But behind the closed doors of a doctor's examining room, it's a common question.
Pediatric urologist Lane S. Palmer, MD, chief of pediatric urology at Cohen Children's Medical Center, New Hyde Park, N.Y., hears it a lot.
"Fathers often will ask it about their sons -- and, while in the office, will ask about themselves," Palmer tells WebMD.
Men worried about their penis size usually have some other excuse for seeing a doctor, says Bruce R. Gilbert, MD, PhD, director of reproductive and sexual medicine at the Smith Institute for Urology in Lake Success, N.Y.
"When they come in for something else, they ask, 'Oh, by the way, I am concerned about the length of my penis,'" Gilbert tells WebMD. "Most are men in the 20-to-40 age group. But some are aged 40 to 50, and some even older men ask."
The vast majority of these men are in for a surprise. Worry about small penis size is common. Abnormally small penis size is not.
Penis Size: What's Normal, What's Not
You might think that as long a man's penis works, he would have no problem with it. You would be wrong.
Penis size is a key element of a man's self-image. Yet when he's not boasting to other men, the average man significantly underestimates the relative size of his penis.
Here's an example: Over a two-year period, 67 men asked an Italian hospital for surgical correction of a small penis. All turned out to have normal-size penises.
"A few days ago, I had a patient who spent an hour taking measurements of his penis and thinking it is too small," Gilbert says. "Yet it was normal."
That man isn't alone. About 45% of his brethren want a bigger penis. Never mind that 85% of heterosexual women say they are satisfied with their partners' penis parameters.
Many men worry about the size of their erections. Many more worry about how their penis looks when it's limp, studies find.
So how can a man know if he's normal, super-sized, or small? Not by his shoe size, a common and disproved myth about estimating penis length. Like so much else in life, direct measurement is the key.
How to Measure a Penis
Men may be surprised to learn that penis length isn't measured on the erect penis. Too many variables are involved.
Instead, the most reliable penis measurement is called SPL -- stretched penis length. The longer a man's SPL, the longer his erect penis length, according to studies done on brave young men who volunteered to have erection-stimulating penis injections.
To learn your SPL, measure the penis while it's flaccid. Press the ruler tight against the pubic bone at the base of the penis. Don't just measure from where the penis separates from the scrotum, or you'll lose precious centimeters. Now gently, but very firmly, stretch the penis as far as it will go. Measure from the pubic bone to the tip of the stretched penis.
Did you get five and a quarter inches? If so, you are exactly normal. Most adult men are within about a half inch of 5.24 inches, according to statistics Palmer has compiled. Nearly all studies of penis length come up with a similar measure.
If you're a little smaller than that, you've got lots of company. Just as many men are below average penis size as above it.
How big is big? According to Palmer's statistics, only 0.6% of men have an SPL of 6.8 inches or more. But too big isn't what men tend to worry about.
Micropenis: When a Penis Really Is Too Small
There is, of course, such a thing as a very small penis. The medical term "micropenis" applies to the 0.6% of men with the smallest penises. According to Palmer's statistics, an SPL of three and two-thirds inches or less indicates a micropenis.
Even then, U.S. doctors hesitate to recommend surgery for a man whose SPL is longer than three inches. That's because surgery is controversial and risky.
Micropenis isn't usually something a man discovers when he's an adult. It's usually caused by genetic or hormonal abnormalities that cause other, more serious health problems early in life.
That's because the penis starts to develop when a fetus is just 8 weeks old. By week 12, the penis has developed and begins to grow. During the second and third trimesters, male sex hormones cause the penis to grow to normal length. Factors that interfere with hormone production and hormone action stunt penis growth.
When discovered in infancy, micropenis can be treated with testosterone, which can stimulate penis growth in childhood, even after puberty. While the safety and long-term efficacy of this treatment remains to be proved, available data suggest the treatment does not affect normal development during puberty.
For adults with micropenis, the options are few.
"For true micropenis, there is not much you can do that is adequate for the adult patient, except for putting in a penile prosthesis," Gilbert says.
Fortunately, micropenis is a rare condition. Far more common is what Palmer and colleagues call "the constellation of conditions that make the penis look diminutive and small" -- inconspicuous penis.
Inconspicuous Penis: When Size Isn't the Only Issue
"Inconspicuous penis means a penis that is hard to see," Palmer says.
Micropenis -- a truly tiny penile shaft -- is the rarest of the conditions under the umbrella term "inconspicuous penis."
Other forms of inconspicuous penis that may have remained untreated until adulthood are webbed penis and buried penis.
