The 5 inch furnace air filter comes in 4.25, 4.38, 4.5, 4.75 and 5 inches wide, depending on furnace manufacturers.
The actual is the true dimensions of the length, width, and thickness of the filter required by the unit.Air filters of actual 19 1/2″ x 19 1/2″ x 3/4″ are nominal 20x20x1.
A nominal piece of lumber 2by4 is actually only 1 1/2″ x 3 1/2″. Furnace air filters called 5 inch wide are nominal size, not actual size. The same is true for lumber.
Actual size air filters differ by make and model of your furnace. So write down the air filter maker and size that fit your particular furnace.
Nominal dimensions refer to the rounded number used on the label packaging of both air filters and lumber. Nominal sizing rounds up the measurements – 1.5 becomes 2, or 10.8 becomes 11. Knowing nominal size is not the exact measurement, but a rounded number instead, makes searching for the filter easier.
Whether you are measuring air filters or lumber, check the 2 dimensions to get the proper size for effectiveness and efficiency.