During puberty, the penis typically grows from a pre-puberty length of around 5 cm (about 2 inches) to an average of roughly 12 to 13 cm (about 5 inches) when stretched. That growth doesn’t happen all at once. It unfolds gradually over several years, with the fastest changes happening in mid-puberty and tapering off by the late teens.
When Growth Starts and How Long It Takes
Penile growth follows the same staged progression as other pubertal changes. Doctors track this using a five-stage system (called Tanner stages) that describes how the genitals develop over time. In the earliest stage, nothing has changed yet. Visible growth begins in stage 2, typically between ages 9 and 14, when the testicles and scrotum start enlarging first. The penis itself begins growing more noticeably in stage 3, between ages 10 and 16, and picks up speed in stage 4.
A 2010 study found that the average growth rate is less than half an inch per year from ages 11 to 15. After 15, growth continues but at a slower pace. By age 18 or 19, most growth in both length and thickness has stopped, though some minor changes can continue into the early 20s. The entire process from first signs of puberty to full adult size usually spans about four to five years.
How Much Growth to Expect at Each Stage
A large cross-sectional study published in The Journal of Urology measured penile length in adolescent boys and organized the results by developmental stage rather than age, which gives a more accurate picture since boys mature at very different rates. Here’s what the averages looked like:
- Stage 2 (early puberty): average stretched length of about 8.3 cm (3.3 inches)
- Stage 3 (mid-puberty): average of about 11.5 cm (4.5 inches)
- Stage 4 (late puberty): average of about 12.4 cm (4.9 inches)
- Stage 5 (adult): average of about 12.3 cm (4.9 inches), consistent with completed growth
The biggest jump happens between stages 2 and 3, when length increases by roughly 3 cm (over an inch). Growth between stages 3 and 4 is smaller, and by stage 5 the penis has reached its adult size. These are stretched measurements taken in a clinical setting, which is the standard method researchers use because it correlates closely with erect length and removes variability from temperature or arousal.
There’s a wide range of normal at every stage. The study found that lengths more than 2.5 standard deviations below the mean for a given stage were considered clinically small (a condition called micropenis). For stage 2, that cutoff was about 3.4 cm; for stage 4, about 9.3 cm. Most boys fall well within normal range even if they feel like they’re behind their peers.
What Drives the Growth
The hormone testosterone is the primary driver, but it doesn’t do the job alone. An enzyme in the body converts testosterone into a more potent form called DHT (dihydrotestosterone), which is specifically responsible for the development of the external genitalia. DHT plays a role even before birth, shaping the formation of the penis and scrotum in the womb. During puberty, rising levels of both testosterone and DHT stimulate tissue in the penis to grow in length and girth.
This is why the testicles grow first. They’re the main production site for testosterone, and their enlargement signals that the hormonal machinery of puberty has switched on. Once testosterone levels rise high enough and DHT production follows, penile growth accelerates.
Why Development Varies So Much
Two boys the same age can look completely different in terms of genital development, and both can be perfectly normal. Puberty can start as early as 9 or as late as 14, which means a 13-year-old who started puberty at 10 might already be at stage 4, while his classmate who started at 13 is still at stage 2. Genetics, nutrition, body composition, and overall health all influence timing.
This is exactly why the Journal of Urology study emphasized that penile length should be evaluated by developmental stage, not by age. A boy who is 13 with a shorter penis isn’t necessarily behind if his puberty simply started later. He has the same total growth ahead of him; it’s just on a different timeline.
Signs That Growth May Be Delayed
Delayed puberty is formally diagnosed when there’s no testicular enlargement by age 13 or 14, or when more than four years pass between the start of genital development and its completion. If puberty seems to stall for more than a year after it has started, that can also warrant evaluation even if the boy hasn’t reached the age threshold.
Doctors typically start by checking bone age with an X-ray of the left hand. This reveals how much skeletal growth remains and can help predict whether puberty is simply late or whether something else is going on. Hormone levels, particularly a hormone called FSH, can indicate whether the testes are functioning properly. In cases of constitutional delay (meaning puberty is late but otherwise normal), a short course of low-dose testosterone can jumpstart the process without affecting final adult size.
The key distinction is between “late” and “absent.” Most boys who are late bloomers catch up fully. A smaller number have underlying hormonal conditions that benefit from treatment, which is why evaluation matters if puberty hasn’t shown any signs by age 14.
After Puberty Ends
Once you’ve reached the final stage of development, typically by 18 or 19, the penis has reached its adult size. No foods, supplements, or exercises will add length or girth after this point. The tissue has matured, and the hormonal signals that drove growth during puberty are no longer active in the same way. Weight changes can affect how much of the penis is visible (a layer of fat at the base can obscure length), but the underlying structure stays the same.