Penis length is measured along the top side, from the pubic bone to the tip, using a rigid ruler or measuring tape. Girth is measured around the thickest part of the shaft. These are the same methods used in clinical research, and getting an accurate number at home comes down to a few specific details most people get wrong.
How to Measure Length
Stand upright and hold the penis straight out, parallel to the floor. Place a rigid ruler or measuring tape along the top of the shaft (the side facing your abdomen, not the underside). Press the end of the ruler firmly against the pubic bone. Fat padding in this area can hide a significant portion of length, so pressing down until you meet resistance gives you what clinicians call “bone-pressed” length, which is the standard used in virtually all medical research.
Measure to the very tip of the glans. Read the measurement in centimeters or inches, whichever you prefer. If the penis curves to one side or upward, a flexible measuring tape laid along the top surface will follow the curve and give a more accurate reading than a straight ruler.
For the most consistent result, measure while fully erect. Flaccid size varies dramatically depending on temperature, stress, arousal level, and time of day, so it tells you very little on its own. If you do want a flaccid measurement that approximates erect length, stretch the penis gently outward (while flaccid) and measure the same way. Stretched flaccid length correlates reasonably well with erect length, which is why urologists use it during office exams when an erection isn’t practical. That said, research published in The Journal of Urology found that the standard clinical stretch technique underestimates true length by an average of 2.7 cm compared to measurements taken at maximum stretch force, so it’s an approximation rather than an exact match.
How to Measure Girth
Girth is the circumference of the shaft. Wrap a flexible measuring tape snugly around the thickest part of the shaft, which for most men is just below the head. Don’t pull the tape tight enough to compress the tissue, but don’t leave slack either. Read where the tape overlaps.
If you don’t have a flexible tape, use a piece of string or a strip of paper. Wrap it around the shaft, mark where it overlaps, then lay it flat against a ruler. Measure while erect for a consistent number, since flaccid girth changes just as much as flaccid length.
Where Most Mistakes Happen
The most common source of error is not pressing the ruler against the pubic bone. Measuring from the surface of the skin, especially if you carry extra weight in the lower abdomen, can shave off a centimeter or more. Every clinical study uses bone-pressed measurement, so if you’re comparing yourself to published averages, you need to do the same.
Measuring along the underside of the penis is another frequent mistake. The underside is longer than the top because of the angle where the shaft meets the body, so it inflates the number. Always measure along the dorsal surface (the top, facing your stomach).
Arousal level matters too. An erection that isn’t fully rigid will give a shorter reading. If you’re measuring out of curiosity or concern, take the measurement a few times on different days at full erection and average the results. A single measurement on a single day isn’t especially reliable.
What the Averages Actually Are
The largest systematic review on the topic, published in BJU International, pooled data from over 15,500 men measured by clinicians (not self-reported). The averages:
- Erect length: 13.12 cm (5.16 inches)
- Erect girth: 11.66 cm (4.59 inches)
- Flaccid length: 9.16 cm (3.61 inches)
- Stretched flaccid length: 13.24 cm (5.21 inches)
- Flaccid girth: 9.31 cm (3.67 inches)
These numbers come from clinician measurements specifically because self-reported data tends to skew higher. Men round up, whether consciously or not. Studies that rely on self-measurement consistently produce inflated averages, which is why the numbers you see cited in rigorous medical literature may be smaller than what you’ve encountered elsewhere online.
The standard deviation for erect length was about 1.66 cm, meaning roughly two-thirds of men fall between 11.5 cm (4.5 inches) and 14.8 cm (5.8 inches). If your measurement lands anywhere in that range, you’re squarely in the middle of the distribution.
When Size Becomes a Medical Question
Micropenis is a formal clinical diagnosis, not just a casual term. It’s defined as a stretched penile length more than 2.5 standard deviations below the mean for age. In adults, that works out to roughly 7 cm (2.75 inches) or shorter when stretched. This is a rare condition, typically identified in infancy or childhood, and it has specific hormonal causes that can sometimes be treated. Simply being below average does not meet the threshold.
Outside of micropenis, there is no medical definition of “too small.” Size concerns are common, but studies consistently find that most men who worry about their size fall within the normal range. If your measurement lines up with the averages above and you still feel distressed, that’s a psychological pattern worth exploring on its own terms rather than a size problem.