The size of the DokLite you will need depends on the size of your dock pilings. Simply measure the diameter of your piling to determine the DokLite size you will need. If the diameter of your piling is between whole inches (8.5 inches, 9.5 inches, etc.) you must choose the larger size DokLite. For example, if your piling has a diamter of 8.5 inches, a 9 inch DokLite will fit your piling perfectly.