Make sure your penis is hard and use a measuring tape to measure the circumference. You do this by wrapping the base of your whole package with your measuring tape behind your balls.
From the beginning to the end we take 13 cm measured (average size) as an example. We divide this circumference by 3.14.
You will get a number in centimeters (4.14 cm). The penis ring must be slightly smaller to do its job properly. From the 4.14 cm we take about 0.3 to 0.4 cm away
So your actual penis ring size should be around 4.14 - 0.3 = 3.84. This means that the best penis ring size is 38/39 mm.