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Guide

How to measure a furnace filter (nominal vs actual size)

Order the rounded number printed on the frame. Confirm the replacement’s actual (cut) size against the brand you are buying, because that is what has to slide into the slot.

Nominal size vs actual size

Nominal size is the shopping label: whole inches, like 16x25x1. It is printed large on the cardboard frame and on the box.

Actual size is the physical cut. For a 1-inch filter it is almost always a quarter to a half inch smaller on each face, and about ¾ inch thick — so the panel drops into a 1-inch rack without binding.

A 16x25x1 is usually about 15.75 × 24.75 × 0.75 inches. That is the figure Amazon Basics and current Honeywell 1-inch 16x25x1 listings publish. Other brands cut it differently (Filtrete closer to 15.69 × 24.69 × 0.81; Nordic Pure 15½ × 24½ × ¾). There is no single industry actual-size standard. Always match the brand’s published actual size to what is in your slot.

Step by step

1. Turn the system off. You will be pulling the panel out of the return or the furnace cabinet.
2. Read the frame first. Look for three numbers, such as 16x25x1 or 20×20×1. If you can read them, that is the size to order. Ignore the tape for ordering; use it only to confirm.
3. If the print is gone, measure the old filter. Measure the two face sides and the thickness to the nearest ⅛ inch. Write all three down.
4. Round the face dimensions up to whole inches. 15.69, 15.75, and 15.50 all shop as 16. 24.5, 24.69, and 24.75 all shop as 25. Thickness near ¾ inch is a 1-inch filter. Thickness near 1¾ is a 2-inch; near 3¾ is a 4-inch.
5. Check actual size on the replacement listing. Compare the brand’s published actual dimensions to your measurement. A 16x25x1 that is 15.50 × 24.50 × 0.75 (Nordic Pure) is a slightly looser fit than one that is 15.69 × 24.69 × 0.81 (many Filtrete 1-inch SKUs). Either can be correct; a large gap around the frame is not.
6. Do not mix thickness classes. A 16x25x1 does not replace a 16x25x4 media cabinet filter. Match the third number, or the OEM model number on 4- and 5-inch cabinets.

Worked example: 16x25x1

Suppose the old filter’s tape reading is 15¾ by 24¾ by ¾. You still buy 16x25x1. Published actual sizes for that label, from manufacturer product copy:

AirFilters.com’s own FAQ states that a 16x25x1 “usually measures about 15.75x24.75x0.75,” which matches the Amazon Basics / current Honeywell cut, not Nordic Pure’s 15.5 × 24.5 cut.

What not to do

• Do not order 15.75x24.75x0.75 as if it were a custom size unless you are actually buying a custom cut. Retail SKUs are sold as 16x25x1.
• Do not assume every brand’s 16x25x1 will measure the same. Compare actual size, not just the label.
• Do not force a filter that is too thick for the rack. The 1-inch class is ~0.75–0.81 inch thick, not a full inch.

16x25x1: actual size, MERV, and where to buy