Penile growth typically reaches its adult size between ages 13 and 18, with most growth finishing by around age 16 or 17. The exact timing depends on when you started puberty, your genetics, and your hormonal profile. If you’re still in puberty, you likely still have growth ahead of you.
How Puberty Drives Growth
The penis grows because of a surge in hormones, primarily testosterone, that begins at puberty. A related hormone converted from testosterone is what actually kicks off genital development in boys. These hormones signal penile tissue to grow first in length and then in width, a process that unfolds over several years rather than all at once.
The first visible sign of puberty isn’t penile growth at all. It’s a near doubling in the size of the testicles, which usually happens between ages 9 and 14. The penis begins growing noticeably about a year or two after that. So if your testicles have recently gotten larger but your penis hasn’t changed much yet, that’s completely normal and on schedule.
The Growth Timeline, Stage by Stage
Doctors describe puberty in five stages. In the first two stages, the testicles begin enlarging and the body starts its hormonal shift, but the penis stays roughly the same size. Real penile growth starts in stage three, when length increases noticeably. In stage four, both length and girth increase, and the body hits its overall peak growth velocity (around age 13 to 14 for most boys). By stage five, the genitals have reached adult size and overall body growth decelerates and stops, typically around age 17.
The key point: a boy can have fully adult-size genitals as early as 13 or as late as 18. That’s a five-year window, and where you fall in it is mostly a matter of when your puberty started. Someone who began puberty at 10 will finish earlier than someone who started at 13.
What If You’re a Late Bloomer
Some boys simply start puberty later than their peers. This is called constitutional delay, and it’s the most common reason a teenager might feel behind. The medical threshold is no testicular enlargement by age 13 or 14. Boys with constitutional delay almost always show clear signs of sexual maturation by age 18 and go on to reach a normal adult size. Their timeline is just shifted later.
Research on boys with constitutionally small penises found that about 87% of those who reached puberty without any treatment ended up within the normal adult range on their own. The body’s catch-up growth was enough. For the small number who needed hormonal support, treatment produced rapid growth that was sustained afterward. In other words, being smaller or later than average during your teen years does not predict where you’ll end up.
What Determines Your Final Size
Genetics plays the largest role. Your genes influence your hormonal environment, your sensitivity to those hormones, and the growth potential of your tissue. You inherit a general blueprint, though small genetic variations unique to you (not identical copies of your parents’ traits) also factor in.
Beyond genetics, a few environmental factors can nudge things in either direction. Adequate nutrition during fetal development and early childhood supports normal organ growth, including reproductive organs. Malnutrition during these windows can impair hormonal function and limit development. Exposure to certain chemicals that mimic estrogen, found in some pesticides, plastic containers, and detergents, has been linked to reduced penile size. These are called endocrine disruptors because they interfere with the body’s hormonal signaling during critical growth periods.
None of these factors are things you can control retroactively, which is why genetics remains the dominant influence for most people.
Does Any Growth Happen After Puberty?
Once puberty is complete and your growth plates have closed, no further significant penile growth occurs. There is no second growth phase in your 20s. Weight changes can affect how much of the penis is visible (fat accumulation around the pubic bone can obscure length), but the underlying tissue does not keep growing.
If you’ve finished puberty and your height has stabilized, your penis has almost certainly reached its final size.
Average Adult Size
A large meta-analysis pooling data from over 55,000 men across studies published between 1942 and 2021 found that the average erect length is about 13.9 cm (roughly 5.5 inches). The average flaccid length is about 8.7 cm (3.4 inches). There’s a wide range of normal on either side of these averages.
If you want to measure accurately, use a ruler or measuring tape while fully erect. Place it on the top side of the penis, press the end firmly against the pubic bone (pushing past any fat pad), and measure in a straight line to the tip. If your penis has a curve, a flexible measuring tape works better than a rigid ruler. Avoid measuring in a cold room, which can temporarily reduce size.
Why Perceived Size Changes Over Time
Teenagers sometimes notice what feels like fluctuation in size. This is usually explained by normal variation in blood flow, arousal level, temperature, and the simple fact that your body proportions are changing rapidly. As your torso, hands, and legs grow, your penis can look relatively smaller even if it’s actually still growing. Comparing yourself to adult bodies (in person or online) during a period when your own body is mid-development gives a skewed picture.
The most reliable way to know if you’re still growing is to track your overall pubertal development. If you’re still getting taller, still developing more body or facial hair, or your voice is still deepening, your body hasn’t finished puberty and penile growth may still be underway.