"The webbed penis indicates the scrotum has connected to the underside of the penis so it pulls the penis inward," Palmer says. "Usually the penis is at right angles to the scrotum. But in webbed penis, the scrotum is high riding and the separation from the penis is not clear."
Buried penis occurs when the penis is hidden below the skin. This can happen because of excessive belly fat in the front of the abdominal wall droops down to conceal the penis. It can also happen when the connection to the scrotum is absent and the penis withdraws inward toward the pelvis. Another form of buried penis occurs when a too-large foreskin makes the penis look buried.
Small Penis Syndrome
It's been dubbed small penis syndrome: the belief that you aren't a real man because you don't have a big penis.
Perhaps one reason men often believe this is that most men have no idea what a normal-size penis looks like. When asked to guess, most guess wrong. About 15% of men just throw up their hands and admit they don't know, one study found.
Another reason is pornography. Palmer tells the story of a teen patient who was in agony over his small penis size. But examination showed he was perfectly normal. It turned out that the boy had been viewing Internet pornography, from which he got a distorted picture of normal male anatomy.
Body dysmorphic disorder and other psychiatric issues that may underlie small penis syndrome may require professional psychological help. But Palmer and Gilbert say that most men need only reassurance that they really are normal.
"I give the patient a physical and get his personal history and social history," Gilbert says. "If he has no medical problem, I speak positively. I tell him that he has a normal phallus that is no different in function from any other healthy man. I give him confidence."
Palmer notes that the law of averages dictates that some men will have smaller penises than others. This does not make them abnormal.
"If a man has perfectly normal penis function, he must reconcile with the fact that his penis is what it is," he says. "You cannot make a short person tall. People have to accommodate to their own anatomy."
Can Surgery Make a Small Penis Larger?
It's hard for a man to open his email without encountering an offer of a drug, device, or surgical procedure that will give him a bigger penis.
Sure, that stuff is just spam. But are there legitimate ways to lengthen a penis?
"There isn't much to make the penis larger," Palmer says. "Surgically, there is not much to significantly increase penis size or girth that is not fraught with significant complications."
Procedures known as "augmentative phalloplasty" promise to make a penis wider in girth. And because these procedures add weight to the penis -- by grafting fat from another part of the body -- they make the penis a half-inch or so longer when flaccid.
"Getting a uniformly even expansion of girth is unpredictable and often does not lead to a good outcome," Palmer says. "You have to hope you do not get a lumpy penis. This is not an easy thing to do."
Most men do not know that only about two-thirds of their penis protrudes from their bodies. The other third, held in place with ligaments, provides the leverage for sexual function.
One surgical technique cuts these tendons and allows the penis to protrude another inch or two. Gilbert takes a dim view of the technique, because the penis loses its leverage.
"When surgeons take the inside part of the penis and move it out, the most common outcome is their erection no longer goes up but down," he says. "If a man is unhappy with his penis length, he is going to be unhappy with a penis he has to pick up and insert. I've had men ask me, 'Can you put it back the way it was?'"
Gilbert says that because of the high complication rate, malpractice insurance rarely covers penis-lengthening procedures.
All of these procedures are considered "experimental surgery." Severe complications include -- but are not limited to -- penis shortening, a lumpy or uneven appearance, scarring, sexual dysfunction, and curvature of the penis. These complications often cannot be corrected.
Can Devices Make a Small Penis Larger?
Traction devices that stretch the penis may actually add an inch or so to penis length, based on small studies and anecdotal reports. Gilbert says he is following two patients who are using these devices in an effort to make their penises longer.
"These devices have to be worn for several hours a day, for many months," he says. "Most people, even if they are highly motivated, don't have that kind of time. So I'm not sure a lot of patients have the time or energy or perseverance to do that."
Vacuum devices may help men with erectile dysfunction achieve an erection -- but they do not make the penis larger.
Can Regenerative Medicine Make a Small Penis Larger?
However, there is one area of research with intriguing possibilities: regenerative medicine. Scientists have been able to grow animal penises in the laboratory by seeding scaffolds with the animals' own cells. These penises then were successfully transplanted.
Already a group of researchers in Beijing, China, have reported using a similar technique to treat 69 patients with what they called small penis syndrome. In this case, the tissues were grown from the patients' own scrotal skin and grafted to their penises to increase their girth.
"With regenerative medicine, the sky is the limit," Palmer says. "We might have this as an option down the line."
"Regenerative medicine is still not ready for prime time," Gilbert says. "These things have a lot of potential, but nothing we can offer to patients in the near future."