The penis typically reaches its adult size between ages 13 and 18, with most growth happening during puberty. The exact timing depends on when puberty starts, how long it lasts, and a combination of genetic and hormonal factors unique to each person.
When Growth Starts and Stops
Penile lengthening usually begins around age 11½ to 13, shortly after the testicles start to enlarge (which is the first visible sign of puberty in boys). From there, growth continues gradually over several years. A boy may have adult-size genitals as early as 13 or as late as 18, according to the American Academy of Pediatrics. The peak of the growth spurt happens during the later stages of sexual maturation, so much of the change occurs in the middle-to-late teenage years rather than right at the start.
Once puberty is complete, growth stops. There is no documented medical evidence that the penis continues to grow after puberty ends. For most males, this means growth is finished somewhere between 16 and 18, though the full range extends a bit in either direction depending on the individual.
What Drives the Growth
The process is controlled by a hormonal chain reaction that starts in the brain. A region called the hypothalamus signals the pituitary gland, which then releases hormones that tell the testicles to produce testosterone. That surge of testosterone is what drives penile and testicular growth, along with other changes like a deeper voice, facial hair, and increased muscle mass.
The amount of testosterone produced and how long the body stays in active puberty both influence final size. Once testosterone levels stabilize at adult levels and the growth plates in the body close, the developmental window shuts. This is why there’s no “second growth phase” later in life.
What Determines Final Size
Genetics is the biggest factor. Interestingly, research suggests that penis size and girth may be more closely tied to genes on the X chromosome (inherited from the mother’s side) than the Y chromosome, which determines male sex development but not necessarily dimensions. A person’s final size comes from a combination of parental genes, their own unique genetic makeup, and several external influences.
Hormones play a critical supporting role. Variations in testosterone exposure, even during pregnancy, can affect genital development. Nutrition also matters: adequate nutrition during childhood and puberty supports normal hormonal function and growth. Environmental pollutants, including certain pesticides and plasticizers, may act as endocrine disruptors and interfere with normal development. A 2022 systematic review found that exposure to these chemicals before or after birth can affect puberty onset and genital development.
Rare genetic conditions like Kallmann syndrome and Klinefelter syndrome can also influence penis size, though these are uncommon and typically identified by a doctor during childhood or adolescence.
What If Puberty Starts Late
Boys who enter puberty later than average often worry they’ll end up with a smaller penis. The research is reassuring. A study tracking boys who had a small or even micropenis before puberty found that most experienced significant catch-up growth once puberty kicked in. Nine patients originally diagnosed with a micropenis no longer met that criteria after going through puberty. On average, penile length increased by roughly 40 to 44% during puberty in these boys, regardless of whether they received hormonal treatment or not.
The boys who started with the smallest measurements relative to their age actually saw the greatest percentage of growth. So a late start doesn’t necessarily mean a smaller outcome. The body’s own testosterone production during puberty was enough to drive normal development in most cases.
Average Adult Size
A large study of over 15,000 men, cited by the Sexual Medicine Society of North America, found the following averages:
- Flaccid length: 3.6 inches
- Flaccid circumference: 3.7 inches
- Erect length: 5.1 inches
- Erect circumference: 4.5 inches
There’s a wide range of normal around these numbers. Flaccid size is particularly variable and doesn’t reliably predict erect size. Some penises grow substantially when erect while others change very little.
Why Enlargement Products Don’t Work
Because growth stops after puberty, no pill, supplement, lotion, or device can restart it. The Mayo Clinic states plainly: “There’s no proven way to make a penis larger.” Most advertised products contain vitamins, minerals, herbs, or hormones with no clinical evidence supporting their claims. Because dietary supplements don’t require FDA approval before being sold, manufacturers don’t have to prove safety or effectiveness.
Some of these products can actually cause harm. The American Sexual Health Association notes that none of the non-medical treatments marketed for enlargement have been fully tested in clinical trials, and none are recommended by reputable physicians. Weight loss, on the other hand, can make the penis appear larger in men carrying excess fat around the lower abdomen, since a fat pad at the base of the penis can obscure visible length. This isn’t growth, but it’s a real and noticeable visual difference.