Introduction

The subject of flaccid and erect penis size is a matter of perennial interest. Flaccid penis size influences male attractiveness,1 and over the course of a man’s life, 45% of men want to have a larger penis.2

The relationship between flaccid and erect penis size remains shrouded in mystery, with multiple competing and sometimes contradictory narratives. Master’s and Johnson reported that “the difference in average erective size increase between the smaller flaccid penis and the larger flaccid penis is not significant.”3 An analysis of the Kinsey data reported that there is an inverse relationship between the length of the flaccid penis and its increase in in size during erection.4 It is a common belief that flaccid penis length is not predictive of erect penis length, with larger flaccid penises colloquially referred to as “showers” and smaller flaccid penises popularly referred to as “growers.” It is taken by many to be the case that flaccid penis length is so much more variable than erect penis length that inferring the latter from the former is impossible.

Here we provide a follow-up to our previously reported estimate for average penis size5 based on a larger sample size and incorporate new data regarding flaccid measurements. This report does not aim to settle the question of what the “true” average penis size is. For a number of practical reasons, this number is very likely to remain unknowable. However, the numbers represented by this report are an excellent representation of penis sizes among men who have an interest in comparing them.

We will also consider the relationship between flaccid and erect penis sizes, and discuss whether an attitude of general skepticism regarding erect penis size is warranted when flaccid size is known.

Methods

These data were collected from users of www.cocksizecontest.com. Only photo-verified and human-curated measurements were included in this analysis. Measurements that reflect self-report only, without photo verification were excluded. Some analyses represent only the subset of users who provided flaccid measurement photos.

For erect length, users are instructed to measure the shortest path along the top of their erect cock, from the base to the tip. For girth, users are instructed to choose a cross-section that is neither the thickest nor the thinnest, but most representative of their girth as a whole. Measurement photos are carefully curated by moderators, and due to the competitive nature of the context of data collection, photo-verification is also carefully scrutinized by the other members of the site.

For flaccid length, users are instructed to measure what they consider to be the “typical size presented at a urinal, public shower or locker room.” This standard was chosen for practical reasons. There are many lengths that a person’s flaccid penis can take, depending on temperature, level of arousal and many other factors. While maximal erect length is fairly straightforward, comparing it to minimal flaccid length is not. (E.g. Should a user be required to bathe their penis in ice beforehand? How long should a user be required to think non-arousing thoughts before measurement?) Rather than trying to measure the “true” flaccid size, users were instructed to measure the size that we take to be most relevant—what others will see in the most common contexts of comparison. For simplicity, only flaccid length, and not girth was measured.

All data points included in this report represent photo-verified and human-curated measurements.

All statistical tests and graphs were done with R version 3.4.16 using the ggplot2 package.7 For statistical testing, we defined \(p\) < 0.05 to be statistically significant. We did not correct for statistical multiplicity and our linear model represents an exploratory analysis.

Results

Sample demographics

Between 2011-09-10 and 2017-07-22, 26,774 users signed up for an account on www.cocksizecontest.com. Of these, 1603 verified their claimed size with measured length and girth photos. A total of 84 users submitted photos measuring their length while flaccid.

Distribution of length and girth

The mean for erect penis length among photo-verified users of www.cocksizecontest.com is 15.1 cm, with standard deviation 2.5 cm. The mean for erect penis girth is 13.3 cm, standard deviation 1.4 cm. The sample size for erect penis length and girth measurements was \(n\) = 1603.

The mean for flaccid penis length is 10.7 cm, standard deviation 2.8 cm. The sample size for flaccid penis length and girth measurements was \(n\) = 84.

To visually represent the distribution of sizes in our sample, we graphed density plots for flaccid measured length and erect measured length among participants in our sample (see blue and red density plots in Figure 1, respectively). Similarly, a density plot of girths of participants was graphed (see green density plot in Figure 2).

Distributions of flaccid (blue) and erect (red) lengths. Vertical lines indicate mean values.

Figure 1: Distributions of flaccid (blue) and erect (red) lengths. Vertical lines indicate mean values.

Distribution of girths (green). Vertical line indicates mean girth.

Figure 2: Distribution of girths (green). Vertical line indicates mean girth.

Flaccid length vs erect length

To investigate the relationship between flaccid and erect lengths, we graphed a density plot of the difference between a user’s erect and flaccid measured lengths (Figure 3, left). The mean difference is 4.6 cm and the standard deviation among those differences is 1.7 cm. We also graphed a density plot of the ratio of erect to flaccid length (Figure 3, right). The mean ratio of erect to flaccid length is 1.5 with standard deviation 0.3.

Density plot of difference between erect and flaccid lengths (left, purple) and ratio of erect to flaccid length (right, orange). Vertical line indicates mean difference and ratio, respectively.

Figure 3: Density plot of difference between erect and flaccid lengths (left, purple) and ratio of erect to flaccid length (right, orange). Vertical line indicates mean difference and ratio, respectively.

To test the hypothesis that penises with smaller flaccid sizes grow more when erect, we calculated a linear regression model of the difference between erect and flaccid length as a function of flaccid length. The slope of the linear model is slightly negative but significantly different from 0 (\(p\) < 0.05). The adjusted \(r^2\) for this model is 0.05.