The nominal filter size is the approximate size up of the filter rounded up to the nearest “whole” inch. This makes the filter easier to identify for the consumer and allows for filters to comfortably fit into specified space in a residential furnace without air by-pass. The tolerances allowed for with this nominal size are +/- 1/8″ for both the width and length. The actual size is just as it sounds; it is literally the exact dimensions of the filter. For example, the nominal size of a Dust Shield filter could be 20x20x1 however its actual size is 19 5/8″x19 5/8″x13/16″. This may not be true for all manufacturers. Be sure to note the actual size required for you unit before purchasing and changing your filter.