IStock Photo 10817557 © Stephan Zabel

“Sex and the City,” Season 2, Episodes 1 and 18 (“Take Me Out to the Ballgame” and “Ex and the City”): Samantha dates men who are on the far ends of the size spectrum—in the first, her boyfriend packs “half a pack of Lifesavers,” while in the second, as Carrie deals with Mr. Big, Samantha must come to grips with Mr. Too Big.

“How long is your penis when it is hard?” –Alfred Kinsey

Of the many sensitive questions asked of thousands of people by Alfred Kinsey, this is one for the history books. The pioneering sexologist wanted to find out what has been hidden since humanity donned the fig leaf: How long is the average penis?

The answer? Six inches.

Here’s how he found out.

In the course of his sexological research, Kinsey asked thousands of American males to estimate the size of their penises. According to Gebhard and Johnson’s The Kinsey Data, “a ruler was often held [by the interviewer] so that the numerals were not visible, and the interviewer's finger moved along it slowly.” The interviewed man would say “Stop” or “There,” or some such answer, at what he felt was his length. Of the 4,000+ responses recorded by Kinsey and his researchers, the most common estimated length was 6 inches—about 25% of all the responses.

The next four ballpark lengths—in descending order—were 6.5 in., 5 in., 7 in., and 5.5 in. Those five lengths comprise almost 90% of all the estimations (7% estimated their penises to be less than 5 inches, and only 3% of men estimated their penises to be over 8 inches). Ever the vigilant statistician, Kinsey noted that “it is obvious that most persons thought in terms of inches and half inches; one-quarter and three-quarters were seldom reported.” Just as, by high school, most boys are not counting their age in half-years (“I'm 18 and a half!”), by college most men are not thinking of their penises in quarter-inches.

As a complement to these numbers, Kinsey then requested what no one had yet gotten on penises: hard data.

Kinsey's measurement method was simple, yet brilliant. He gave each of the men who had been interviewed a card, along with a pre-addressed, stamped envelope, and one simple instruction: during a full erection, lay the card along the top of the penis, and mark in ink where the tip was. The brilliance lay in the method’s anonymity:

• No penis was measured in a clinical setting, by another person; such a situation might have caused many a man to wilt under the pressure.
• All identities were kept secret, carefully coded by Kinsey. Each man was explicitly instructed NOT to write his name on the card or envelope.
• The cards were blank white ones, not ruled or lined—the absence of a visual reference (e.g. a line marking each inch) provided little incentive for exaggeration.
• What ended up in the return mail—a blank card with two marks on it, one for flaccid length and the other for erect—gave no clue to the data it contained. If such a card ended up in the wrong mailbox, no one would be the wiser.
• Over half of the respondents were white men 18 - 24 enrolled in college: 2,376 of them. In that same age range, they also received measurements from 59 black collegians and from 143 white non-collegians.

Over 2,500 of the men returned their cards. Although the average measured length was 6 inches—corresponding with the greatest percentage of estimates on length—it turned out that many of the men had underestimated their size. Of the white college students, 36.9% had estimated they were under 6 inches while only 30.5% turned out to be. For white men not in college, the percentages were 38.8% compared to 27.3%. And among black college students, 26% estimated their penis size to be under 6 inches, while the actual measured percentage was 15.3%.

Although this was the largest study of its kind, subsequent studies have returned similar results: Here are some odds, based upon the Kinsey research:

The odds a white man 18 - 24 enrolled in college has an erect penis that is...

...less than 5 inches long are 1 in 31.25

...5 - 6 inches long are 1 in 3.66

...6 - 7 inches long are 1 in 1.9 (53%)

...7 - 8 inches long are 1 in 6.58

...at least 8 inches long are 1 in 55.56

Each one-inch deviation down is about twice as likely as a deviation up. Translation: smaller is likelier than larger. A white, college-age male is twice as likely to have a penis under half-a-foot long than to have one between 7 and 8 inches. He is also roughly twice as likely to have a penis less than 5 inches long as he is to have one over 8 inches. For the size-obsessed man: you are about as likely to break the 8-inch mark as you are to live in Maryland.

And you’re as likely to be average (6 to 7 inches) as you are to be, well, married.

And no matter what her age, a woman is very likely to be content with her partner’s penis size. The odds a woman is satisfied with her partner’s penis size ranges from 1 in 1.25 women age 56 - 65 (79%) to 1 in 1.16 women age 18 - 25 (86%).

Have questions? For instance: Why only men in college? Why only 18- to 24-year-olds? What about black men, Asian men, Hispanic men, etc? When did Kinsey obtain these measurements? What about those other studies? Click here.