When it comes to air filters, the exact size is always slightly smaller than the nominal size to ensure it fits properly into the slot or housing. This is a common practice in the manufacturing of air filters. Generally, the length and width dimensions are interchangeable, with the largest measurement being the width and the shortest being the length. The smallest dimension is usually the depth. Nominal dimensions refer to the rounded number used on air filter and wood labels.
There are thousands of sizes of air filters, and manufacturers of heating, ventilation and air conditioning systems often make it as complicated as possible, so that you have to purchase expensive replacements. As for depth measurement, standard air filters are nominally 1 (0.75 inches), 2 (1.75 inches) and 4 (3.75 inches) in depth. In some cases, the length and width are interchangeable, but you must always measure those three specific dimensions of the air filter or oven filter. If you find yourself having to force an air filter into or out of its slot, it's not the right size, so standard sizes are usually half an inch smaller than advertised. The actual size is the actual dimensions of the filter in length, width, and thickness.
Not understanding these numbers often leads consumers to buy a filter that doesn't fit their oven or air conditioning system. For example, a size of 20x30x1 may appear on the filter label, but if you measure the filter, you will see that the actual size is smaller than the nominal air filter size of 20x30x1. Building a custom air conditioner or oven filter allows you to choose specific dimensions if standard sizes aren't suitable for your needs. Since air filters are usually sold for their nominal dimensions, it is essential to learn how to measure them correctly. Incorrectly sized air filters can place excessive stress on your air conditioning system, damage it, and increase your energy bills.
