The researchers found that when erect, the average penis length was 13.12 cm (5.16 inches).
Dr. David Veale, of King's College London and South London and Maudsley NHS Foundation Trust in the UK, and colleagues say their findings may help health care providers counsel men who approach them with worries about their penis size - known as "small penis anxiety" or "small penis syndrome."
"Men may present to urologists or sexual medicine clinics with a concern with their penis size, despite their size falling within a normal range," the authors note.
"Some men who are preoccupied and severely distressed with the size of their penis may also be diagnosed with body dysmorphic disorder (BDD)," they add, "where the preoccupation, excessive self-consciousness and distress is focussed on their penis size or shape."
BDD is estimated to affect around 1% of the US population, and people diagnosed with the condition are also more likely to suffer from obsessive-compulsive disorder (OCD), social anxiety disorder, depression and eating disorders.
While past studies have measured penile size in a variety of samples, Dr. Veale and colleagues note there has never been a formal systematic review of penile size measurements.
What is more, the team says there has been no attempt to use such data to develop graphical diagrams, or "nomograms," that illustrate the average length and circumference of a flaccid and erect penis across all ages and races. With this in mind, the researchers set out to create such graphs.
Nomograms reveal average erect penis length 5.16 inches
The team reviewed 17 studies of penile size measurements involving up to 15,521 men aged 17 and older. Using standard procedures, all men had the length and circumference, or girth, of their penis measured by health professionals while it was flaccid and erect.
All data from the studies were combined to create nomograms that depict the average penis length and girth.
The results of the nomograms revealed that the average length of a flaccid penis was 9.16 cm (3.6 inches) and 13.24 cm (5.21 inches) when stretched. When erect, the average penis length was 13.12 cm (5.16 inches). The researchers identified a small correlation between height and erect penis length.
The average penis girth was 9.31 cm (3.66 inches) when flaccid and 11.66 cm (4.59 inches) when erect.
The team believes the nomograms created in this study will be useful in clinical settings and for future research. Dr. Veale adds:
"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."
The team notes, however, that when it comes to assessing men with small penis anxiety, measurement of penile size is only one of many factors that should be taken into consideration.
"Equally as important is male genital image satisfaction, an understanding of the beliefs and attitudes about penile size, the frequency of avoidance and safety-seeking behaviors to prevent the risk of shame or humiliation, and the need to screen for BDD either by a questionnaire or structured diagnostic interview," they add.
Our Knowledge Center article - "What is the average penis size?" - looks at what drives men's concerns about penis size and provides information on other studies that have set out to determine the average penis size.