What scientific studies define average penile size and their methods?

1. The body of evidence: systematic reviews and large studies set the baseline

Meta-analyses and systematic reviews synthesize decades of small studies to derive pooled averages; a 2015 systematic review that prioritized health professional measurements reported a mean erect length of about 13.12 cm and an average erect circumference of 11.66 cm, and more recent meta-analyses have produced similar pooled ranges while noting regional variation and study heterogeneity ^{[1] [2] [3]}.

2. Measurement types: erect, stretched, flaccid — why they matter

Studies measure penile size in three common states—fully erect, flaccid-stretched (manually stretched while flaccid), and non-stretched flaccid—and each yields systematically different values; stretched/flaccid measurements typically underestimate erect size by roughly 20% on average, and erect measurements are considered the least biased when they can be obtained reliably ^{[7] [8] [4]}.

3. How length is defined: bone‑pressed versus skin‑to‑tip changes the number

A recurring methodological axis concerns whether length is measured from the pubic bone (bone‑to‑tip; BTT) with the fat pad compressed or from the penopubic skin junction (skin‑to‑tip; STT); bone‑to‑tip measurements are recommended as more accurate and reduce underestimation—especially in overweight men—so studies using STT vs BTT are not directly comparable ^{[4] [7] [5]}.

4. Observer and protocol variability: the invisible source of noise

Inter‑observer variability and nonstandardized stretching force create measurement error; large multicenter studies and methodological reviews recommend a single trained evaluator per study and standardized protocols (body position, instrument, examiner) because differences in technique and who measures explain much of the spread in published averages ^{[4] [5] [9]}.

5. Sampling, publication bias and external validity

Many datasets derive from clinical samples, volunteers, or convenience cohorts (urology clinics, students, regional populations), and some geographic areas are underrepresented; systematic reviewers warn that selection bias and publication bias (studies with striking or positive findings more likely to be published) limit generalizability and may skew pooled averages by region and era ^{[2] [3] [10]}.

6. Girth (circumference) and anthropometric correlations

Girth is less often studied and historically measured less consistently than length, but pooled figures put average erect circumference near 11.66–12.2 cm in several analyses; some prospective large studies that also collected anthropometrics (height, weight, foot length) did not find reliable, clinically useful predictors of penile size, contrary to cultural myths ^{[1] [11]}.

7. Best-practice recommendations emerging from the literature

Consensus reviews and methodological papers urge standardized reporting: state whether measurement is erect/stretched/flaccid, use bone‑to‑tip for length, report sample selection and examiner training, and prefer erected measurements when ethically and practically possible; without such standards, comparisons across studies remain fraught ^{[6] [5] [9]}.

8. What remains uncertain and how to read the numbers

While pooled averages provide useful reference ranges, the literature still lacks universal measurement standards and adequate sampling in some regions, and many studies exclude men unable to achieve an erection or rely on self-selection, so reported means should be interpreted as estimates conditioned on study methods rather than absolute truths ^{[2] [8] [3]}.