How does penis size typically change by Tanner stage during puberty?
Executive summary
Most clinical references say male genital changes during puberty follow the Tanner (sexual maturity) stages: little or no change in Stage I, testicular/scrotal enlargement in Stage II, penis lengthening beginning in Stage III with continued growth through Stage IV, and adult size reached by Stage V (examples: ScienceDirect, Cleveland Clinic, GetLabTest) [1] [2] [3]. Exact numeric penis-length values are variably reported and not consistently included in modern summaries; some educational pages and historical descriptions give approximate centimetre ranges for mid-to-late stages, but reporting is inconsistent across sources [4] [5].
1. What the Tanner stages describe for male genital development
The Tanner scale rates external sexual development in five stages; for boys this focuses on genital appearance (penis and scrotum) and pubic hair pattern rather than strict ages, because progression varies widely between individuals [5] [6]. Classic resources describe Stage I as prepubertal with childlike genitalia and no pubic hair, Stage II as initial enlargement of testes and scrotum with little pubic hair, Stage III as the stage where the penis begins to lengthen, Stage IV as continued growth with increased breadth and darkening of the scrotum, and Stage V as adult morphology [1] [2] [3].
2. When penis lengthening typically starts and how it progresses
Multiple clinical summaries state penis lengthening generally begins in Tanner Stage III and continues through Stage IV into Stage V, with testes and scrotum enlarging earlier (Stage II) and then in parallel as puberty advances [1] [3] [2]. In other words, early puberty often shows testicular change first; measurable penile length growth tends to accelerate by mid‑puberty (Stage III) and reaches adult size by Stage V [1] [2].
3. Numeric measurements: inconsistent, historical, and approximate figures
Some sources derived from older or non‑standard educational pages report approximate penis-length values (for example, a prepubertal penis of ~3 cm or less and reported increases to about 6 cm in Stage III and about 10 cm by Stage IV in one embryology summary), but these numeric ranges are not uniformly cited in clinical overviews and are variably presented across web pages [4]. Wikipedia’s adapted Tanner descriptions emphasize that while testicular volume is quantified, many summaries state “penis length unchanged” for certain stages or do not give precise length figures, illustrating inconsistency in how length is described [5]. Available sources do not provide a single, universally accepted table of penis length by Tanner stage.
4. Clinical practice: why clinicians focus on appearance and testicular volume
Practitioners commonly assess genital Tanner stage by observing penis/scrotum appearance and pubic hair rather than relying solely on penile length measurements; testicular volume (measurable with an orchidometer) is often used because it can be more reliable for staging and biologic function, and because penile and testicular changes can follow different pathways in some conditions [6] [7]. A recent review argues for separate assessment of penile/scrotal changes and testicular size to avoid misclassification—showing clinical nuance beyond a single length number [7].
5. Timeline and variation: age ranges and individual differences
The Tanner stages are maturity stages, not strict age markers; sources note wide variation in timing. Typical age windows are shared in educational materials (for example, puberty often begins around 9–11 in boys and Tanner Stage V often occurs in mid‑ to late adolescence), but individuals progress at different rates so a given penile length or stage does not map neatly to chronological age [5] [8] [9]. Pediatricians treat Tanner staging as a clinical snapshot of maturity, not an exact timetable [6].
6. Limitations, disagreements, and what’s not settled
Sources disagree on whether to present precise penis‑length numbers by stage: some educational summaries include centimetre estimates [4], while mainstream clinical resources focus on descriptive change without consistent numeric values [5] [2]. Reform literature warns that combining testicular and penile descriptors can mislead clinicians in certain medical contexts [7]. Available sources do not offer a standardized, evidence‑based chart of penis length by Tanner stage agreed upon across major clinical authorities.
7. Practical takeaways for readers
If you want to know where a boy’s genital development stands, clinicians use Tanner stage descriptions and testicular volume rather than relying solely on length measurements; expect penis lengthening to begin in Stage III and continue through IV to Stage V, with large individual variability [1] [2] [3]. For specific concerns about timing or measurements, pediatric endocrinology texts and a pediatrician’s exam (including testicular volume assessment) are the appropriate next steps; current public sources show descriptive consensus but lack a single, universally adopted numeric length table [7] [5].