Which large-scale scientific studies measured erect penis length and what were their methods?

1. Big, pooled studies: what was measured and how they chose studies

The largest recent synthesis is a systematic review and meta‑analysis that screened literature to February 2024 and included 33 studies totaling 36,883 men; the authors explicitly included only studies in which a healthcare professional evaluated penile size and then meta‑analysed flaccid, stretched and erect measurements — for erect length they pooled 5,669 men and reported a mean erect length of 13.84 cm (SE 0.94) ^{[1] [6]}. That review grouped outcomes by WHO region and rated the included studies’ risk of bias as moderate/low while noting limitations tied to measurement heterogeneity ^{[1] [7]}.

2. Large single‑country and cohort studies: examples and numbers

Several large single‑country analyses provide comparable data. A 2024 Andrology paper pooled Chinese studies (23 studies, 34,060 men) and international comparators, reporting a mean erect length 12.42 cm (SD ±1.63) for Chinese men and noting a larger percentage increase from flaccid to erect among that sample ^[2]. In the U.S., a 2014 study of 1,661 men who measured erect dimensions to obtain correctly sized condoms reported a mean erect length of 14.15 cm and mean erect circumference of 12.23 cm; that was a large, motivated cohort measuring erect size directly ^[3].

3. How “erect length” has been defined and measured across studies

Studies vary in the basic measurement approach. A common clinical standard is to measure length dorsally from the pubic bone (bone‑pressed or pubic bone‑to‑tip/BTT) to the glans tip; many studies instead use the pubopenile skin junction to tip (skin‑to‑tip/STT) or rely on stretched flaccid length as a proxy for erect length ^{[5] [4]}. The systematic review explicitly included stretched, erect and flaccid measures and notes the difficulty of erect measurements, which explains why erect samples are often smaller within pooled data ^{[1] [4]}.

4. Who does the measuring — clinician vs. self‑report — and why it matters

Measurement validity depends on who measures: the meta‑analysis restricted inclusion to studies where a healthcare professional measured penile size to improve reliability ^[1]. By contrast, many population surveys rely on self‑measurement or self‑report; those are more prone to bias and typically show wider variability (available sources do not mention specific self‑reported studies beyond general surveys). The U.S. condom study used participant self‑measurement motivated by receiving properly sized condoms, which may reduce but not eliminate reporting bias ^[3].

5. Methodological challenges highlighted by methodological reviews

Systematic methodological reviews emphasise heterogeneity and recommend bone‑pressed measurements from pubic bone to tip for comparability; they also document that studies differ in state measured (flaccid, stretched, erect) and that overweight/obesity and fat pad can reduce visible length unless bone‑pressed technique is used ^{[4] [5]}. The methodological review reported that only about 27% of studies measured penile length in an erect state, explaining why erect‑state samples are smaller and more variable ^[4].

6. What these methods mean for interpreting “average erect length”

Because measurement technique, population sampling and who performs the measurement vary, pooled averages (e.g., 13.84 cm from the 2024–2025 meta‑analysis) are useful benchmarks but not precise universal truths — they reflect the subset of studies that measured clinically and the particular measurement definitions used ^{[1] [5]}. Regional syntheses (e.g., the Chinese nomogram) show some population differences but also emphasise methodological drivers, such as choice of stretched vs. erect measurement and bone‑pressed technique ^{[2] [1]}.

7. How to read future claims and what’s missing

Future reports should state: sample size for the erect measurement specifically, whether measurements were bone‑pressed or skin‑to‑tip, who measured, and whether stretched flaccid length was used as a surrogate — the systematic review adopted exactly these inclusion criteria to reduce heterogeneity ^[1]. Available sources do not mention a single global, standardized protocol universally enforced across all studies; methodological reviews argue that standardization remains incomplete ^{[4] [5]}.

Sources: Mostafaei et al. systematic review/meta‑analysis (Urology Research and Practice / PubMed) and PDFs summarizing pooled counts and methods ^{[1] [7] [6]}; Chinese meta‑analysis in Andrology ^[2]; U.S. condom‑sizing cohort (J Sex Med 2014) ^[3]; methodological reviews of measurement technique and recommendations ^{[4] [5]}.