How to Find Your Furnace Filter Size (Step by Step)

You need three numbers: width × height × depth in inches (for example, 16×25×1). The surest way is to read the old filter or measure the slot—not guess from the furnace model alone.

Before you start

Turn the thermostat to Off (or set fan to Auto and wait for the blower to stop).
Have a flashlight and a tape measure ready.
Plan to write the size down on the furnace cabinet or in your phone so you only do this once.

Step 1: Find the filter

Most homes keep the filter in one of these places:

At a wall or ceiling return grille — a large vent cover, often in a hallway or living area. Many pull open or unscrew so the filter sits behind the grille.
In a slot on the furnace or air handler — look for a thin door or slot on the side or bottom of the metal cabinet, usually marked “filter.”
In a media cabinet — a thicker box next to the furnace that holds a 4" or 5" filter.

Some homes have more than one return. Check every large return grille and the furnace itself so you catch every filter.

Step 2: Read the size printed on the frame

Slide the filter out carefully (dust side faces the room/return; keep it level so debris does not spill).
Look along the cardboard edge for a printed size such as 16x25x1 or 20x25x1.
You may also see a smaller “actual” size (for example 15½ × 24½ × ¾). Stores sell by the larger nominal size—the rounded numbers.
If the print is worn off, go to Step 3 and measure.

Step 3: Measure if nothing is printed

Measure the filter’s width (side to side), height (top to bottom), and depth/thickness.
Or measure the open slot or grille opening the same way.
Round each length up to the nearest whole inch for the nominal size. Example: 19½ × 24½ × ¾ becomes 20×25×1.
Depth is usually 1", 2", 4", or 5". Match the depth your slot or cabinet was built for—do not force a thicker filter into a 1" slot.

Step 4: Confirm orientation and reinstall

The arrow on the frame points in the direction of airflow: toward the furnace/blower, away from the room.
Seat the filter fully so air cannot bypass the edges.
Close the grille or door and restore power.

Quick checks that save a return trip

Write the full size (for example 16x25x1) on the cabinet with a marker.
If you have multiple returns, note each size—they are not always the same.
For thick media filters, also note the cabinet brand/model if it is on a sticker; some 4"/5" filters are brand-specific.
When in doubt, take a clear photo of the old filter’s edge label before you throw it away.

For a list of common residential sizes and how nominal vs. actual sizes relate, see Common Furnace Filter Sizes and How to Read Them.

Shop common sizes

After you write your size down, these Amazon searches cover the sizes most homes use. As an Amazon Associate we earn from qualifying purchases.