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How To Measure Air Filter

What you need

A tape measure, a phone camera and a few minutes. Turn the thermostat to off, or switch the system off, before pulling a filter so the blower is not drawing dust into the equipment while the slot is open. If the filter sits in a furnace cabinet, you may need to remove a small access door or slide a cover sideways.

Step 1: Find every filter location

Many homes have one filter at the furnace or air handler. Others have filters in one or more return grilles on walls or ceilings, and no filter at the equipment. A few have both, which is usually unnecessary and can restrict airflow. Walk the house and look at each large return grille: a filter grille has hinged louvres or latches and a filter visible behind it. Write down where each one is.

Step 2: Photograph the old filter

Before measuring, photograph the frame label. Even a faded or dusty label often shows the nominal size, the MERV rating and the airflow arrow. That photo is your fastest path to the right replacement and a record of the arrow direction.

Step 3: Measure the old filter

• Lay the filter flat and measure the longer side edge to edge.
• Measure the shorter side.
• Measure the depth across the frame, not the pleats.
• Write them down to the nearest ⅛ inch.

Round each face dimension up to the next whole inch. A filter measuring 15½ × 24⅝ inches is nominal 16 × 25. Depth around ¾ inch is sold as 1 inch; around 1¾ is 2 inch; about 3½ to 4⅜ is a 4- or 5-inch media filter depending on the cabinet. The filter size finder does this rounding for you.

Step 4: Measure an empty slot or grille

If the filter is missing or damaged, measure the opening it slides into. For a furnace slot, measure the inside length and height and the width of the slot gap. For a return grille, open the grille door and measure the inside of the frame that holds the filter, not the outer decorative face. A grille that looks 30 × 20 from the room may hold a 30 × 20 nominal filter or a slightly smaller one, depending on design. Slot dimensions usually exceed the nominal size by a fraction of an inch; round down to the nearest whole inch. If you get 20⅛ × 25¼ inches, buy 20x25.

Step 5: Check the depth

Depth matters as much as face size. A 1-inch filter in a 4-inch media cabinet leaves room for air to go around it; a 4-inch filter will not fit a 1-inch slot. Media cabinets often have a model number on the door; their manuals list the exact replacement, which may be brand-specific.

Step 6: Note the airflow arrow

The arrow on the frame points the way air travels: toward the furnace or air handler and away from the return duct. In a ceiling grille it points up; in a wall grille it points into the wall. Mark the direction on the photo.

Worked example

In a townhouse, the furnace slot is empty and the old filter was thrown away. The homeowner measures the slot at 16⅛ inches tall and 25¼ inches wide, with a 1-inch gap. Rounding down gives a nominal 16x25x1. They also check the hallway ceiling and find a second filter grille holding a 20x20x1 filter. Because filtering at both points adds resistance, they ask the building's HVAC contractor which location is intended; the answer is the ceiling grille only, so the furnace slot gets a cover instead of a filter.

Illustrative note (2026-10-29): This example was prepared on 2026-10-29 for illustration. The home layout and measurements are hypothetical; whether to use one or two filter locations depends on your system, so ask a qualified HVAC technician.

Common measuring mistakes

• Measuring the outside of a grille instead of the inside filter frame.
• Rounding down a filter measurement — this gives a filter that is too small.
• Ignoring depth and buying 1-inch filters for a 4-inch cabinet.
• Reading the actual size on a label as the nominal size.
• Assuming every return grille has the same filter size.

Unusual equipment

Some heat pumps, ductless air handlers, through-wall units and mobile home furnaces use proprietary filters or non-standard sizes. If your measurement lands between common sizes, check the unit's data plate or manual. For mobile homes, see mobile home filters; for apartments, apartment filter sizes.

Making the next purchase easy

Once you know your size, write it on the furnace cabinet or the inside of the grille door with a marker, along with the date you installed the filter. Many people also keep a note in their phone listing each location, size and MERV. Our replacement schedule can turn that into reminder dates.

Safety while measuring

Never reach into a furnace cabinet past the filter area while the system is powered. Some cabinets have a door switch; others do not. Avoid touching burner or electrical compartments.

Keep a spare

Once measured, buying two filters at a time means the next change happens on schedule instead of waiting for a shopping trip.

Frequently asked questions

Do I measure in inches or centimetres?

Residential filters in Canada and the U.S. are sold in inches. Convert metric measurements before rounding.

Which number comes first?

Conventionally the smaller face dimension, then the larger, then depth, such as 16x25x1. A 25x16x1 is the same filter.

What if my measurement is exactly a whole number?

Measure the slot. A filter measuring exactly 16 inches may be a near-nominal brand, or a 16½-inch custom size.

Related pages

• Nominal vs actual
• Filter size finder
• Return grille sizer
• Size chart