Measuring your air filter is easier than you think. One of the best ways to find the size of your air filter, is to check the outside of the filter to find the part number and size. The other way to find the size of your filter is to measure it yourself. The measurements of the air filter go by length, width and depth. It’s also important to know that the length and width are interchangeable. For example, if your filter is 14 X 20 X 1, it can also be 20 X 14 X 1. Be sure to measure your air filter to the nearest whole number in order to find your correct size.