To measure your penis accurately, you need a rigid ruler for length and a flexible tape (or a piece of string and a ruler) for girth. The technique matters more than most people realize: pressing the ruler against the pubic bone, measuring along the top, and using the right state of arousal all affect your number. Here’s exactly how to do it so your measurement is consistent and comparable to published averages.
How to Measure Length
Stand upright with your penis fully erect and pointing straight out from your body, roughly perpendicular to the floor. Place a rigid ruler along the top of the shaft, pressing the end firmly into the pubic bone just above the base. Read the measurement at the very tip of the head. This is called the “bone-pressed” method, and it’s the standard used in virtually all clinical research.
Pressing into the pubic bone matters because it eliminates the variable of body fat. Two people with the same underlying penile length can get different numbers if one carries more weight in the lower abdomen. The fat pad in that area compresses when you press the ruler in, giving you a measurement that reflects actual size rather than how much padding sits in front of it. A systematic review of measurement studies confirmed that bone-to-tip readings are more accurate and reliable, with the biggest discrepancies showing up in overweight individuals.
Always measure along the top (the dorsal side), not the underside or the sides. If your penis curves, hold the ruler along the top and follow the curve as best you can, or use a flexible tape pressed along the surface to capture the true length along the outer curve.
How to Measure Girth
Girth is the circumference of your shaft. While fully erect, wrap a soft measuring tape snugly around the thickest part of the shaft, which for most people 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 meets its starting point.
If you don’t have a flexible measuring tape, use a strip of paper or a piece of string. Wrap it around the shaft, mark or pinch where it overlaps, then lay it flat against a ruler to get your circumference. This string method is surprisingly accurate and avoids the awkwardness of trying to read a floppy tape while holding it in place.
If You Can’t Measure While Erect
Getting a full erection on demand isn’t always easy, especially when you’re focused on holding a ruler. The good news: stretched flaccid length is a reliable stand-in. Research published in The Journal of Urology found that stretched flaccid length predicted erect length with a correlation of about 0.79, which is quite strong. Regular flaccid length, by contrast, was a much weaker predictor at 0.68, because flaccid size varies dramatically with temperature, stress, and blood flow.
To measure stretched length, hold the head of your flaccid penis between your thumb and forefinger and gently extend it outward as far as it will comfortably go. With your other hand, place a ruler along the top, pressed against the pubic bone, and read the measurement at the tip. This won’t be exact, but it gives you a reasonable estimate of your erect length.
Factors That Affect Your Measurement
Your penis isn’t the same size every time you measure. Several things cause real, temporary changes in size:
- Temperature: Cold air causes the penis to retract. Clinical studies take measurements in rooms set to around 21°C (72°F) to standardize this. If you’ve just come in from the cold, your number will be lower.
- Arousal level: A partial erection will give you a shorter and thinner reading than a full one. If you’re anxious or distracted, you may not reach your maximum size.
- Time of day: Testosterone levels peak in the morning, and many people find erections are firmest then.
- Recent activity: Exercise increases blood flow and may give you a slightly different reading than sitting at a desk for hours.
For a consistent number, measure a few times across different days, ideally in a warm room with a full erection, and average the results.
How Your Numbers Compare
A large meta-analysis combining data from over 15,000 men found these averages: erect length was 13.12 cm (5.16 inches) with most men falling within about 1.6 cm of that number in either direction. Erect circumference averaged 11.66 cm (4.59 inches). Flaccid length averaged 9.16 cm (3.6 inches), though this number varies widely from person to person and even hour to hour.
Stretched flaccid length averaged 13.24 cm (5.21 inches), which lines up closely with the erect average, reinforcing why stretched length works as a proxy. If your erect or stretched measurement falls within a couple of centimeters of these averages, you’re solidly in the normal range. The clinical threshold for micropenis is a stretched length more than 2.5 standard deviations below the mean for age, which in adults works out to roughly 9.3 cm (3.7 inches) or less. Below that point, medical evaluation may be appropriate.
Common Measurement Mistakes
The most frequent error is measuring along the side or underside of the shaft, which gives an inconsistent number because curvature and anatomy vary. Always use the top. Another common mistake is not pressing against the pubic bone. If you simply place the ruler against the skin surface without pressing in, you’ll get what’s called the “non-bone-pressed” measurement. That number is valid in its own right, but it won’t match the figures used in medical studies or published averages, so comparisons won’t be meaningful.
Using a rigid ruler for girth is another pitfall. A ruler can only measure diameter, and converting diameter to circumference with math introduces error because the cross-section of a penis isn’t a perfect circle. Stick with a flexible tape or the string method for girth.
Finally, measuring from the side or from underneath the shaft will almost always produce a longer number because of how the penis attaches to the body. The standard is the top surface, from pubic bone to tip. If you see a measurement technique suggesting otherwise, it’s not following clinical conventions.