How to read and measure your filter size

The size printed on the frame, like 20x25x1, is a rounded label called the nominal size. The filter itself is built a little smaller so it slides into the slot without binding. A new 20x25x1 often measures about 19.5 x 24.5 x 0.75 inches, and the gap is usually a quarter to a half inch per side.^[1] Each size page on this site lists the actual size each brand gives, so you can see how much they differ.

If you have the old filter

Read the printed size on the edge of the frame and buy that. If the label is gone, measure width, height and thickness to the nearest eighth of an inch, then round each one up to the next whole inch. Rounding down leaves a gap that lets air go around the filter.^[1]

If the slot is empty

Measure the opening from frame edge to frame edge and round to the nearest whole inch. The slot is built close to the nominal size, so the filter you order will measure a bit smaller than the opening.^[1]

Depth is not a choice

A 1-inch slot takes a 1-inch filter. Thicker filters need a slot or cabinet built for them. If your system has a media cabinet, its model page lists the filter the manual calls for.

MERV

MERV rates how well a filter catches particles between 0.3 and 10 microns.^[2] EPA suggests going as high as your system fan and filter slot can handle, and says an HVAC technician can help you find that limit.^[2]