To measure penis size accurately, you need a rigid ruler for length and a flexible measuring tape (or a piece of string) for girth. Length is measured along the top of the shaft from the pubic bone to the tip, and girth is measured around the thickest part of the shaft. Getting a reliable number comes down to using the right tools, the right technique, and consistent conditions.
How to Measure Length
Stand upright and hold the penis straight out, parallel to the floor. Place a rigid ruler 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, pushing into the fat pad above the base of the penis. Read the measurement at the very tip of the head.
This method is called bone-pressed length, and it’s the standard used in medical research because it eliminates the variable of body fat. The fat pad above the pubic bone gets thicker as body weight increases, so measuring from the skin surface produces a shorter, less consistent number. Pressing to the bone gives you the same reference point regardless of weight fluctuations. Research from the NIH confirms that bone-to-tip measurement is more accurate and reliable than skin-to-tip, with the biggest discrepancy showing up in men who carry extra weight.
A rigid ruler works best for length because it compresses the fat pad effectively with one hand. A flexible tape can bend along the shaft and give a slightly inflated reading, and it’s harder to press into the fat pad with its soft edge. The one exception: if you have noticeable curvature, a rigid ruler can underestimate your length because it measures in a straight line. In that case, a flexible tape laid along the curve of the top surface gives a more accurate result.
Erect vs. Stretched: Which to Measure
Erect length is the most commonly referenced number and the one most men are looking for. Measure at full erection for the most representative result. If you can’t maintain a full erection during measurement, stretched flaccid length is the clinical alternative. To do this, gently stretch the flaccid penis to its maximum comfortable length while pressing the ruler to the pubic bone, and read the measurement at the tip.
Stretched flaccid length closely approximates erect length in most men, which is why urologists use it in clinical settings. The largest meta-analysis on the topic, covering over 15,500 men, found a mean stretched length of 13.24 cm (5.21 inches) compared to a mean erect length of 13.12 cm (5.17 inches), essentially the same number.
Flaccid length on its own is not a useful benchmark. It varies significantly throughout the day based on temperature, physical activity, and stress levels. Clinical studies go to considerable lengths to control for this, measuring immediately after undressing in temperature-controlled rooms (23 to 25°C) to minimize shrinkage from cold exposure. Your flaccid size at any given moment tells you very little about your erect size.
How to Measure Girth
Girth is the circumference of the shaft. Wrap a flexible measuring tape snugly around the thickest part of the shaft, typically just below the head, while fully erect. Read the number where the tape overlaps. If you don’t have a measuring tape, wrap a piece of string or a strip of paper around the shaft, mark where it meets, then lay it flat against a ruler.
“Snugly” means the tape touches the skin all the way around without compressing it. You’re measuring circumference, not squeezing.
Average Size for Reference
The most widely cited data comes from a 2015 meta-analysis published in BJU International that pooled measurements from over 15,500 men across multiple countries. The averages:
- Erect length: 13.12 cm / 5.17 inches
- Erect girth: 11.66 cm / 4.59 inches
- Flaccid length: 9.16 cm / 3.61 inches
- Flaccid girth: 9.31 cm / 3.67 inches
These numbers were taken by clinicians, not self-reported. That distinction matters. A clinical study on self-assessment found that about 73% of men overestimate their erect length, with the average overestimation coming in at roughly 1 cm (just under half an inch). If your own measurement lands close to the averages above, you’re right in the middle of the distribution.
Common Mistakes That Skew Results
Measuring from the side or underside of the shaft adds length because the base of the penis sits farther from the skin surface on those sides. Always measure along the dorsal surface (the top, facing your belly).
Not pressing to the pubic bone is the other big one. If you just rest the ruler against the skin, you’re measuring the visible portion only, which can be a centimeter or more shorter than the bone-pressed measurement depending on your body composition. Bone-pressed length is the standard in every major study, so if you want to compare your number to published averages, you need to use the same method.
Measuring when not fully erect will undercount. Arousal level, time of day, room temperature, and recent physical activity all influence erection quality. For the most consistent result, measure at peak erection in a warm, comfortable environment. If you want extra confidence in the number, measure on three separate occasions and average the results.
When Measurement Has Medical Relevance
For most men, measuring is a matter of curiosity. But there are situations where accurate measurement becomes clinically important. Micropenis is a medical diagnosis defined as a stretched penile length more than 2.5 standard deviations below the average. In practical terms for an adult, that translates to a stretched length under roughly 7 cm (2.75 inches). This is a rare condition, and the diagnosis is made by a physician using standardized technique.
Men with Peyronie’s disease, a condition that causes penile curvature from scar tissue, are often asked to document their length before and after treatment. The American Urological Association recommends measuring stretched length from the skin junction to the tip to establish a baseline. Tracking changes in length, curvature angle, and girth over time helps guide treatment decisions. If you’re measuring for this purpose, consistency in technique is more important than any single number, since you’re tracking change rather than comparing to an average.