There are two ways to measure the circumference of the penis when it is erect, which can be categorized into the direct measurement method and the thin line measurement method. 1. Direct Measurement: When the penis is erect, place the thickest part of the penis directly on a ruler, visually measure the diameter of the penis, and then multiply this value by 3.14 to get the circumference of the penis. 2. Thin line measurement method: when the penis is erect, first use a thin line around the thickest part of the penis for a week, and record the length of the thin line, and then measure the length of the thin line, that is, the circumference of the penis. Pay attention to the measurement, to slightly force against the skin around the penis, because some people have more abdominal fat, subcutaneous fat covers part of the penis size, which may cause errors in the measurement. In addition to the two methods mentioned above, you can also go to the hospital to find a specialist for measurement.