Science goes to great lengths to measure average penis size

Researchers comb through measurements of more than 15,000 men to see what qualifies as a 'normal' penis

Researchers have finally answered one of the most compelling questions in the field of medicine: What is a normal size for a penis?

The answer, according to anatomically precise measurements of up to 15,521 men from around the world, is 3.6 inches (9.16 centimeters) in the resting state and 5.2 inches (13.12 centimeters) when erect.

These average lengths are just two of the findings reported in BJU International, the official journal of the British Assn. of Urological Surgeons and a handful of other medical societies. But before we move on to more of their results, let’s pause for another question:

Why should anyone spend any amount of time trying to figure this out?

The research team – from King’s College London, King's College NHS Foundation Trust and South London and Maudsley NHS Foundation Trust – offered several reasons. A scientifically validated answer may help doctors reassure men with conditions like “small penis anxiety” or body dysmorphic disorder who suffer considerable distress due to the size of their organ. It could also help manufacturers design better condoms with lower failure rates.

So the group conducted an exhaustive search for published studies with reliable assessments of penile length and girth. To qualify, studies had to include 50 or more male volunteers who were at least 17 years old. Would-be participants were excluded if they suffered from erectile dysfunction or had any physical abnormalities, including implants. Only studies with careful measurements taken by health professionals were included in the analysis.

The researchers wound up with 20 studies that evaluated thousands of men. They assumed that if they plotted all the measurements, the distribution of penis sizes would fall along a bell curve. Then they used that to come up with charts that displayed length and girth by percentile.

It takes 3.6 inches from the root of the penis to the tip of the glans to hit the 50th percentile for flaccid length. That means 68% of men measure between 3 and 4.2 inches, and 95% are in the 2.4- to 4.8-inch range.

Furthermore, with an average length of 5.2 inches for an erect penis, 68% of men measure between 4.5 and 5.8 inches and 95% are in the 3.9- and 6.5-inch range.

The average circumference, or girth, of a resting penis is 3.7 inches (9.31 cm); for an erect penis, it's 4.6 inches (11.66 cm).

Their investigation presented an opportunity for the researchers to test the validity of the folklore surrounding penis size. They found no convincing support in the data for the idea that penis size is correlated with the length of one’s index finger or the ratio of the length of the second and fourth fingers. Nor was there any good evidence that foot size was a predictor of penis size. Age, body mass index and testicular volume were also unrelated.

However, the data did show that height offered some predictive value for estimating penis length. The correlation was weak but statistically significant, according to the study.

The overwhelming majority of men included in the analysis were Caucasian or Middle Eastern, and only a few hundred were of African or Asian descent. That made it impossible “to draw any conclusions about any differences in penile size across different races,” the researchers wrote.

Despite the large number of men included in the study, the researchers said their results might be biased if well-endowed men were more inclined to be measured. They also acknowledged that the findings – though intended to provide reassurance to most people – could prompt up to half of all men to feel “defective” or “abnormal” upon learning that their measurements are below average.

If so, a 2006 study from the journal Psychology of Men & Masculinity might make some of them feel better. After surveying more than 52,000 people, researchers from UCLA and CSULA found that while 45% of men wished they had a larger penis, 85% of women thought their partner’s organs were just fine.

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