How to measure your AC filter

Nominal size vs. actual size

Every filter has two sizes. The nominal size is the rounded number printed on the frame — 20x25x1, 16x20x1, 20x20x4. It is the size filters are labelled and sold by, so it is the number you shop with.

The actual size is what the filter really measures, and it is usually about ¼″ to ½″ smaller on each side so the filter can slide into its slot. A filter labelled 16x25x1 might measure roughly 15½ × 24½ × ¾ inches. The exact difference varies by manufacturer, so two filters with the same nominal size are not always identical.

Step 1: Read the size on the frame

Before you pull the old filter, note which way its airflow arrow points so the new one goes in the same way. Then look along the cardboard edge of the frame.

• The size is printed as width × height × depth, for example 20x25x1. The depth is the small number at the end.
• Some frames also print the actual size, often in brackets beside it — for example 20x25x1 (19½ x 24½ x ¾). The whole-number size is the nominal one.
• A MERV number printed nearby is the filter's rating, not part of its size.
• 20x25 and 25x20 are the same filter turned around. The depth is not interchangeable: a 20x25x1 and a 20x25x4 are different filters for different cabinets.

Many homes have more than one filter. Check every return grille and the air handler itself, and write each size down separately.

Step 2: No label? Measure the filter

If the print has worn off, lay the filter flat and measure it with a tape measure that reads to at least ⅛″. Write down three numbers in order: length, width and depth.

As a rule of thumb, the nominal size is each measurement rounded up to the next whole inch — 19½ × 24½ × ¾ is usually sold as 20x25x1. Round each dimension on its own. If a measurement already lands on a whole inch, the rule cannot tell you which way to go — measure the slot too (step 3).

Step 3: No filter at all? Measure the slot

Measure the inside of the opening where the filter sits — the slot in the air handler or the frame behind the return grille. Take the width, the height, and the deepest filter the space will hold.

The filter's actual size needs to be a little smaller than the opening, not larger. Before you buy, compare the actual dimensions the seller lists against your measurements.

How often to check it

ENERGY STAR suggests checking your filter every month, especially in heavy-use months, changing it when it looks dirty, and changing it at least every three months. In South Florida the air conditioner runs for much of the year, so a monthly look is a sensible habit. Your filter's and your system's own instructions come first.

Still not sure?

FilterJungle reads the size printed on your filter's label from a photo. It reads the label; it does not turn a tape-measure reading into a size for you.