10 Facts About the Penis You've Always Wanted to Know

As straightforward as it may appear, the penis can be pretty mysterious. Why is it always hard in the morning? What makes it go soft in the middle of sex sometimes?

To help you get a handle on all things penis-related, we consulted healthcare providers who specialize in male sexuality and urology and asked them to shed light on the questions and topics you may not want to ask about. Behold, 10 fascinating facts that may surprise you.

Most Erect Penises Range From 4 to 6 Inches

The average length of an erect penis is between 5.1 and 5.5 inches, according to research published in the Journal of Sex & Marital Therapy. The average erection girth is 4.5 inches, according to the Sexual Medicine Society of North America (SMSNA).

When men are smaller or larger than average, it's not by much. "Almost every man is between 4 and 6 inches, and maybe 15% of men have a penis over 7 inches in length," said Michael Reitano, MD, a physician in residence at men's health company Roman.

As for non-erect penises, the average flaccid length is 3.606 inches, and the average flaccid girth is 3.665 inches. Dr. Reitano explained that there's no correlation between how big a penis is when it's not erect and when it is.

The Male Orgasm Is a Short Experience

The typical female orgasm lasts about 20 seconds, said Dr. Reitano, which makes it three times as long as the average male orgasm—which clocks in at just six seconds. Like the ability to have multiple orgasms and G-spot orgasms, consider this one more orgasmic advantage women have over men.

Sleep Hard-Ons Happen Up to Six Times Each Night

No, it's not because of sexual dreams. Though the exact reason isn't known, the four to six sleep erections one has each night may be the body's way of keeping things healthy, said Natalya Lopushnyan, MD, a urologist at Greater Boston Urology. "If men don't get regular erections and the penis doesn't expand regularly, it starts to scar down and actually becomes shorter." A person with prolonged erectile dysfunction will likely find his penis to be shorter than it used to be, she said.

Smoking Can Shave an Inch Off a Penis

As if there aren't enough health risks associated with cigarettes, it's also poison to penises. "Smoking can affect the blood vessels, meaning it can affect (a man's) ability to have an erection," said Dr. Reitano. "Even men who have an erection may not [get] as full an erection as they would be if they weren't smoking." In fact, a smoker can expect their erect penis to be up to an inch shorter than if they gave up the cigs, said Dr. Reitano. Once they quit, the true size of their erection will likely return.

The Penis Is Similar to This Female Organ

In the early stages of embryonic development, the clitoris develops from the same tissue as the penis. "We all have the same parts; it just depends on what hormones we have in our bodies that influence male vs. female part development," said Dr. Lopushnyan. When a destined-to-be-male embryo releases testosterone, the male sex hormone, that sexual tissue grows and becomes a penis. In the absence of testosterone, the same tissue stays small and becomes the clitoris.

"The tip of the clitoris is also very similar to the head of the penis," said Dr. Lopushnyan. "It's very sensitive, and honestly, if you look at it, it kind of looks the same, just smaller." It also functions in a similar way. "When a woman gets excited, the clitoris becomes larger and gets filled with blood," she added.

Circumcised vs. Uncircumcised: Very Little Difference

A circumcised penis is one that no longer has a foreskin—a retractable sheath of skin that covers the head of the penis and is surgically removed within days of a boy's birth. Circumcision is common in many cultures around the world, though rates are on the decline in the United States. According to The American Academy of Pediatrics, there are reasons in favor of and against circumcision, and since it is not essential to a child's health, parents should choose what is best for their child by looking at the benefits and risks.

Other than appearance, said Dr. Lopushnyan, there's not much difference between circumcised and uncircumcised penises. When an uncircumcised person gets an erection, the foreskin folds back and the head is exposed, so you won't notice a difference during sex.

A Broken Penis Is Serious

Penile fracture is a real (and very, very painful) thing. Though the penis doesn't have a bone, the fibrous, blood-filled tissue inside an erect penis can tear if it is thrust against something too forcefully, said Dr. Lopushnyan. When it tears, "all of that blood comes out and stays under the skin, so you very quickly get this huge bruise and a lot of swelling," she explained. A fractured penis turns purple, swells, and resembles an eggplant.

Sexual positions that have a partner coming straight down on the penis put a penis at the greatest risk for fracture. If a broken penis isn't addressed immediately, it can result in permanent damage.

An Erection Offers Clues to Health

Two key things are needed to have an erection: healthy blood vessels to carry blood to the penis and healthy nerves to signal those blood vessels to start pumping. Without these, the result can be erectile dysfunction (ED)—which means trouble having or maintaining an erection.

According to the National Library of Medicine, ED is common and becomes more frequent with age. But it's not a natural part of aging.

Heart disease, diabetes, high blood pressure, and other potentially life-threatening issues can cause ED because they affect blood vessels, so anyone who suffers from ED should see a healthcare provider. ED can also be the result of stress, depression, and anxiety. If a physical workup comes out normal, a mental health evaluation may be in order.

Morning Erection Is a Good Sign

Waking up with a hard-on is normal, since testosterone levels are highest in the morning. If having morning erections stops, however, it could mean that something's up health-wise. Two weeks without one necessitates a trip to a healthcare provider, said Dr. Reitano.

Sometimes, hormone levels, blood vessels, or mental health are to blame. "It's so important that men realize an erection is more than just something that happens based on their being able to be sexually active, but it's based on being healthy individuals who are taking care of every organ system," said Dr. Reitano.

A Semen Allergy Is Rare—but Real

It's very uncommon, but an allergy to semen (or, more precisely, an allergy to the proteins in semen) is a real medical condition. A partner should look for signs of burning, itching, redness, and vaginal discomfort 10 to 30 minutes following unprotected sex, said Dr. Lopushnyan. It might happen the first time you have sex, or it could develop years later.

A semen allergy is often misdiagnosed as a yeast infection or an STI, according to the International Society for Sexual Medicine. If you're noticing these signs, try having sex with a condom. If the symptoms don't appear, semen might be the culprit. While it can be a major relationship buzzkill, it is possible to conceive with someone whose semen you're allergic to with the help of in vitro fertilization, said Dr. Lopushnyan.

Being aware of some of the basic facts about penises can help you and your partner stay safe and healthy and have a fulfilling sex life too.