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. As of 2015, a systematic review of 20 studies, of up to 15,521 men, who were measured by health professionals rather than themselves, concluded that the average length of an erect human penis is 13.12 cm (5.17 in), while the average circumference of an erect human penis is 11.66 cm (4.59 in). A 1996 study of flaccid length found a mean of 8.8 cm (3.5 in) 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, with the cutoff usually being defined as around 3 inches (7.6 cm).

Limited to 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.

While results vary slightly across reputable studies, the consensus is that the mean human penis, when erect, is in the range 5–6 in (13–15 cm) in length.

The systematic review by Veale et al. (2015), of 20 studies of 15,521 men, where subjects were measured by staff, rather then self measured, found the average flaccid, stretched, and erect lengths of 9.16 cm (3.61 in), 13.24 cm (5.21 in), and 13.12 cm (5.17 in), respectively, and average flaccid and erect circumferences of 9.31 cm (3.67 in) and 11.66 cm (4.59 in), respectively. Erect lengths in the included studies were 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.

One study (published in 1996) found the mean flaccid penis length to be 3.5 in (8.9 cm) (measured by staff). A review of several studies found average flaccid length to be 9–10 cm (3.5–3.9 in). 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, anxious or nervous 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.

Neither the age nor 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 that the average length of a stretched flaccid penis was 13.24 cm (5.21 inches) long, which is near identical to the average length of an erect human penis which is 13.12 cm (5.17 inches) long.
• A 2001 study of about 3,300 men published in European Urology concluded that flaccid stretched length was measured on average to about 12.5 cm (4.9 in). In addition, they checked 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. They also reported a few non-significant correlations.

Scientific studies have been performed on the erect length of the adult penis. Studies that have relied on self-measurement, including those from Internet surveys, consistently reported a higher average length than those that used medical or scientific methods to obtain measurements.

The following staff-measured studies are composed of different subgroups of the human population (in other words, specific age range or race; selection of those with sexual medical concerns or self-selection) that could cause a sample bias.

• In a study of 80 healthy males published in the September 1996 Journal of Urology an average erect penis length of 12.9 cm (5.1 in) was measured. The purpose of the study was to "provide guidelines of penile length and circumference to assist in counseling patients considering penile augmentation." Erection was pharmacologically induced in 80 physically normal American men (varying ethnicity, average age 54). It was concluded: "Neither patient age nor size of the flaccid penis accurately predicted erectile length."
• A study published in the December 2000 International Journal of Impotence Research found that average erect penis length in 50 Jewish Caucasian males was 13.6 cm (5.4 in) (measured by staff). The study intended "to identify clinical and engineering parameters of the flaccid penis for prediction of penile size during erection." Erection was pharmacologically induced in 50 Jewish Caucasian patients who had been evaluated for erectile dysfunction (ED) (average age 47±14y). Patients with penis abnormalities or whose ED could be attributed to more than one psychological origin were omitted from the study.
• A review published in the 2007 issue of BJU International showed the average erect penis length to be 14–16 cm (5.5–6.3 in) and girth to be 12–13 cm (4.7–5.1 in). The paper compared results of twelve studies conducted on different populations in several countries. Various methods of measurements were included in the review.
• A 2015 study by BJU International concluded the average erect penis length to be 13.12 cm (5.16 inches).
• An Indian study (published in 2007) of 301 men ages 18 to 60 published in the International Journal of Impotence Research found flaccid, stretched and erect length to be 8.21 cm (3.23 in), 10.88 cm (4.28 in) and 13.01 cm (5.12 in), respectively.
• A Korean study (published in 1971) of 702 men ages 21 to 31 identified the average erect penis length to be 12.70 cm (5.00 in). Another study (from 1998) of 150 Koreans found the average erect penis length to be 13.42 cm (5.28 in). The most recent study (published in 2016) of 248 Korean men identified the average erect penis length to be 13.53 cm (5.33 in).
• A 2020 review in the Journal of Sex & Marital Therapy found that the majority of men believed that 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.
• A 2023 review and meta-analysis analyzed 75 studies published between 1942 and 2021. The results showed that the average length of the erect penis has increased by 24% over the past 29 years.

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).

The average stretched penile length at birth is about 4 cm (1.6 in), and the length for 90% of newborn boys is between 2.4 and 5.5 cm (0.94 and 2.17 in). 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 6 cm (2.4 in), with adult size reached about 5 years later.

