Measurements listed on filters are usually not exact. For example a 16x20x1 (nominal size) filter could actually measure 15-1/2 x 19-1/2 x 3/4 (actual size). If your filter does not have a size listed we recommend double checking the measurements before placing your order. This difference between nominal size measurements and actual size measurements allows for variations in filter rack sizes and also ensures that the filter will fit into a standard size filter rack. All filters on our website are listed as the nominal size. Listing nominal size instead of actual size is an industry standard for most air filter brands so if your current filter says 16x20x1 in most cases this will be a nominal size. In some older homes, and in other cases you may require the actual size. If you require the actual size you will need to place a custom order. If you are unsure whether you need actual or nominal size please measure your air filter before placing your order.