Penis length is measured along the top of the shaft, from the pubic bone to the tip of the glans, using a rigid ruler pressed firmly against the body. This “bone-pressed” method is the standard used in medical research because it accounts for differences in body fat and gives the most consistent, comparable result. Girth is measured around the thickest part of the shaft with a flexible measuring tape.
The Bone-Pressed Method
The key detail that separates a reliable measurement from a rough guess is pressing the ruler against the pubic bone. Here’s how it works: you hold the penis straight out from the body (or measure while erect), place a rigid ruler on top of the shaft, and push it back until it meets the pubic bone. You then read the measurement at the tip of the glans.
Pressing into the pubic bone matters because everyone carries a different amount of fat in the area just above the penis, sometimes called the fat pad. Without pressing through that tissue, two men with identical penile anatomy could get very different numbers simply based on body composition. The bone-pressed technique removes that variable and captures what researchers call “functional length,” the full length of the penile shaft regardless of how much soft tissue sits on top of it.
Girth uses a different tool. A flexible tape or a strip of string (which you then hold against a ruler) wraps around the shaft at its widest point. Most studies measure circumference rather than diameter, since a tape conforms to the natural shape more accurately than trying to measure straight across.
Erect, Flaccid, and Stretched Measurements
Studies typically report three types of length: flaccid, stretched flaccid, and erect. Each tells you something different, and they don’t always line up the way you’d expect.
Flaccid length is the least reliable indicator of erect size. Research published in The Journal of Urology found that neither a man’s age nor his flaccid length accurately predicted how long his penis would be when erect. Some men experience significant growth from flaccid to erect, while others change relatively little.
Stretched flaccid length, on the other hand, closely correlates with erect length. To take this measurement, you gently pull the flaccid penis forward, extending it until it’s fully stretched but not uncomfortable, and measure from the pubic bone to the tip. This is the measurement most commonly used in clinical settings because it doesn’t require the patient to have an erection, which makes data collection far more practical. The correlation is strong enough that doctors use stretched length to counsel patients about surgical outcomes, including penile implants.
What the Numbers Look Like
The largest systematic review on the topic, published in BJU International in 2015, pooled data from 17 studies covering up to 15,521 men whose measurements were taken by health professionals using standardized methods. The averages were:
- Flaccid length: 9.16 cm (3.6 inches)
- Stretched flaccid 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 how close the stretched flaccid and erect lengths are. That tight match, just 0.12 cm apart in this dataset, is exactly why stretched measurements work as a stand-in for erect ones. A separate study found a similar pattern, with a mean stretched length of 12.4 cm compared to an erect length of 12.8 cm.
Why Measurements Vary So Much
One reason you’ll see different average sizes reported across studies is that there is no single universally agreed-upon protocol. A 2020 systematic review of measurement methods noted that the penis is a “dynamic organ” affected by both internal and external factors. Room temperature, anxiety, time of day, and degree of arousal all influence size at the moment of measurement. Two readings on the same person taken hours apart can differ noticeably.
Measurement technique also introduces variability. Whether a researcher uses a rigid ruler or a flexible tape, measures from the top of the shaft or the side, and whether they press to the bone or measure from the skin surface all shift the final number. Studies that rely on self-reported measurements tend to produce higher averages than those where a clinician does the measuring, which is why researcher-collected data is considered more trustworthy.
How Body Weight Affects Visible Length
Weight gain deposits fat in the area above the penis, known as the suprapubic fat pad. As this pad grows, it can partially or fully cover the base of the shaft, making the penis appear significantly shorter even though the underlying anatomy hasn’t changed. In clinical literature, this is described as a “buried” or “concealed” penis, where the shaft is normal length but hidden beneath skin and fat. The condition can limit the usable length during sex and cause psychological distress.
This is another reason the bone-pressed measurement exists. It captures the true length of the shaft beneath the fat pad. If you measure without pressing, you’re measuring visible length, which changes with weight fluctuations. For someone tracking their own size consistently, using the bone-pressed technique eliminates that noise. And for men whose visible length has decreased due to weight gain, losing weight can partially or fully reverse the concealment by reducing the fat pad.
Getting a Consistent Measurement at Home
If you’re measuring yourself, a few details will help you get a number that’s comparable to what clinical studies report. Use a rigid ruler rather than a flexible tape for length, since a tape can follow the curve of the shaft and overestimate. Place the ruler on the top of the penis (the side facing your abdomen), not the underside, and press it firmly to the pubic bone. Measure to the very tip of the glans.
For girth, switch to a flexible tape or a piece of string. Wrap it around the thickest part of the shaft and mark where it overlaps, then measure the string against a ruler. Take your measurement at roughly the same time of day and at the same level of arousal each time. Temperature, stress, and even recent physical activity can cause enough variation to make comparisons unreliable if conditions aren’t somewhat consistent.