The average ring size for a man is between 10 and 11, but a smaller man might wear a size 7 and a large man a size 13. To find the correct size, you need to know the circumference of your ring finger. It's best to measure at the end of the day, when your finger is its largest. Also, don't measure your finger when it's cold, since cold contracts and will give you a false reading.
Cut a piece of string to about 2 inches in length. If you don't have string, cut out a strip of paper about 1/4-inch wide and 2 inches long.
Wrap the piece of string or paper around the base of your ring finger. Wrap around both the base of your finger and your knuckle if your knuckle is noticeably larger than the base of your finger.
Mark the string or paper at the exact point that it overlaps to make a compete circle around your finger. Do this for where it overlaps on your knuckle as well, if you needed to measure it.
Unroll the string or paper and place it against a ruler that is divided in millimetres. The starting end of the string or paper (the end you first put against your finger) must be at the base of the ruler and marked 0. The point at which the mark on your string or paper meets the ruler is the circumference of your ring finger. If you needed two marks because of your knuckle, make a new mark exactly between the two and use that mark as your measure.
Refer to a ring chart to get your ring size (see Resources). For example, if the circumference measures 62mm, your ring size is a 10.
Take the measurements twice to ensure accuracy.