To measure a penis accurately, you need a full erection, a ruler or flexible measuring tape, and a consistent technique. The method used in clinical research is called bone-pressed erect length, and it gives the most reliable, reproducible number. Here’s exactly how to do it for both length and girth.
How to Measure Length
Start with a full erection. Partial erections produce significantly smaller readings and won’t give you a number you can compare to anything meaningful. Stand upright or stand against a wall for consistency.
Place a rigid ruler or measuring tape along the top of the penis, starting at the base where the shaft meets the pubic bone. Press the end of 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 in urology research because it removes body fat as a variable. Measure in a straight line from the pubic bone to the very tip of the head (glans). That number is your bone-pressed erect length.
If you don’t press into the pubic bone, you’re getting what’s called “non-bone-pressed” length. This measurement is typically about an inch shorter, depending on how much fat sits above the pubic bone. Non-bone-pressed length reflects what’s visible or usable during sex, while bone-pressed length captures the full anatomical size. Both numbers are valid. Just know which one you’re taking so you can compare it meaningfully.
If your penis curves noticeably to one side or upward, measure along the curve using a flexible tape rather than a rigid ruler. A rigid ruler held in a straight line will undercount the actual length of a curved shaft.
How to Measure Girth
Girth is the circumference of the shaft. You’ll need a flexible cloth or plastic measuring tape for this. If you don’t have one, wrap a piece of string around the shaft, mark where it overlaps, then lay the string flat against a ruler.
Take the measurement at mid-shaft, roughly halfway between the base and the head. This is the standard location because it gives the most consistent reading. The base tends to be thicker, so measuring there will skew the number higher than what’s typically reported in studies. Wrap the tape perpendicular to the shaft, keeping it level rather than angled. It should be snug against the skin but not compressed into it. Note where the tape meets itself.
Common Mistakes That Skew Results
The most frequent error is measuring without a full erection. Even a 90% erection can shave off noticeable length and girth. If you lose firmness during the process, wait and try again.
Other common mistakes:
- Measuring along the underside. The standard is along the top (dorsal side) of the penis. The underside includes the frenulum area and often gives a slightly different number.
- Skipping the bone press. Without pressing into the pubic bone, your result is a non-bone-pressed measurement. That’s fine if you know that’s what you’re tracking, but it won’t match clinical averages, which almost always use bone-pressed length.
- Wrapping girth tape at an angle. If the tape spirals even slightly rather than sitting perpendicular to the shaft, you’ll get a larger circumference reading than the actual girth.
- Pressing girth tape too hard. Compressing the tape into the skin inflates the number. Keep it snug, not tight.
- Cold conditions. Room temperature affects size. Clinical studies standardize measurement environments at around 24°C (75°F). A cold room will give you a smaller reading than your typical erect size.
Why Doctors Use Stretched Length Instead
If you’ve had a measurement taken at a urology appointment, the doctor likely measured stretched flaccid length rather than erect length. The technique involves gently stretching the flaccid penis to its maximum length and measuring from the pubic bone to the tip. Research shows this produces results nearly identical to erect length in most men, and it’s considered the clinical gold standard because it doesn’t require an erection in a medical setting. It’s not a perfect substitute in every case, but it’s close enough that urologists rely on it for diagnostic decisions.
How Your Numbers Compare to Averages
A 2023 systematic review and meta-analysis published in the World Journal of Men’s Health, pooling data from studies across multiple countries, found the average erect length to be 13.93 cm, which is about 5.5 inches. The average stretched flaccid length was 12.93 cm (roughly 5.1 inches), and the average resting flaccid length was 8.70 cm (about 3.4 inches). For girth, the most commonly cited average from large-scale studies is approximately 4.5 to 4.7 inches at mid-shaft, though this varies somewhat by study population.
Keep in mind that a single measurement is just a snapshot. Erection quality fluctuates with arousal level, stress, temperature, time of day, and overall health. If you want a reliable baseline, measure on three or four separate occasions under similar conditions and average the results. That gives you a much more accurate picture than any single reading.
Picking the Right Tool
A rigid ruler works well for length because it stays straight and presses easily into the pubic bone. A flexible measuring tape is better for girth and for measuring a curved shaft. Tailoring tape from a sewing kit is ideal. Avoid construction-style retractable tape measures, which are too stiff and wide to wrap accurately around the shaft. If all you have is a piece of string and a ruler, that combination works perfectly fine for both measurements.