How To Find Your Air Filter Size: 3 Easy Methods

Buying the wrong size air filter is one of the most common and most avoidable HVAC mistakes. A filter that is too big won't seat, and one that is too small lets unfiltered air slip around the edges. The good news: there are three reliable ways to nail down the size you need, and none of them require a technician.

1. Read the size printed on your current filter

The fastest method is to pull your existing filter and look at the cardboard frame. Manufacturers almost always print the dimensions along one edge, usually as three numbers such as 16x25x1 (width, height, and depth). If the filter is dusty from a few months of service, wipe the frame with a dry rag so the numbers are easy to read.

Those three numbers are your nominal size, the label size you order by. Once you have them, matching a replacement is simple. Popular residential sizes like 16x25x1, 20x25x1, and 16x20x1 are stocked and ready to ship, so most homeowners never have to measure at all.

2. Measure the filter yourself

If the frame is blank or too worn to read, grab a tape measure. You'll take three measurements in inches: width, height, and depth (thickness). Measure the filter itself, not the slot, and record the numbers in the same order every time to avoid mixing up width and height.

Here's the part that trips people up. The number you measure is the actual size, and it is often an eighth to a quarter inch smaller than the nominal size you shop by. For example, a filter that measures 15.75″ x 24.75″ x 0.75″ is sold as a nominal 16x25x1. That small gap is intentional, it lets the filter slide in and out without binding. So if your measurement lands just under a round number, round up to the nearest whole inch and order that nominal size.

• Width and height: round to the nearest whole inch for the nominal size.
• Depth: common thicknesses are 1″, 2″, 4″, and 5″. If you're unsure whether a thicker filter will fit, our guide to 1-inch vs. 2-inch vs. 4-inch filters explains the tradeoffs.
• Order matters, but the pair doesn't: a 20x25x1 and a 25x20x1 are the same filter, just rotated. The same is true for oddballs like a 23x20x1 and a 20x23x1.

For a step-by-step walkthrough with photos of where to measure, see our complete guide to measuring a furnace filter.

3. Check your HVAC owner's manual

If you don't have a tape measure handy, your system's owner's manual will usually list the recommended filter dimensions in the specifications. Lost the manual? Note the model number stamped on the furnace or air handler and call the manufacturer, or reach out to our filtration specialists. Either way you can confirm the size without guesswork.

What if my size isn't standard?

Not every system takes an off-the-shelf size. Older homes, additions, and some high-efficiency cabinets use in-between dimensions that big-box stores never carry. When that happens you have two solid options: order a custom cut-to-fit filter built to your exact measurements, or read our custom air filter size guide and tips for measuring an odd-size furnace filter before you buy. Curious how your size stacks up against the rest of the country? Our roundup of the most common air filter sizes for homes is a helpful reference.

Once you know your number, browse the full range of air filters by size and MERV rating and set up auto-delivery so the right filter shows up before the old one is due.

Frequently asked questions

What do the three numbers on an air filter mean?

They are the width, height, and depth in inches, in that order, for example 16x25x1. The first two describe the face of the filter and the third is its thickness. This is the nominal size you order by.

Is the nominal size the same as the actual size?

Not exactly. The nominal size is the rounded label size, while the actual size is the filter's true measurement, typically an eighth to a quarter inch smaller so it slides into the slot easily. Order by the nominal size unless your system specifically calls for an exact custom size.

What if I measure a size no store carries?

You likely have a non-standard opening. Order a custom-size filter cut to your exact dimensions rather than forcing a filter that doesn't fit, which would let air bypass the media entirely.