The penis typically reaches its full adult size by around age 16 to 17, though the exact timing depends on when puberty started. Growth begins when the testes and penis start increasing in size, usually around age 11 or 12, and continues through the final stage of puberty. Most males complete this process within about five years of puberty’s onset.
How Penile Growth Progresses Through Puberty
Penile growth doesn’t happen all at once. It follows a predictable sequence that doctors track using a five-stage framework. In the early phase, between roughly ages 10 and 14, the testes enlarge first, followed by gradual lengthening of the penis. Pubic hair begins to appear and darken during this time.
The most noticeable growth happens during the middle and later stages, typically between ages 11 and 16. During this period, the penis increases in both length and girth, the skin of the scrotum darkens, and body hair fills in to adult levels. This coincides with the peak height growth spurt, when boys grow an average of nearly 4 inches taller per year. Voice cracking, acne, and increased muscle mass also develop during these stages.
The final stage marks the end of puberty entirely. According to data from the University of Cincinnati, the penis reaches its mature size by about age 16.5 on average. Some boys finish earlier, some later. Once this final stage is complete, no further growth occurs.
What Drives Growth and What Stops It
Penile development is driven primarily by hormones that ramp up during puberty. The testes produce testosterone, which the body converts into a more potent form that directly stimulates genital tissue growth. Researchers have also identified a secondary hormonal pathway, independent of testicular testosterone, that contributes to penis development. This involves the hormone androsterone, which is converted into the same active compound in tissues like the liver and placenta during fetal development.
Growth stops when these hormone-driven processes complete their course. Once the tissues have fully matured and the growth plates in the bones have closed (signaling the end of puberty overall), the penis does not continue to change in size. This is a one-time developmental window.
What Determines Final Size
Genetics are the strongest predictor of penis size. Interestingly, the relevant genes appear to sit largely on the X chromosome, which carries around 900 genes compared to the Y chromosome’s 50 to 60. This may explain why penis size can vary even among brothers with the same biological parents.
Nutrition also plays a role, particularly during fetal development and adolescence. Malnutrition during these periods can delay puberty and temporarily result in smaller genital size, though boys with delayed puberty typically catch up once development resumes. Environmental chemicals like pesticides and plasticizers can act as endocrine disruptors and affect genital development during fetal life. These factors only matter during the developmental window, however. They have no effect on penis size in adults.
Average Adult Size
A 2025 meta-analysis of studies across multiple global regions found these averages for adult men:
- Erect length: about 5.45 inches (13.84 cm)
- Erect girth: about 4.69 inches (11.91 cm)
- Flaccid length: about 3.63 inches (9.22 cm)
- Flaccid girth: about 3.58 inches (9.10 cm)
There’s a notable measurement quirk worth knowing: studies where men measure themselves tend to report slightly larger numbers than studies where a healthcare provider takes the measurement.
Early and Late Bloomers
Puberty normally starts anywhere between ages 8 and 14. A boy who begins puberty at age 10 will likely finish penile growth earlier than one who starts at 14, but both typically reach the same endpoint relative to their genetics. Starting late doesn’t mean ending up smaller.
That said, there are thresholds where the timing warrants a medical evaluation. If there’s no testicular enlargement by age 13 or 14, this meets the criteria for delayed puberty. The same applies if puberty starts but then stalls for more than a year without progressing. In these situations, a short course of hormone therapy can help kickstart development. Signs of early puberty, those appearing before age 8, also warrant evaluation.
Can Anything Increase Size After Puberty?
Once puberty is complete, the penis does not grow further, and no supplement, exercise, or device has been proven to change that. The Mayo Clinic states plainly that there is no guaranteed safe and effective method to enlarge the penis. Pills and lotions marketed for this purpose, typically containing vitamins, herbs, or hormones, have no evidence supporting their claims. Some can be harmful.
A technique called jelqing, which uses a hand-over-hand motion to push blood through the penis, also lacks scientific support. Rather than producing growth, it can cause scar tissue, pain, and structural damage. The only approaches with any measurable effect involve surgical procedures, which carry significant risks and are generally reserved for medical conditions rather than cosmetic concerns.