To get an accurate measurement, you need a ruler or flexible measuring tape, a consistent method, and ideally a full erection. The technique matters more than people realize: measuring from the wrong spot or at the wrong angle can easily add or subtract a centimeter, which skews your result and any comparison to published averages.
How to Measure Length
Stand upright with a full erection. Place a rigid ruler or measuring tape along the top of the penis, pressing the end firmly into the pubic bone at the base. This compression pushes past the fat pad that sits above the shaft and gives you what clinicians call “bone-pressed erect length,” the standard used in virtually every peer-reviewed study on penis size. Measure in a straight line from the pubic bone to the tip of the glans.
You measure along the top (the dorsal side) rather than the underside because the underside includes extra tissue where the urethra runs, which inflates the number and doesn’t reflect functional length. If your penis curves noticeably to one side or upward, use a flexible measuring tape instead of a ruler so it can follow the curve. Lay the tape along the outer curve and read the distance from base to tip. A rigid ruler held against a curved shaft will underestimate the true length.
One practical tip: don’t pull a fabric or plastic measuring tape too taut. Some tapes stretch under tension, which gives you a reading that’s slightly longer than reality.
How to Measure Girth
Girth is the circumference of the shaft, and you need a flexible measuring tape for this. Wrap it around the thickest part of the erect shaft, typically near the midpoint, without compressing the skin. Read the number where the tape overlaps itself. If you don’t have a measuring tape, wrap a piece of non-stretchy string around the shaft, mark where it overlaps, then lay the string flat against a ruler to get your circumference.
A rigid ruler cannot measure girth directly. Trying to calculate circumference from a diameter measurement introduces rounding errors because the shaft isn’t a perfect circle.
Measuring Without an Erection
If measuring erect isn’t practical, the most reliable alternative is a stretched flaccid measurement. Gently stretch the flaccid penis to its full length and measure along the top from the pubic bone to the tip, just as you would when erect. Research published in the Journal of Urology confirms that stretched flaccid length correlates closely with erect length, making it a suitable stand-in.
A non-stretched flaccid measurement, on the other hand, is not a reliable indicator of erect size. Flaccid length varies significantly with temperature, arousal level, time of day, and blood flow, so the correlation between a resting flaccid measurement and erect length is too loose to be meaningful.
Common Mistakes That Skew Results
Measuring from the side or underside of the shaft adds length that doesn’t appear in any clinical dataset, so your number won’t compare accurately to published averages. Skipping the bone press (not pushing the ruler into the pubic bone) will underestimate length, especially if you carry extra weight in the pubic area. Measuring at partial erection gives you a shorter and less consistent result: arousal level can fluctuate, so aim for full erection and take two or three measurements on different occasions to find a consistent number.
Ambient temperature matters too. A cold room constricts blood vessels and can reduce both length and girth slightly. A neutral or warm environment gives the most representative measurement.
How Your Measurements Compare
A large meta-analysis from King’s College London, covering over 15,000 men, found the following averages:
- Erect length: 13.12 cm (5.16 inches)
- Erect circumference: 11.66 cm (4.59 inches)
- Flaccid length: 9.16 cm (3.61 inches)
- Stretched flaccid length: 13.24 cm (5.21 inches)
- Flaccid circumference: 9.31 cm (3.66 inches)
Notice that the average stretched flaccid length (13.24 cm) is almost identical to the average erect length (13.12 cm), which reinforces why stretched measurement works as a proxy. Most men fall within about 1.5 cm (roughly half an inch) above or below these averages. If your measurement is anywhere in that range, you are statistically normal.
Self-reported measurements in online surveys tend to run about a centimeter longer than measurements taken by clinicians, likely a combination of optimistic technique and rounding up. If you follow the bone-pressed, dorsal-surface method described above, your number should align closely with the research data.
Why Consistency Matters More Than Precision
If you’re tracking size over time for any reason, the single most important factor is using the same method every time. Same position (standing), same surface (top of the shaft), same tool, same arousal level. A half-centimeter difference between sessions is normal and reflects blood flow variation, not an actual change in size. Only a consistent trend across multiple measurements taken weeks apart tells you anything real.