Human penis size varies on a number of measures, including length and circumference when flaccid and erect. Besides the natural variability of human penises in general, there are factors that lead to minor variations in a particular male, such as the level of arousal, time of day, ambient temperature, anxiety level, physical activity, and frequency of sexual activity. 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. Most human penis growth occurs in two stages: the first between infancy and the age of five; and then between about one year after the onset of puberty and, at the latest, approximately 17 years of age.
Measurements vary, with studies that rely on self-measurement reporting a significantly higher average than those with a health professional measuring. A 2015 systematic review measured by health professionals rather than self-reporting, found an average erect length of , and average erect circumference of . A 1996 study of flaccid length found a mean of when measured by staff. Flaccid penis length can sometimes be a poor predictor of erect length. An adult penis that is abnormally small but otherwise normally formed is referred to in medicine as a micropenis.
No statistically significant correlation between penis size and the size of other body parts has been found in research. Some environmental factors in addition to genetics, such as the presence of endocrine disruptors, can affect penis growth.
Studies
While results vary slightly across reputable studies, the consensus is that the mean human penis, when erect, is in the range in length.
The 2015 review (measured by staff) found average flaccid, stretched, and erect lengths of , , and , respectively, and average flaccid and erect circumferences of and , respectively. Erect length in the studies was measured by pushing the pre-pubic fat pad to the bone, and flaccid or erect girth (circumference) was measured at the base or mid-shaft of the penis.
Measurement methods Reported averages vary partly because penile size is affected by erection state, measurement technique and whether the measurement is self-reported or taken by a clinician. In clinical studies, length is commonly measured along the dorsal side from the pubic bone to the tip of the glans, with the pre-pubic fat pad compressed to the bone. Circumference is usually measured at the base or mid-shaft. Stretched flaccid length is also used in clinical assessment, although it is not identical to erect length in all men and can be affected by the amount of traction applied during measurement.
Length
Flaccid A review of studies found average flaccid length to be . Length of the flaccid penis does not necessarily correspond to length of the erect penis; some smaller flaccid penises grow much longer, while some larger flaccid penises grow comparatively less.
The penis and scrotum can contract involuntarily in reaction to cold temperatures, anxiety level and participation in sports. This decrease of flaccid penis size is referred to by the slang term "shrinkage", due to action by the cremaster muscle. The same phenomenon affects cyclists and exercise bike users, with prolonged pressure on the perineum from the bicycle saddle and the straining of the exercise causing the penis and scrotum to contract involuntarily. An incorrect saddle may ultimately cause erectile dysfunction (see crotch pressure for more information). Individuals with hard flaccid syndrome or other pelvic floor disorders may temporarily have an abnormally small penis.
Stretched Neither age or size of the flaccid penis accurately predicted erectile length. Stretched length has correlated with erect length in some cases. However, studies have also shown drastic differences between stretched and erect length. One study found that a minimal tension force of approximately 450 g during stretching of the penis was required to reach a full potential erection length. This study also found that tension forces exerted in this study by the urologist were shown to be significantly (P<0.01) lower than 450g. This may account for differences between stretched and erect length. The 2015 study of 15,521 men found an average stretched flaccid length of 13.24 cm (5.21 inches), nearly identical to the average erect length of 13.12 cm (5.17 inches). A 2001 study of about 3,300 men found an average flaccid stretched length of . Checking for correlations in a random subset of the sample consisting of 325 men, they found a few statistically significant Spearman's correlations: between flaccid length and height of 0.208, −0.140 with weight, and −0.238 with BMI, flaccid circumference and height 0.156, stretched length and height 0.221, weight −0.136, BMI −0.169.
Erect Scientific studies have been performed on the erect length of the adult penis. Studies that have relied on self-measurement, including from Internet surveys, consistently reported a higher average length than those that used medical or scientific methods to obtain measurements.
A 2020 review found most men believed the average erect penis length is more than 15.24 cm (6 inches). This inaccurate belief has likely been fed by inaccurate and exaggerated data presented in studies where the size of a participant's erect penis is self-reported. Participants may report overestimates of the size of their penis in the belief that a larger penis is more socially desirable. The same review analyzed the results from ten prior studies where measurements of erect penis size were made by researchers. They reported an erect penis to be between 12.95 and 13.92 cm (5.1 and 5.5 inches, respectively) in length, a result significantly below the average obtained in self-reported studies. The authors commented that results of such measurement studies may still be inflated due to volunteer bias — the possibility that men with larger penises may be more likely to choose to participate in such studies.
