How to measure your air filter

Most filters have the size printed on the cardboard frame. Start there.

Step 1: Read the size off the frame

Look for three numbers in this order: length, width, thickness. A common example is 16 x 25 x 1. This is the nominal size, the rounded name used when you shop.

Step 2: Know nominal versus actual

The actual size is usually a little smaller than the nominal size so the filter slides in easily. A filter named 16 x 25 x 1 often measures close to 15 3/4 x 24 3/4 x 3/4. Shop by the nominal size unless a listing says otherwise.

Step 3: Measure if the frame has no label

Use a tape measure on length, width, and thickness. Round each number to the nearest whole inch to get the nominal size. Check the thickness carefully, because a 1 inch filter will not seat properly in a 4 inch slot.

Step 4: Check the airflow arrow

The arrow on the frame should point toward the blower or furnace, in the direction the air travels. A filter installed backward works poorly.

Step 5: Write it down

Mark the size and the replacement date on the new filter's frame with a marker. Next time you will not need to measure at all.