It's an age-old worry for the men the world over.

Now, however, scientists have revealed what's considered 'normal' for penis size and length.

The average length of an erect penis is 5.2 inches (13.12 cm), they found.

Meanwhile, the average length of a flaccid penis is 3.6 inches (9.16 cm) and 5.2 inches (13.24 cm) when flaccid but stretched.

And when it comes to girth, the average erect circumference was 4.6 inches (11.66 cm) and 3.7 inches (9.31 cm) when flaccid.

The British research also found there was a small correlation between the erect length of a penis and a man's height.

The average length of a flaccid penis is 3.6 inches (9.16 cm) and 5.2 inches (13.24 cm) when flaccid but stretched, the researchers found (file photo)

The researchers say the graphs they have plotted, which show the size distributions of a penis, will help clinicians dealing with Body Dysmorphic Disorder (BDD), a serious anxiety disorder relating to body image.

The study could help to counsel men worried about their size, or with investigations into how condom failure relates to penis size and girth.

Some men are concerned about their penis size, and those who are preoccupied and severely distressed may even be diagnosed with BDD.

The researchers created a graph (or a nomogram) depicting the size distributions of penis' among men of all ages and many races.

The research included 17 studies of 15,521 men who underwent penis size measurements by health professionals using a standard procedure.

Men compare themselves in locker rooms and with pornography on the internet and believe they are inferior Dr David Veale, Institute of Psychiatry, King's College London

Before this research, there had been no formal systematic reviews of these measurements, and no attempt to create a nomogram showing the distribution of size.

Lead author Dr David Veale from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN), King's College London and South London and Maudsley NHS Foundation Trust, told MailOnline: 'Men compare themselves in locker rooms and with pornography on the internet and believe they are inferior.

'Some men have been teased by sexual partners about their length.

'There are lots of men who might be worried - and we think reassuring men they are in the normal range will help.

'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.

'We have a specialist Body Dysmophic Clinic and some men are very worried and distressed about their size.

Experts plotted graphs showing the size distribution of the length of the average penis when flaccid, flaccid and stretched and erect. The graph above shows average penis length of men - in centimetres - in each percentile

The researchers also plotted a nomogram showing the size distribution of the girth of the average penis. The graph shows the average penis girth of men in each percentile - again in centimetres

'They are unlikely to be helped just by looking at a size distribution graph, they will need more formal treatment like Cognitive Behaviourhal Therapy or medication too.'

Dr Martin Baggaley, medical director at South London and Maudsley NHS Foundation Trust, said: 'BDD causes a person to have a distorted view of how they look and they can spend an obsessive amount of time worrying about their appearance.

'This can include worries over their weight, specific parts of their body and, for men, the size of their penis.

'This can take over someone's life and cause a great deal of distress.

'Hopefully this new study will help reassure those many men who are concerned with their penis size and assist clinicians dealing with BDD.'

'BDD occurs alongside Obsessive Compulsive Disorder (OCD) and a person with BDD may constantly compare their looks to other people's, spend a long time in front of the mirror or avoid the mirror altogether, constantly conceal a perceived defect or feel anxious when around other people or in social situations.'