The easiest way to determine a man's ring size is to use the string method along with a ring size chart. A ring size chart can be found at Zales.com. Wrap a piece of string snugly around the finger to be measured, and compare the measurement with a sizing chart.
Do not wrap the string too tightly, and measure when hands are warm rather than cold. In order for the measurement to be as accurate as possible, someone should help measure. Remember to double-check for the most accurate measurement. Each finger may have a different size, so fingers should be measured separately.