The most accurate measurement of the size of a human penis can be derived from several readings at different times since there is natural minor variability in size depending upon arousal level, time of day, room temperature, frequency of sexual activity, and reliability of measurement. When compared to other primates, including large examples such as the gorilla, the human penis is thickest, both in absolute terms and relative to the rest of the body.
Measurements vary, with studies that rely on self-measurement reporting a significantly higher average than those with a health professional measuring. As of 2015, a systematic review of 15,521 men, and the best research to date on the topic, as the subjects were measured by health professionals, rather than self-measured, has concluded that the average length of an erect human penis is 13.12 cm (5.17 inches) long, while the average circumference of an erect human penis is 11.66 cm (4.59 inches). Flaccid penis length can sometimes be a poor predictor of erect length.
Most human penis growth occurs 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 genetics, such as the presence of endocrine disruptors, can affect penis growth. An adult penis with an erect length of less than 7 cm (2.8 in), but otherwise formed normally, is referred to in medicine as a micropenis.