What size air filter do I need?

Filter sizing sounds like it should be trivial, and then you are standing in a hardware store holding a tape measure. There are three numbers involved, and knowing which one you need makes the whole thing simple.

Read the old filter first

The easiest answer is already in the system. Slide the current filter out and read the cardboard frame — the size is printed along one edge, as three numbers: length, width, and depth. For example, 20x25x1 means a 20-by-25-inch filter, one inch thick. Buy that.

Nominal vs. actual size

Here is the part that confuses everyone. If you measure a “20x25” filter with a tape, you will find it is closer to 19.5 by 24.5 inches. That is not a defect. The printed size is the nominal size — the size of the opening it fits — and the filter is made slightly undersized so it slides in without binding. So do not round your tape measurement to the nearest filter; buy the nominal size printed on the old one.

If there is no old filter to read

• Measure the slot opening, not the filter, and round up to the nearest standard size.
• Common sizes are 16x20, 16x25, 20x20, 20x25, and 20x30 — one of those is very likely your answer.
• Depth matters as much as the other two: a 1-inch filter in a 4-inch slot will rattle around and let air bypass it entirely.

Which way does it go in?

Every filter has an airflow arrow printed on the frame. It points in the direction air travels — toward the furnace or air handler, away from the return duct. Installing one backwards will not destroy anything, but the filter media is built directionally and works noticeably worse the wrong way round.

One practical trick: once you know the size, write it in marker on the filter slot cover, or photograph the frame. It is the single most reliably forgotten number in home maintenance.