To measure penis size accurately, you need a ruler or flexible measuring tape, a full erection, and a consistent technique called the “bone-pressed” method. This is the same approach used in clinical studies, and it gives the most reliable, repeatable result. Here’s exactly how to do it, what tools work best, and how your numbers compare to published averages.
How to Measure Length
Start with a full erection. Place a rigid ruler or measuring tape along the top of the penis (the side facing your belly when you stand). Position the end of the ruler at the base where the shaft meets your body, right against the pubic bone. Press the ruler firmly into the pubic bone, pushing past any fat pad or pubic hair. Measure to the tip of the glans (the head). This is called bone-pressed erect length, and it’s the standard used in virtually all clinical research.
Pressing into the pubic bone matters more than you might think. The fat pad above the base of the penis can be half an inch thick or more, especially with weight gain, and it obscures the true starting point. Skipping this step is one of the most common reasons self-measurements come up short compared to clinical ones. If the foreskin is intact, retract it before measuring so you’re capturing shaft and glans only.
Measure along the top surface, not the underside or along a curve. If your erection has a noticeable curve, a flexible tailor’s tape held snug against the top surface will follow the contour more accurately than a rigid ruler.
How to Measure Girth
Girth is the circumference, the distance around the shaft. Use a flexible measuring tape (the kind used for sewing) and wrap it around the thickest part of the erect shaft. Pull it snug but not tight enough to compress the tissue. Read the number where the tape overlaps itself.
If you don’t have a flexible tape, wrap a strip of paper or a piece of string around the shaft, mark where it meets, then lay it flat against a ruler. Some men find the thickest point is mid-shaft, others near the base. Measure at the widest point for the most useful number, particularly if you’re sizing for condoms.
Factors That Affect Your Measurement
Temperature is the biggest variable. Cold air causes significant shrinkage in the flaccid state and can reduce erect size slightly. Clinicians take measurements immediately after patients undress to minimize the effect of room temperature. For consistency, measure in a comfortably warm room.
Body weight changes your visible length. A thicker pubic fat pad buries the base of the penis, reducing what’s externally visible. This is why the bone-pressed method exists: it measures the full anatomical length regardless of body composition. Losing weight won’t make the penis longer, but it will make more of it visible and accessible.
Arousal level matters too. A partial erection will give you a shorter reading. Wait until you’re fully erect. If you lose your erection during the process, start over rather than estimating. Erection quality can also vary by time of day, stress, and hydration.
Age plays a role over the long term. Penile extensibility decreases with age as the connective tissue surrounding the erectile chambers loses elasticity. This is gradual and typically not noticeable from one year to the next.
The Stretched Flaccid Method
If getting a full erection during measurement isn’t practical, there’s an alternative clinicians use regularly: stretched flaccid length. This is the most common metric in urology guidelines. Hold the tip of the flaccid penis (with foreskin retracted if applicable) and stretch it outward to its maximum comfortable extension. Measure from the pubic bone to the tip of the glans, pressing into the fat pad just as you would for an erect measurement.
Stretched flaccid length closely approximates erect length for most men. A large meta-analysis 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), nearly identical numbers across thousands of measurements.
How Your Numbers Compare
The largest systematic review on the topic, published in BJU International, compiled data from over 15,500 men. The averages:
- Erect length: 13.12 cm (5.16 inches), with a standard deviation of 1.66 cm
- Erect girth: 11.66 cm (4.59 inches), with a standard deviation of 1.10 cm
- Flaccid length: 9.16 cm (3.61 inches)
- Flaccid girth: 9.31 cm (3.67 inches)
Standard deviation tells you how spread out the data is. About 68% of men fall within one standard deviation of the mean. For erect length, that means roughly two-thirds of men measure between 11.46 cm (4.51 inches) and 14.78 cm (5.82 inches). Falling outside that range in either direction is completely normal, just less common.
Common Measurement Mistakes
Measuring from the side or underside of the penis adds length because the distance from base to tip is longer along those surfaces. Clinical measurements are always taken from the top (dorsal) surface. Measuring from the underside can add a centimeter or more, which is why it gives results that don’t match published data.
Not pressing into the pubic bone is the other frequent error. Studies have specifically noted that failing to depress the fat pad leads to inconsistent, shorter readings. The bone is your fixed anatomical landmark. Without it, your measurement changes every time your weight fluctuates.
Self-reported measurements also tend to run higher than clinician-measured ones. Research has documented significant variability between the two, likely a combination of optimistic rounding, inconsistent technique, and measuring at a slight angle. If you follow the bone-pressed method described above, your number should be close to what a clinician would record.