How do researchers measure penis size in studies?
Executive summary
Researchers measure penis size using a few distinct physical states (flaccid, stretched/flaccid-stretched, and erect), standard anatomical start–end points (most commonly from the pubic bone/penopubic skin junction to the tip of the glans), and simple instruments such as rulers or flexible tapes, but there is no universally agreed global standard and methodology variation drives much of the literature’s heterogeneity [1] [2] [3].
1. What, exactly, is being measured: flaccid, stretched and erect lengths
Studies report three principal length metrics—flaccid length, stretched flaccid length (SPL), and erect length—with SPL used by many researchers because it correlates reasonably well with erect length and is easier to obtain in a clinical setting; reported frequencies show SPL used in roughly 60% of studies, flaccid-only in about 53%, and erect measurements in roughly 27% of studies [1] [4].
2. Where measurements start and stop: STT versus BTT and “bone-pressed” technique
Length is typically measured along the dorsal surface from a defined base point to the glans tip; many researchers use the skin-to-tip (STT) method starting at the penopubic skin junction, while others prefer bone-to-tip (BTT), pressing the ruler into the pubic bone to exclude variable fat pad—published reviews show BTT is generally considered more reliable, especially in overweight subjects [2] [5] [6].
3. Girth (circumference) and where it’s measured
Circumference is commonly measured with a flexible tape at the penile base or mid-shaft; large meta-analyses treated base and mid-shaft circumference as equivalent for pooled reporting and note that girth increases with erection, so site and state must be recorded to make valid comparisons [7] [6].
4. Instruments, positioning and examiner procedures that matter
Researchers typically use rigid rulers for length and non-stretch tapes for circumference, with instructions to press to the pubic bone for BTT and to retract foreskin when needed; systematic reviews stress documenting patient position, room conditions, the instrument used, and which examiner performed the measurement because these variables affect accuracy and reproducibility [1] [8].
5. Sources of bias: self-reporting, sample selection and measurement variability
Studies relying on self-measurement or self-report consistently report larger values than clinician-measured studies, reflecting social desirability and measurement error, and many historical datasets (e.g., internet surveys) are thus inflated compared with in-person assessments—a recurring caveat in reviews and encyclopedic summaries of the field [9] [10] [4].
6. Why there’s no single gold standard and what experts recommend
Multiple systematic reviews and specialty society statements conclude there is no definitive single method favored by all researchers; instead they recommend transparency—reporting whether measurements were flaccid, stretched or erect, whether STT or BTT was used, the instrument, the examiner and conditions—and many advocate BTT (pubic bone to glans tip) plus standardized examiner training to reduce interstudy heterogeneity [3] [5] [1].
7. Practical implications for interpreting study numbers
When comparing reported averages or “worldwide” numbers, readers must check how lengths were obtained (self-report vs clinician-measured), which state (flaccid/stretched/erect) and whether bone-pressed methods were used, because those methodological choices explain much of the variation between studies and regions rather than biological extremes alone [7] [6] [9].