furnace Locations, Filterbuy (US air filter manufacturer). Checked October 9, 2026.

Every outside number on this page names its source and the date we checked it. See them all on the sources page. Spotted an error? Tell us.