The book Sex by Numbers : What statistics can tell us about sexual behaviour was released this week and it 's a must-read for anyone fascinated by who really does what , where , when and how in bed . The author , David Spielgalhalter , a Cambridge University professor , sets out to investigate various sexual statistical claims and separate myth from reality . It 's prompted me to do the same . Scroll down for video . Scientists who reviewed 20 different studies of penis size , including 15,521 measurements of penises from around the world found some surprising results . Since no other part of the human body is more shrouded in myth and folklore than men 's most precious part , it seemed the penis was the most deserving subject to study . Here 's what the latest research has to say : . So , it is really six inches ? A new paper in the British Journal of Urology International is based on the results of scientists who reviewed 20 different studies of penis size , including 15,521 measurements of penises from around the world . Most importantly , it was based only on studies where the penis had been measured by a trained health professional ( surprise , surprise : men are n't terribly honest or accurate when they measure it themselves ) . The result : most men fall within just a couple of centimetres of each other , with most measuring around 5.2 inches in length when erect with a 4.6 inch circumference . If he wants to DIY , the correct way to measure a penis is to use a cloth measuring tape , not a straight-edged ruler . He should measure while it 's erect and measure the side facing his stomach , pushing the tape down to press against the pubic bone ( so he 's not measuring fat ! ) . Tracey investigate various sexual statistical claims and separates myth from reality . The average time between penetration and him ejaculating ( for heterosexual couples ) is nine minutes . This was the result of a European study of 1,100 couples from 44 countries . Each couple attempted intercourse at least twice a week for eight weeks and used a stop watch , clicking it on the minute he penetrated and clicking stop immediately after ejaculation . Previous studies settled on between two or three minutes duration ; another rated the 'ideal time ' of intercourse for both men and women at between three and 13 minutes . The more he exercises , the stronger his erection . A new study in the Journal of Sexual Medicine looked at how physical activity affected erectile functioning . Men who exercised the most ( 18 hours a week ) had the highest sexual functioning scores . Watching porn does n't affect his ability to perform . Two reputable online surveys found no link between the use of pornography and erection problems , despite popular perception that watching too much porn leads to ED ( erectile dysfunction ) . What does make sense ( though , again not proven ) is that men who have difficulties turn to porn as a way of experiencing sex without embarrassment . Great sex does not depend on a big penis . A study including opinions of 25 highly respected sex therapists identified eight elements that make up the best sex you can possibly expect . Guess what ? The penis did n't even get a look in . The myth that big hands/big feet/big nose means a big penis has no scientific proof or evidence supporting it whatsoever . But height did make a difference : the taller the man , the longer his penis is likely to be . Bald guys have a stronger sex drive . This may well have legs . The myth says bald men are more virile than men with hair because they have more testosterone in their body ( which can cause hair loss ) . The testosterone/hair loss link is proven . Baldness is caused when hair follicles become exposed to too much dihydrotestosterone ( DHT ) which is a chemical produced by the male hormone testosterone . Testosterone is also the chemical responsible for his libido . The average number of erections per day for most men is seven . But five of these occur while he 's asleep . The penis has a safety-valve that ensures a man ca n't ejaculate and urinate at the same time . The average quantity of semen ejaculated is 3ml . The average speed of ejaculation is 28 miles per hour . The amount of blood in an erect penis is ten times more than when it 's flaccid . It shoots from 9ml flaccid to 90ml erect . One testicle hangs lower than the other to avoid them getting squashed when he walks ! Eighty-five percent of men find their left testicle is lower and larger . Find more fascinating sex stats and info in all of Tracey 's books on amazon .
A Numbers professor has written a book about sex statistics . Sex by Cambridge University investigates what the real average penis size is . Tracey Cox takes a look at the surprising findings .