Penis size is measured using two key dimensions: length and circumference (girth). Length is taken along the top of the penis from the pubic bone to the tip, while girth is measured around the thickest part of the shaft. Both can be measured in either the flaccid or erect state, though erect measurements are the standard for comparison. Getting an accurate number depends on technique, and small details like where you place the ruler and how much you press into the skin make a real difference.
How Length Is Measured
The standard clinical method is called bone-pressed penile length. You place a rigid ruler along the top (dorsal) surface of the penis, pressing it firmly into the pubic bone at the base. The measurement runs from that point to the very tip of the glans. Pressing into the pubic bone is the critical step because it accounts for the fat pad that sits over the bone and can obscure a significant amount of length, especially in men who carry extra weight around the midsection.
Measuring from the top, not the underside or sides, is important for consistency. The underside includes extra tissue where the shaft meets the scrotum, which inflates the number. Clinical measurements in research settings are taken by trained professionals using a millimeter-graduated ruler, with the subject standing upright. For self-measurement at home, a rigid ruler works better than a flexible tape measure, since tape can follow curves and add length that isn’t really there.
Stretched Length as an Alternative
In clinical settings, doctors often measure stretched penile length instead of erect length. This involves gently pulling the flaccid penis to its maximum extension and measuring the same way: ruler on top, pressed against the pubic bone. It’s a practical substitute because achieving a full erection in a doctor’s office isn’t always feasible.
Research supports this approach. A prospective study of Vietnamese men found a positive correlation (r = 0.73) between stretched length and erect length, confirming that a stretched measurement is a reasonable proxy. The two numbers won’t be identical, but they track closely enough for clinical purposes like evaluating anatomy before surgical procedures or diagnosing conditions related to penile length.
How Girth Is Measured
Circumference requires a flexible measuring tape, not a ruler. Wrap the tape snugly around the thickest part of the shaft, which for most men sits just below the head of the penis. Read the number where the tape meets itself. If you don’t have a measuring tape, wrap a piece of non-stretchy string around the shaft, mark where the ends overlap, then lay the string flat against a ruler.
Two common mistakes throw off girth measurements. First, using a string or tape that has some stretch to it. Pulling a stretchy material tight will compress the tissue and give a smaller number. Second, measuring at an inconsistent location on the shaft. The widest point is the standard, and measuring closer to the base or mid-shaft can produce a different number.
What Affects Measurement Accuracy
Flaccid penis size is highly variable from one moment to the next in the same person. Temperature is the biggest factor. Cold air triggers the muscles around the penis to contract, pulling it closer to the body and reducing both length and girth. Clinical guidelines from urology research recommend measuring immediately after undressing to minimize the effects of room temperature, and at least one study protocol specified a controlled room temperature of 25°C (77°F).
Stress, recent physical activity, time of day, and arousal level also influence flaccid size. This is why erect measurements are far more consistent and useful for comparison. If you’re measuring while flaccid, understand that the number is a snapshot, not a fixed value.
Body composition plays a major role, too. The pubic fat pad can partially or even fully bury the visible portion of the penis without changing its actual size. This is why the bone-pressed technique exists. By pressing the ruler to the bone, you measure the true length of the penile shaft regardless of how much fat sits on top. Men with higher body fat will often see a bigger difference between their visible length and their bone-pressed length.
Self-Measurement vs. Clinical Measurement
Studies that rely on self-reported penis size consistently produce larger averages than studies where a clinician does the measuring. The reasons are straightforward: some men round up, some measure from a non-standard starting point (like the side or underside), and some don’t press firmly enough against the pubic bone. There’s also a psychological incentive to report a favorable number, even on an anonymous survey.
If you want a measurement that’s genuinely comparable to published research data, follow the clinical protocol exactly. Ruler on top, pressed to the bone, measured to the tip while fully erect. For girth, non-stretchy tape around the widest point. Doing it the same way researchers do is the only way the averages mean anything as a reference point.
How Your Numbers Compare to Averages
The largest systematic review on the topic, published in BJU International and drawing from over 15,000 men, established these averages:
- Flaccid length: 9.16 cm (3.6 inches)
- Stretched length: 13.24 cm (5.2 inches)
- Erect length: 13.12 cm (5.2 inches)
- Flaccid circumference: 9.31 cm (3.7 inches)
- Erect circumference: 11.66 cm (4.6 inches)
Notice that stretched length and erect length are nearly identical in this dataset, reinforcing the usefulness of a stretched measurement. Most men fall within about 2 cm (roughly 0.75 inches) of these averages in either direction. Flaccid size is a poor predictor of erect size. Some men experience significant growth during an erection while others start closer to their full length.
These averages come from measurements taken by healthcare professionals, not self-reports. That distinction matters. If you measure yourself correctly using the bone-pressed method and find you’re close to these numbers, you’re squarely in the normal range.