Erect circumference Similar results exist regarding studies of the circumference of the adult fully erect penis, with the measurement usually taken mid-shaft. As with length, studies that relied on self-measurement consistently reported a significantly higher average than those with staff measuring. In a study of penis size where measurements were taken in a laboratory setting, the average penis circumference when erect was 11.66 cm (4.59 inches).
Size at birth and during childhood
The average stretched penile length at birth is about , and the length for 90% of newborn boys is between . Limited growth of the penis occurs between birth and 5 years of age, but very little occurs between 5 years and the onset of puberty. The average size at the beginning of puberty is , with adult size reached about 5 years later.
W. A. Schonfeld published a penis growth curve in 1943.
Size in older men Age is not believed to negatively correlate with penis size.
Size and height Height is very weakly correlated with flaccid stretched length.
Size and hands A study investigating the relationship with digit ratio and found men with longer ring fingers than index fingers had longer penises, and it was the only significant predictor of both stretched and erect penis length. However, the common misconception that overall hand size predicts penis size has been widely discredited.
Size and other body parts There is no correlation between shoe size and stretched penis size.
Nose size was highly related to stretched penile length in Japanese male cadavers.
There may be a link between the malformation of the genitalia and the human limbs. The development of the penis in an embryo is controlled by some of the same Hox genes (in particular HOXA13 and HOXD13) as those that control the development of the limbs. Mutations of some Hox genes that control the growth of limbs cause malformed genitalia (hand–foot–genital syndrome).
Obesity and perceived size
Excess adipose tissue in the pubic region can obscure penile length, a condition known as buried penis. In such cases, the penis is of normal size but is partially or completely hidden by the surrounding fat pad. A cross-sectional study of 680 obese boys found that 30.88% had buried penis, while 9.12% met criteria for micropenis based on stretched penile length measurements. Standard clinical measurement of penile length involves pushing the pre-pubic fat pad to the pubic bone to obtain an accurate reading, distinguishing true penile length from apparent length.
Size, race, and ethnicity Alleged differences in races have led to the creation of sexual myths. There is no scientific background to support the alleged "oversized" penis in black people. There is no indication that penis size differs between ethnicities.
According to urologist Aaron Spitz, many websites and studies promoting variation of penis size between races use unscientific methods of collecting information and often ignore contradictory evidence. He concludes that "when you really take a good look at the naked data, there's not a whole lot there [showing racial variation in penis size]."
A 2024 meta-analysis of 34,060 Chinese men found that while flaccid penis length was shorter on average than the global reference population, erect length showed no statistically significant difference. The study found that Chinese men had a higher average increase from flaccid to erect state (67%) compared to the global average (43%), meaning a greater proportion fell into the category colloquially known as "growers" rather than "showers." The authors noted this has implications for older cross-population studies, many of which relied on flaccid rather than erect measurements: populations with more "growers" would appear smaller in flaccid-based datasets even when erect dimensions are comparable.
While some meta-analyses have identified modest average differences across geographic regions, the variation in penis size within any racial or ethnic group vastly exceeds the differences between groups. Individual variation within any racial group spans over 7 cm (3 inches) in some datasets, a range that is 15 to 30 times greater than the alleged average differences between racial groups, which typically measure less than 0.5 cm (0.2 inches) in clinician-measured studies. Factually race is therefore a poor predictor of any individual's penis size, and reported racial averages do not account for the substantial overlap between group distributions.
Size preferences among sexual partners In a 1994 cover story by Psychology Today, 1,500 readers (about two-thirds women) were surveyed about male body image. Many of the women were not particularly concerned with penis size, and over 71% thought men overemphasized the importance of penis size and shape. Generally, the women polled cared more about width than men thought, and less about length than men thought, although the strength of preference for either among women showed a similar pattern.
A small study of 50 undergraduate women, surveyed by two popular male athletes on campus about their perceptions of sexual satisfaction; found the width of a penis feels better than the length of a penis, when subjects are asked to choose between the two (size was left unspecified). It was also concluded that this may show that penis size overall affects sexual satisfaction, since women chose between the two options they were given.
A 2002 study at Groningen University Hospital, asked 375 sexually active women (who had recently given birth) the importance of penis size. The results showed that 21% of women thought length was important and 32% thought that girth was important.