The average thickness of an erect penis, measured as circumference (the distance around the shaft), is about 4.6 inches or 11.66 cm. When flaccid, the average circumference drops to roughly 3.7 inches or 9.31 cm. These figures come from a systematic review of over 15,500 men, the largest analysis of its kind, published in the British Journal of Urology International.
Circumference vs. Diameter
Most people who search for penis “thickness” are really asking about girth, which is the circumference wrapped around the shaft. That’s the number researchers report in nearly every major study. If you want the actual diameter (the distance straight across), you can divide the circumference by pi (3.14). For an average erect circumference of 4.6 inches, the diameter works out to about 1.46 inches, or roughly 3.7 cm.
One large study of over 14,500 men directly measured mid-shaft diameter in the flaccid state and found it to be about 2.86 cm (just over 1.1 inches). During erection, the diameter increases as blood flow expands the tissue, which is why the erect circumference is noticeably larger than the flaccid measurement.
What the Numbers Look Like in Practice
The standard deviation for erect circumference is about 1.1 cm (roughly 0.4 inches). That means about two-thirds of men fall between 4.2 and 5.0 inches of erect girth. If you’re within that window, you’re squarely in the middle of the bell curve. Men at the higher or lower ends of the range, more than two standard deviations from the mean, account for less than 5% of the population on each side.
Flaccid girth has a tighter spread, with a standard deviation of 0.9 cm. The flaccid measurement is less consistent overall because temperature, arousal level, time of day, and even stress can temporarily change how much blood the penis holds at rest.
How Girth Is Measured
Researchers typically measure circumference at the mid-shaft using a flexible tape or a strip of paper. For self-measurement at home, the most reliable approach is to wrap a soft measuring tape snugly around the thickest part of the shaft, usually just below the head. If you don’t have a flexible tape, wrap a piece of string around the shaft, mark where the ends meet, then lay the string flat against a ruler.
Consistency matters more than technique. Measure while fully erect, at roughly the same time of day, and take a few readings on different days to get a reliable average. A single measurement can be off depending on your level of arousal or how tightly you pull the tape.
Girth and Length Don’t Always Correlate
A longer penis isn’t necessarily a thicker one. Research on penile dimensions has found only a weak relationship between length and circumference. Some men have above-average length with average girth, and vice versa. The two measurements are largely independent, which is why studies report them separately rather than using one to predict the other.
For context on length alongside girth: a 2023 meta-analysis covering over 55,000 men across 75 studies found the average erect length to be about 5.5 inches (13.93 cm). That analysis also identified a modest increase in average erect length over the past three decades, though no similar trend appeared for circumference.
When Size Falls Outside the Typical Range
Clinical concern about penis size centers almost entirely on length rather than girth. The medical diagnosis of micropenis is based on stretched penile length falling more than 2.5 standard deviations below the mean, which works out to about 3.7 inches (9.3 cm) or less in adults. There is no equivalent clinical threshold for circumference alone.
That said, many men who worry about their size fall well within the normal range. Studies on body image consistently find that men tend to underestimate their own measurements compared to clinical averages. The fact that the data set behind these averages includes thousands of men measured by clinicians, not self-reported numbers, makes the figures more reliable than what you’d find in informal surveys, which tend to skew higher due to reporting bias.