The nominal size is a shorthand number rounded to the nearest whole or half inch, normally in a larger font (ex: 16 x 20 x 1). Whereas the actual size is an exact measurement, normally in a smaller font, that depicts the filter's precise dimensions (ex: 15.50 x 19.50 x 0.75). All filters have both.

If your current air filter does not have a dimension listed on the frame, use a tape measure to record the width x length x depth to find the actual size. Then round these measurements up to the nearest inch to find a corresponding nominal size, which will be easier to locate when searching online.