Penis size measurement methodologies are not standardized and thus we’ll try to provide as much information as possible for each study discussed. All the studies presented here measure at least one of the following measurements: flaccid length, flaccid circumference, erect length, erect circumference or stretched length.
The usual procedure to measure flaccid penis length is to take the measurement with the penis fully flaccid hanging down, from the pubopenile skin junction to the tip of the glans as shown in the first figure.
The erect length can be measured in two ways:
The Non Bone Pressed method, in which the measurement is taken on the upper side of the fully erected penis forming a 90º angle with the body and from the pubopenile skin junction to the tip of the glans. Shown in the second image.
The Bone Pressed method is performed the same way than the Non Bone Pressed method but pushing the ruler into the pubis gently as shown in the third image. The aim of this method is to avoid differences given by the fat pad thickness of subjects, which is related with overweight. Fat pad variability is independent from penis length variability and thus this method studies only the penis size inherent to subjects regardless of their weight. Some authors also refer to this measure as the Functional Length, since the fat pad can move and compress making this part of the penis functional during intercourse. For this reasons, the Bone Pressed method is the most used one in clinical studies.
Another important measure often taken is the circumference of both the flaccid and erect penis. The circumference is measured wrapping a measuring tape around the shaft of the flaccid or erect penis as shown in the first image. Methodologies differ in where the circumference is measured, while most studies take it around the thickest part of the penis some take it at the base, some at the middle of the shaft and some at the glans. This difference will be indicated if mentioned in the study.
As it is not always easy to get subjects to participate in studies that involve erect measures (due to the difficulty of getting and maintaining an erection and/or modesty issues) some studies take the stretched penile length as a first approximation of the erect length. Although the stretched length is not equal to the erect length, this measure has been proven to correlate much better to the erect length than the flaccid size. This measure is done by holding the glans and pulling the flaccid penis away from the body at its maximum without pain and taking the measure on the upper side of the penis as shown in the second image. Again this can be done Bone Pressed or Non Bone Pressed, but it’s mostly done Bone Pressed.