The penis typically grows during puberty, starting as early as age 9 and finishing by age 16 to 18 for most males. Some individuals see continued minor changes into their early twenties, but the vast majority of growth happens in a roughly five-to-seven-year window during adolescence.
When Growth Starts and Stops
Puberty in boys begins between the ages of 9 and 14, and the penis doesn’t start growing right away. The first visible sign of puberty is enlargement of the testicles and scrotum, not the penis itself. Noticeable penile growth begins slightly later, typically between ages 10 and 16, once the body has ramped up production of testosterone and a related hormone that specifically drives genital development.
Growth happens in phases. Early on, the penis mainly grows in length. In the next stage, it gains width and the head becomes more defined. By the final stage, the penis reaches its adult size. For most males, this process wraps up around age 16 or 17. A smaller number continue to see gradual changes through age 18 to 21, but significant growth after 18 is uncommon.
What Drives the Growth
Testosterone is the primary engine of puberty, but the penis and scrotum depend heavily on a more potent form of testosterone that the body converts from regular testosterone using a specific enzyme. This converted hormone is essential for external genital development both before birth and again during puberty. Boys born with a genetic inability to produce this conversion enzyme still experience voice deepening and muscle growth (driven by regular testosterone) but have noticeably reduced genital development, which underscores how important this particular hormonal pathway is for penile growth.
Typical Size at Different Stages
By age 16, the average flaccid length is about 3.75 inches, while the average erect length falls between 5 and 7 inches. Average erect girth is around 4.5 inches. These numbers represent a wide normal range. Two people of the same age can be at very different points in puberty, so comparisons at 13 or 14 are essentially meaningless.
It’s also worth noting that flaccid size varies a lot based on temperature, blood flow, and arousal state, and is a poor predictor of erect size. Some penises grow substantially when erect (“growers”), while others change very little (“showers”).
Late Bloomers and Catch-Up Growth
If puberty starts later, growth simply happens later. A boy who begins puberty at 13 instead of 10 isn’t at a disadvantage for final size; he’s just on a delayed timeline. Research tracking boys with smaller-than-average penises before puberty found that most experienced significant catch-up growth once puberty kicked in. In one study, the average length nearly doubled from about 4 cm at the initial visit (around age 10) to 7.3 cm by puberty. Nine patients who had been diagnosed with a micropenis no longer met that diagnosis after going through puberty.
Interestingly, boys who started smallest relative to their age group tended to see the largest percentage increase during puberty. The body appears to have a strong biological drive toward reaching a genetically determined size, and puberty provides the hormonal fuel to get there. In the same study, boys who received hormonal treatment and those who didn’t ended up with statistically identical growth rates (about 42% increase in both groups), suggesting that for most boys, the body’s own hormone production during puberty is sufficient.
What Determines Final Size
Genetics is the dominant factor. Just as height runs in families, penile size is largely inherited. Hormone levels during puberty play a supporting role, but in the absence of a genuine hormonal disorder, most boys produce enough testosterone to reach their genetic potential.
Conditions that can interfere with normal growth include hormonal deficiencies (particularly low testosterone or problems converting it), chromosomal differences, and certain childhood health conditions that disrupt puberty. A micropenis, defined clinically as a stretched length more than 2.5 standard deviations below average for age, is diagnosed by a doctor and is distinct from simply being on the smaller end of normal. In newborns, this threshold is about 2 cm; in older boys, the cutoff is roughly 4 cm stretched length. True micropenis is rare and typically identified early in life.
After Puberty Ends
Once the body reaches physical maturity in the late teens or early twenties, no supplement, device, or exercise produces meaningful changes in penile size. The tissue has finished its hormonally driven growth phase. Products marketed for “enhancement” have no credible evidence behind them.
Weight changes can affect the appearance of size, though. Excess fat in the pubic area can obscure the base of the penis, making it look shorter than it is. Losing that fat doesn’t create new growth, but it can reveal length that was always there. This is one of the few practical ways perceived size changes in adulthood.