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The Penis

The penis is an external organ in the male reproductive system. The penis has two main functions; sexual intercourse and urination.

The penis can vary in size and shape. There is also large variation in the angle of an erect penis. Some erect penises point straight up, others straight down. Some have a slight bend to the left or right. There is no right shape. If you have a more significant bend in your penis that may cause you pain or difficulty having sex, see your GP.

Penis Size

Studies have found the average size of the penis is 5.16 inches in length when erect, and 4.59 inches in circumference when erect.  

There is no average size for teenage boys because people grow at different rates during puberty and the penis doesn't stop growing until the age of 21.

If you have any concerns or problems, you should visit your GP. You can also visit the NHS website for Penis Problems: a visual guide.



Erections can occur for a variety of reasons, most commonly when a man is sexually aroused. Erections can also occur when a man isn't sexually aroused, for example, an average man will have between three to five erections during a full night's sleep.

During an erection, blood builds up inside the penis, causing the penis to swell and stiffen. The swelling blocks off the veins that normally take blood away from the penis. As an erection disappears, the arteries in the penis narrow again, allowing blood to drain away from the penis.

It is common for most men to fail to get or maintain an erection at some point in their lives. However, if erection problems become frequent, see your GP. If erection problems occur frequently, this is known as erectile dysfunction.


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