Can stretched flaccid length predict erect length — how accurate is the predictor?
Executive summary
Multiple clinical studies and systematic reviews find a positive, often strong correlation between stretched flaccid penile length (SFPL) and erect penile length (EPL), with reported correlation coefficients commonly from ~0.7 to 0.8 and some regression equations offered to predict erection length from stretched length (for example r = 0.73 and erect = 13.34 + 0.01 × stretched in a 2022–23 Vietnamese study) [1] [2] [3]. However, variability between individuals, measurement technique differences, and inconsistent stretching force mean SFPL is an imperfect predictor — useful for population estimates and clinical planning but less reliable for exact prediction in a given man [4] [5] [6].
1. SFPL generally tracks EPL — the weight of evidence
Multiple prospective and comparative studies report that stretched length correlates most closely with erect length, with correlation coefficients frequently around 0.7–0.8, and some studies using regression to estimate erect length from stretched length (for example r = 0.73 with the equation erect = 13.34 + 0.01 × stretched in a Vietnamese cohort) [1] [2] [3]. Systematic reviews and nomogram projects including thousands of men also show mean stretched length and mean erect length to be very similar at the group level (stretched ≈ 13.2 cm, erect ≈ 13.1–13.8 cm across reviews), supporting SFPL’s value as a population-level proxy [7] [8].
2. How strong is “good” correlation? Interpreting r = 0.7–0.8
A correlation coefficient of ~0.7–0.8 indicates a strong relationship but not perfect agreement — it explains a sizable portion of variance in erect length but leaves meaningful individual variation unexplained. Several clinical papers therefore use SFPL for preoperative planning or epidemiology because it is practical and reasonably predictive, while acknowledging it won’t exactly match every man’s erect measurement [2] [3].
3. Measurement technique and operator factors create real-world noise
Studies emphasize that measurement method matters: whether the pre-pubic fat pad is compressed, force applied during stretching, single vs multiple measurers, and whether erection is pharmacologically induced all change numbers and correlations [3] [6]. One engineering-minded study showed that clinical stretching forces are often less than the force needed to reach full potential erection length, and that the flaccid-to-stretched ratio can be informative — meaning inconsistent stretching force across clinicians will alter SFPL’s predictive value [6].
4. Population averages vs individual prediction — different use cases
Large reviews and nomograms used thousands of measurements to create average and percentile charts where mean stretched and erect lengths are nearly identical; this makes SFPL useful for counseling, research, and some surgical planning at the group level [7] [8]. For an individual wanting an exact erect-length prediction, sources show important scatter remains — some men (“growers”) gain much length on erection while others (“showers”) do not, so SFPL can misestimate individual erect length [5] [4].
5. Clinical takeaway: when clinicians rely on SFPL and its limits
Urologic practice commonly accepts SFPL as the best simple bedside predictor of erect length and uses it in preoperative counselling; pharmacologic erection measurement is considered even closer when available (studies show similar high correlations between pharmacologically induced erection and SFPL with postoperative outcomes) [3]. Yet authors warn against assuming exact equivalence — surgical decisions, device sizing, or personal expectations should factor in the known variability [2] [3].
6. Conflicting findings and areas lacking clarity
While many studies and reviews support SFPL as the closest non-erect proxy, some reports note “drastic differences” between stretched and erect values in subsets of men and emphasize the role of measurement technique and stretching force in producing discrepant results [5] [6]. Available sources do not mention universally accepted standards for the force to apply during SFPL measurement in routine clinical practice, which contributes to inconsistent findings [6].
7. Practical guidance based on the evidence
If you need a quick, clinic-friendly estimate of erect length, SFPL is the best validated non-erect measure and will usually approximate erection length within the range observed in studies (correlations ~0.7–0.8) [2] [3]. For precise individual prediction (e.g., before device implantation or cosmetic procedures), consider pharmacologically induced erection measurement or acknowledge the margin of error inherent in SFPL and discuss that with patients [3] [4].
Limitations of this summary: I used only the cited studies and reviews provided; other literature may add nuances not covered here. All factual points above are drawn from the listed sources [1] [7] [4] [5] [2] [8] [3] [6].