If you have a filter that fits well into your air return, then the easiest way to determine what size filter you need is to measure the filter you currently have. You should note that in most cases, the size that is printed on the side of the filter is not usually the exact size of the filter. The size printed on the filter will be referred to as the “nominal size” and the exact size is known as the “exact size”. For example a filter that is marked 23 x 19 x 1 will have a nominal size of 23 x 19 but it will actually measure 22 ¾ x 18 ¾ x ¾, which is its exact size.
If you do not have a filter that fits well, you can measure the inside of the filter frame in your air return. Be sure the measure height, width, and depth. When you order a custom filter, the filter will be made to your exact size with a +0 to -1/8" tolerance. That means the filter will not be more than the size specified but may be up to 1/8th an inch less.
It is important to measure carefully when ordering custom filters, as they are not returnable or refundable.