What is the correlation coefficient between flaccid and erect penis length in large population studies?

1. Why this question matters and what researchers measure

Clinical and population studies distinguish three common measures — natural flaccid, stretched flaccid (SFPL), and erect — because the predictive value of one state for another affects counseling, research and surgical decision‑making; major systematic reviews and meta‑analyses therefore focused on studies where clinicians made standardized measurements rather than self‑report ^{[4] [5]}.

2. The headline numbers: stretched flaccid vs erect

When flaccid length is stretched under standardized conditions, the correlation with erect length in several large or pooled samples is reliably moderate to strong: Veale et al.’s systematic review reports a pooled correlation between stretched and erect length of about r = 0.61 in some study subsets ^[1], while more recent clinical series report Pearson r values in the range r = 0.57–0.65 for SFPL versus erect length ^{[2] [3]}. These figures indicate that the stretched measure explains a meaningful portion of erect variability but is not a perfect one‑to‑one predictor.

3. The weaker relationship: natural (unstretched) flaccid vs erect

By contrast, unstretched flaccid length shows weaker and less consistent correlations with erect length across studies: several papers and reviews conclude that natural flaccid length “is not sufficiently close” to erect length to be used alone for prediction ^{[6] [3]}. Large meta‑analyses emphasize that flaccid lengths are more variable between observers and settings, and that flaccid measures often fail to predict erect size reliably ^{[3] [4]}.

4. Why correlations vary: methods, samples and bias

Heterogeneity in reported correlation coefficients stems from methodological differences — whether erection was pharmacologically induced, how much force examiners applied when stretching, sample composition (general population vs erectile dysfunction clinics or self‑selected volunteers), and inter‑observer variability — all of which drive study‑to‑study variation and can inflate or attenuate r estimates ^{[3] [6] [5]}.

5. Clinical interpretation: what an r in the 0.57–0.61 range means

A correlation of roughly 0.57–0.61 between stretched flaccid and erect length implies moderate predictive power: stretched length accounts for about 33–37% of the variance in erect length (R2 ≈ 0.33–0.37), leaving the majority of variance unexplained by that single measure ^{[2] [3]}. Practically, SFPL is a useful clinical surrogate but not a perfect stand‑in; unstretched flaccid length performs substantially worse.

6. Alternative viewpoints and limits of the evidence

Some individual studies report stronger or weaker correlations depending on protocol, and certain clinical populations (e.g., erectile dysfunction patients) can shift prevalence of “growers” versus “showers,” changing average PLR (penile lengthening ratio) behavior ^[7]. Major reviews note limited numbers of truly large, standardized erect measurements and caution about generalizing results across ethnic and clinical groups ^{[4] [5]}. Where sources do not report a single pooled correlation for unstretched flaccid versus erect across every large dataset, it is not asserted here that a single universal r exists for that pairing ^[4].

7. Bottom line answer

Large population and pooled studies converge on a clear pattern: unstretched (natural) flaccid length correlates only weakly to modestly with erect length and is an unreliable sole predictor (varies by study and protocol) ^{[6] [3]}, whereas stretched flaccid length shows a consistent, moderate‑to‑strong correlation with erect length, typically reported around r ≈ 0.57–0.61 in contemporary clinical series and systematic analyses ^{[1] [2]}.