How to Find Your Air Filter Size

Buying the right air filter starts with one thing: knowing your size. The good news is you almost never need a tape measure — the size is printed right on the filter you already have. Here's how to find it in about a minute.

The fastest way: read the printed size

Your filter's size is printed on the cardboard edge (the frame) of your current filter. Follow three steps:

1. Turn off your AC/heating system at the thermostat so air isn't rushing through while you work.
2. Find your filter and slide it out. It sits in one of two places — a slot in the return-air vent (the big grille on a wall or ceiling that pulls air in), or a slot near your air handler / furnace (often in a closet, garage, attic, or hallway). Filters usually slide straight out; no tools needed.
3. Read the numbers on the frame. You're looking for three numbers, like 20 × 25 × 1. That's width × height × thickness, in inches.

That's it. Write those three numbers down and you're done.

How to read the numbers

A size like 16x25x1 means:

• 16 = width (inches)
• 25 = height (inches)
• 1 = thickness / depth (inches)

Most home filters are 1 inch thick. The order of width and height doesn't have to be perfect — a "16x25" and a "25x16" filter are the same thing — but it helps to write them the way they're printed.

You may see two kinds of numbers on the frame:

• Nominal size — the rounded, labeled size (e.g. 20x25x1). This is the one you want.
• Actual size — the true measured size, slightly smaller (e.g. 19.75 x 24.75 x 0.75). You'll usually see it in smaller print. You don't need this number to order.

When in doubt, go with the clean, rounded numbers — that's the nominal size stores and subscriptions use.

Can't read the size? Measure it yourself

If the printing is worn off, grab a tape measure and measure the filter itself (not the vent opening):

• Measure the width (side to side), the height (top to bottom), and the thickness (how deep the frame is).
• Round each measurement to the nearest inch. If you measure 19.75 inches, that's a 20. If you measure 0.75 inch thick, that's a 1.

Those rounded numbers are your nominal size.

A few things that trip people up

"I have more than one filter size." Totally normal — many homes have different-sized returns in different rooms. Just note each size. With VentWell, one plan can cover multiple sizes; we keep them all on file and include the right filters in every shipment.

"My filter is thicker than 1 inch." Some systems use 2", 4", or 5" media filters near the air handler. Measure the thickness so you order the right depth — a too-thin filter won't seal properly.

"There's no filter in the slot." If you can't find a filter at all, check both the return vents and the air handler. If there's genuinely none installed, measure the slot opening and round down slightly for the frame size.