W.A. Schonfeld published a penis growth curve in 1943.

Age is not believed to negatively correlate with penis size.

Height is correlated with flaccid stretched length and erect length.

One study investigated the relationship with digit ratio and found that men with longer ring fingers than index fingers had slightly longer penises. However, the common misconception that hand size predicts penis size has been widely discredited.

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).

Alleged differences in races have led to the creation of sexual myths. A 2005 study reported that "there is no scientific background to support the alleged 'oversized' penis in black people".

A study of 253 males from Tanzania found that the average stretched flaccid penis length of 93 Tanzanian men is 11.5 cm (4.5 in).

A 1990 study of 109 Chinese men identified the average flaccid stretched penis length of 13.9 cm (5.5 in), and among 14 of the men, the average erect length found was 13.7 cm (5.4 in). Another study (from 1993) of 104 Chinese men found the average stretched penis length of 13.8 cm (5.4 in), and an average erect length of 13.5 cm (5.3 in). A 2014 study on 5196 Chinese males reported an average flaccid length of 6.5 cm (2.6 in) and average stretched length of 12.9 cm (5.1 in) under the method of "gentle painless extension of the penis", which is a measurement method differing from other studies that stretch until maximal extension. A subgroup of 311 men reported an average erect circumference of 10.5 cm (4.1 in) and average erect length of 12.9 cm (5.1 in). They stated that they had "also found that penile dimensions are different in different ethnicities, but further investigations are needed to validate this". However, the 'different ethnicities' referred to in the study were ethnicities within the Chinese population.

A study (published in 2021) of 14,597 Vietnamese men identified an average flaccid penis length of 9.03 cm (3.56 in) and an average stretched penis length of 14.67 cm (5.78 in) as well as an average non-aroused circumference, at the mid-shaft, of 8.39 cm (3.30 in).

A Korean study (published in 1971) of 702 men ages 21 to 31 identified the average erect penis length to be 12.7 cm (5.0 in) and the average non-erect penis length to be 8.0 cm (3.1 in). Another study (from 1998) of 150 Koreans found the average flaccid penis circumference to be 8.34 cm (3.28 in) and the average flaccid penis length to be 8.26 cm (3.25 in) as well as the average erect penis circumference to be 11.17 cm (4.40 in) and the average erect penis length to be 13.42 cm (5.28 in). A 2016 study of 248 Korean men identified the average flaccid penis length to be 7.39 cm (2.91 in) and the average erect penis length to be 13.53 cm (5.33 in).

A 2007 study of 115 men from Nigeria found that the average stretched penis length of Nigerian males was 13.37 cm (5.26 in), and the median stretched penis length found was 13 cm (5.1 in).

A 2014 American study by Herbenick et al. of 1,661 sexually active men involving Asian American, Black American, White American, Pacific Islander/Hawaiian, and Native American men, found average racial differences in erect penile length and circumference to be generally less than one centimeter, with averages in length being: 14.14 cm (5.57 in) for Asian Americans, 14.66 cm (5.77 in) for Black Americans, 14.88 cm (5.86 in) for Pacific Islanders/Hawaiians, 12.86 cm (5.06 in) for Native Americans, 14.18 cm (5.58 in) for White Americans, and 13.79 cm (5.43 in) for Americans reported as "Other" race; with averages in circumference being: 12.10 cm (4.76 in) for Asian Americans, 12.29 cm (4.84 in) for Black Americans, 11.88 cm (4.68 in) for Pacific Islanders/Hawaiians, 11.36 cm (4.47 in) for Native Americans, 12.25 cm (4.82 in) for White Americans, and 12.03 cm (4.74 in) for Americans reported as "Other" race. Unlike previously cited studies, findings were based on self-reported measurements with participants using paper rulers and "detailed, illustrated instructions" to measure their erect penis.

A 2015 systematic review of up to 15,521 men from 20 studies found "no indications of differences in racial variability", and stated that, based on their criteria for including and excluding studies, it was not possible to draw any conclusions about size and race and that further research needed to be conducted.

According to Aaron Spitz, a urologist, 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]."

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 caring for either among women showed a similar pattern.

In a small study conducted by University of Texas–Pan American and published in 2001 in BMC Women's Health, 50 undergraduate women were surveyed by two popular male athletes on campus about their perceptions of sexual satisfaction and it was concluded that 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 study published in 2002, conducted 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 felt length was important and 32% felt that girth was important.

