To measure penis size accurately, you need a rigid ruler or flexible measuring tape, an erection, and the correct starting point: the pubic bone. The method used in clinical research is called the “bone-pressed” technique, and it’s the only way to get a measurement you can meaningfully compare to published averages.
How to Measure Length
Place a rigid ruler along the top of the penis (the side facing your stomach when erect). Press the end of the ruler firmly against the pubic bone at the base, pushing past any fat pad in the area. Measure in a straight line from the pubic bone to the very tip. This is your bone-pressed erect length.
Pressing to the pubic bone matters because the fat pad above the base varies from person to person and can hide a significant amount of length. Without pressing in, two people with identical penile anatomy could get very different numbers depending on body composition. Clinical studies use the bone-pressed method for exactly this reason: it’s the most consistent and reproducible measurement.
If you have a curve, keep the ruler along the top surface and measure the straight-line distance rather than tracing the curve. Some people prefer a flexible measuring tape for this, following the curve along the top of the shaft. Both approaches are acceptable, but know that following a curve will produce a slightly longer number than a straight-line measurement.
How to Measure Girth
Use a soft measuring tape (the kind used in sewing) and wrap it around the thickest part of the shaft, typically just below the head. Pull it snug but not tight, and note where the tape meets the starting end. That number is your circumference.
If you don’t have a measuring tape, wrap a piece of string or a strip of paper around the shaft instead. Pinch or mark where the end meets the wrap, then lay the string flat against a ruler to get your number. Avoid using stretchy tape or pulling too hard, as either can inflate the result.
Getting a Consistent Reading
A few things can skew your measurement if you’re not aware of them. Cold temperatures cause temporary shrinkage, so measure at a comfortable room temperature. Your level of arousal also matters. You want a full erection, not a partial one, since even a slight difference in firmness can change both length and girth. If you’re curious about your typical size, measure on a few different occasions and average the results, because erection quality naturally fluctuates day to day.
Measuring from the side or the underside of the penis will give you a different (and usually longer) number than measuring from the top. The top, dorsal surface pressed to the pubic bone is the standard. If you measure any other way, you won’t be able to compare your result to averages from studies.
What About Flaccid Measurements?
Flaccid size is not a reliable indicator of erect size. Some penises grow substantially when erect, while others start closer to their full length. If you’ve heard the terms “grower” and “shower,” that’s the real-world version of what researchers have documented.
Clinicians sometimes use a “stretched flaccid” measurement as a stand-in for erect length, gently stretching the flaccid penis and measuring from pubic bone to tip. Research published in the Journal of Urology found this approach is less accurate than many assume: in a study of men whose erect length was directly measured, 83% had an erect length longer than their stretched flaccid length, and nearly half showed a discrepancy of 1 centimeter or more. The average stretched flaccid length was 10.8 cm compared to an average erect length of 11.8 cm. So if you’re measuring stretched and flaccid, expect the number to underestimate your erect size.
How You Compare to Averages
A large systematic review analyzing data from over 15,000 men found the average erect length is about 5.2 inches (13.12 cm), with an average erect circumference around 4.5 inches. A separate analysis looking at 75 studies and over 55,000 men, published in the World Journal of Men’s Health, found the average erect length had increased from 4.8 inches in 1992 to 6 inches in 2021, a 24% jump over 29 years. Researchers speculate this trend could be related to environmental factors, including exposure to hormone-disrupting chemicals in food and consumer products, though the cause is not confirmed.
These numbers represent midpoints across large populations. Most men fall within about an inch above or below the average. The range of normal is wide, and outliers on either end are uncommon.
When Size Is Clinically Relevant
There is one clinical threshold worth knowing. A micropenis diagnosis applies when erect length is 2.5 standard deviations below the mean. In practical terms for adults, that means an erect length of about 2.67 inches (6.8 cm) or less, or a stretched flaccid length of about 2.95 inches (7.5 cm) or less. This is a medical diagnosis, not a judgment call. It’s relevant because it can sometimes indicate an underlying hormonal condition, and treatment options exist for those who want them. The vast majority of men who feel they are “small” fall well above this threshold.