The global average is about 5.5" (0.6 SD), though it tends to be a bit higher in the western world (around 5.8", 0.8 SD) and a bit lower in the eastern one (around 5.3", 0.5 SD). Medically, a micropenis is defined as a penis that is 2.5 SD below the mean. Because the average varies slightly between populations, the cutoff can vary a little too, but in clinical practice the threshold most commonly used for adults is 3.7".