A study conducted at the Australian National University, published in early 2013, showed that penis size influences a man's sex appeal, and the taller the man, the bigger the effect. The study showed life-sized 3D computer-generated images, altering the height and other physical attributes, with women typically registering preferences in under 3 seconds. A preference for taller men's larger penis size was indicated.

A US study published in 2015 of the stated preferences of a panel of 75 women using 3D-printed models as scale references showed a preferred penis length of 16 cm (6.3 in) and a preferred circumference of 12.2 cm (4.8 in) for long-term sexual partners, with slightly larger preferred sizes of a length of 16.3 cm (6.4 in) and circumference of 12.7 cm (5.0 in) for one-time sexual encounters.

According to the study, however, when asked to estimate the length of their partner's penis, most women would say a size significantly smaller than what their partner was recorded to be. This suggests that perception of size is not entirely accurate. The visual impression of the size is not necessarily in correlation with the feeling in the vulva and vagina. A very long penis can cause dyspareunia, if the man doesn't understand how to use it carefully.

One Australian study of 184 men looked at penis length and circumference in relationship to condom breakage or slippage. 3,658 condoms were used. The study found that when used correctly, condoms had a breakage rate of 1.34% and a slippage rate of 2.05%, for a total failure rate of 3.39%. Penile dimensions did not influence slippage, although penis circumference and broken condoms were strongly correlated, with larger sizes increasing the rate of breakage.

Androgens like testosterone are responsible for penis enlargement and elongation during puberty. Penis size is positively correlated with increasing testosterone levels during puberty. But after puberty, administration of testosterone does not affect penis size, and androgen deficiency in adult men only results in a small decrease in size. Growth hormone (GH) and insulin-like growth factor 1 (IGF-1) are also involved in penis size, with deficiency (such as that observed in growth hormone deficiency or Laron syndrome) at critical developmental stages having the potential to result in micropenis.

There are certain genes, like homeobox (Hox A and D) genes, which may have a role in regulating penis size. In humans, the AR gene, located on the X chromosome at Xq11-12, may affect penis size. The SRY gene located on the Y chromosome may have a role to play. Variance in size can often be attributed to de novo mutations. Deficiency of pituitary growth hormone or gonadotropins or mild degrees of androgen insensitivity can cause small penis size in males and can be addressed with growth hormone or testosterone treatment in early childhood.

An adult penis with an erect length of less than 7 cm (2.8 in) but otherwise formed normally is referred to in a medical context as having the micropenis condition. The condition affects 0.6% of men. Some of the identifiable causes are deficiency of pituitary growth hormone or gonadotropins, mild degrees of androgen insensitivity, a variety of genetic syndromes and variations in certain homeobox genes. Some types of micropenis can be addressed with growth hormone or testosterone treatment in early childhood. Operations are also available to increase penis size in cases of micropenis in adults.

It has been suggested that differences in penis size between individuals are caused not only by genetics, but also by environmental factors such as culture, diet and chemical or pollution exposure. Endocrine disruption resulting from chemical exposure has been linked to genital deformation in both sexes (among many other problems). Chemicals from both synthetic (e.g., pesticides, anti-bacterial triclosan, plasticizers for plastics) and natural (e.g., chemicals found in tea tree oil and lavender oil) sources have been linked to various degrees of endocrine disruption.

Both PCBs and the plasticizer DEHP have been associated with smaller penis size. DEHP metabolites measured from the urine of pregnant women have been significantly associated with the decreased penis width, shorter anogenital distance and the incomplete descent of testicles of their newborn sons, replicating effects identified in animals. According to a 2008 study published by the US National Library of Medicine, approximately 25% of US women have phthalate levels similar to those observed in animals.

A 2007 study by the University of Ankara, Faculty of Medicine, found that penile size may decrease as a result of some hormonal therapy combined with external beam radiation therapy. In addition, some estrogen-based fertility drugs like diethylstilbestrol (DES) have been linked to genital abnormalities or a smaller than normal penis (microphallus).

Perceptions of penis size are culture-specific. Some prehistoric sculptures and petroglyphs depict male figures with exaggerated erect penises. Ancient Egyptian cultural and artistic conventions generally prevented large penises from being shown in art, as they were considered obscene, but the scruffy, balding male figures in the Turin Erotic Papyrus are shown with exaggeratedly large genitals. The Egyptian god Geb is sometimes shown with a massive erect penis and the god Min is almost always shown with an erection.