Human penis size is the measured length and width of the human penis. The most accurate measurement of the human penis comes from several measurements at different times since there is natural minor variability in size due to arousal level, time of day, room temperature, frequency of sexual activity, and reliability of measurement. When compared to other primates, including large primates such as the gorilla, the human penis is largest, both in absolute terms and in relative size to the rest of the body.
Measurements vary, with studies that rely on self-measurement reporting a significantly higher average than those with staff measuring. However, the mean of an erect human penis is approximately 12.9–15.0 cm (5.1–5.9 in) in length. Flaccid penis length is a poor estimate of erect length. Most of human penis growth happens between infancy and the age of five, and between about one year after the onset of puberty and, at latest, approximately 17 years of age.
A statistically significant correlation between penis size and the size of other body parts has not been found in research. Some environmental factors in addition to genetic, such as the presence of endocrine disruptors, can affect penis growth. An adult penis with an erect length of less than 7 cm or just under 3 inches, but otherwise formed normally, is referred to in medicine as a micropenis