The average penis length when erect is just over 5in, scientists have discovered by measuring more than 15,000 men of all ages and races.
Published in the British Journal of Urology International, scientists from King's College London and the south London and Maudsley NHS Foundation Trust were looking to find out exactly what is "normal" when it comes to penis size and circumference.
Authors say their findings could help provide counselling for men who are worried about the size of their penis – a relatively common problem that can lead to severe distress and Body Dysmorphic Disorder.
David Veale looked to create a nomogram of male penis sizes. He used 17 studies that included the penis measurements of 15,521 men undertaken by health professionals using the standard procedure – eliminating the chance men overestimating themselves.
Findings showed the average length in a flaccid state was 9.16cm (3.6in) in length and 9.31cm in circumference. Erect, the average length was 13.12cm and 11.66cm in circumference.
There was a small correlation between erect length and height, the authors said, adding there was no real link with shoe size – a common misconception.
"We believe these graphs will help doctors reassure the large majority of men that the size of their penis is in the normal range. We will also use the graphs to examine the discrepancy between what a man believes to be their position on the graph and their actual position or what they think they should be" Veale said.
Findings showed it was rare for men to have a penis smaller than 10cm when erect, while only five in 100 would have a penis larger than 16cm.
Measurements were made from the pubic bone to the tip of the penis glans, excluding any length from foreskin. Circumference was measured at the base of the penis or the middle of the shaft.
Commenting on the study, Debra Herbenick, a behavioural scientist from Indiana University, told Science magazine: "It still just strikes me how many men have questions and insecurities and concerns about their own penis size. We actually do need good data on it."