To measure penis size accurately, you need a ruler or flexible measuring tape and a full erection. The method used in clinical studies is called the “bone-pressed” technique, and it takes about 30 seconds once you know the steps. Here’s exactly how to do it for both length and girth.
How to Measure Length
Stand upright and hold your erect penis so it points straight out from your body, parallel to the floor. Place a rigid ruler or measuring tape along the top of the shaft (the side facing the ceiling, not the underside). Position the end of the ruler at the base where the shaft meets your pubic bone.
Here’s the key step most people skip: press the ruler firmly into the pubic bone, pushing past any fat pad or pubic hair. This is the “bone-pressed” method, and it’s the standard used in virtually all medical research. Without it, the fat pad above the base absorbs anywhere from a quarter inch to over an inch of actual length, depending on body composition. Measure in a straight line from the pubic bone to the tip of the head.
If your penis has a noticeable curve, either naturally or from a condition like Peyronie’s disease, use a flexible measuring tape instead of a ruler. Lay the tape along the top surface, following the curve, for a more accurate reading.
How to Measure Girth
Girth is the circumference, the distance around the shaft. Use a flexible measuring tape or a strip of paper you can mark and then hold against a ruler. Wrap it around the thickest part of the mid-shaft while fully erect. Don’t pull the tape tight enough to compress the tissue, just snug against the skin. If the base or head is noticeably thicker or thinner than the middle, you can measure at all three points, but mid-shaft is the standard reference.
Why Erect Measurements Matter
Flaccid size is not a reliable indicator of erect size. A study published in The Journal of Urology found that the correlation between flaccid and erect length is not close enough to use one as a stand-in for the other. Two men with identical flaccid measurements can differ significantly when erect. Flaccid dimensions also shift throughout the day based on temperature, blood flow, stress, and physical activity. If you’ve ever noticed your penis looks smaller after a cold shower or a hard workout, that’s normal physiology, not your actual size.
For a consistent measurement, take it while fully erect in a warm room. If you want to confirm your results, measure on two or three separate occasions and average the numbers.
What the Averages Actually Are
A widely cited analysis of over 15,000 men, referenced by the Sexual Medicine Society of North America, found these averages:
- Erect length: 5.1 inches (13 cm)
- Erect girth: 4.5 inches (11.4 cm)
- Flaccid length: 3.6 inches (9.1 cm)
- Flaccid girth: 3.7 inches (9.4 cm)
These numbers come from researcher-measured data, not self-reports, which tend to skew higher. Most men fall within roughly an inch above or below those averages. Being slightly above or below the mean is statistically ordinary.
When Size Falls Outside the Typical Range
The clinical definition of micropenis is a stretched or erect length of 2.95 inches (7.5 cm) or less in an adult, which corresponds to 2.5 standard deviations below the mean. This is a specific medical diagnosis, not a casual label, and it applies to a very small percentage of the population. Micropenis is typically identified in infancy (under 0.75 inches when gently stretched) and is linked to hormonal factors during fetal development. Adults who meet the clinical threshold can discuss treatment options with a urologist.
Common Measurement Mistakes
Measuring from the side or underside of the shaft adds length artificially because the base sits at a different angle relative to the body there. Always measure along the top. Pressing the ruler at a diagonal rather than straight back into the pubic bone also inflates the number. The ruler should be perpendicular to your body, not angled upward or downward.
Partial erections give inconsistent results. Blood flow determines both length and girth, so anything less than full firmness will undercount. If you lose your erection mid-measurement, wait and try again rather than estimating.
Finally, measuring right after sex, vigorous exercise, or exposure to cold will give you a reading that doesn’t represent your typical size. Room temperature, at rest, with a full erection is the most reproducible condition.