The standard method used in clinical research is called the bone-pressed measurement: press a rigid ruler against the pubic bone at the base of the penis and measure along the top (dorsal) surface to the tip of the head. This technique accounts for the fat pad above the pubic bone and gives the most consistent, comparable result. Here’s how to do it correctly for both length and girth, along with the details that affect accuracy.
How to Measure Length
You’ll get the most accurate length measurement while fully erect. Stand upright, hold the penis straight out perpendicular to your body, and place a rigid ruler along the top surface. Press the end of the ruler firmly into the pubic bone, pushing past any fat or pubic hair. Measure in a straight line from that point to the very tip of the head. This is the bone-pressed erect length, and it’s the measurement used in virtually all major studies on penis size.
A rigid ruler works better than a flexible tape for length because soft measuring tapes can stretch slightly under tension, giving you a reading that’s a bit too generous. The one exception is if your penis has a noticeable curve, either naturally or from a condition like Peyronie’s disease. In that case, a flexible tape placed along the curve of the top surface will capture the actual length more accurately than a straight ruler that ignores the arc.
How to Measure Girth
Girth is the circumference, and you need a soft measuring tape for this. While fully erect, wrap the tape around the thickest part of the shaft. For most people that’s at the midpoint, but anatomy varies. Don’t pull the tape tight enough to compress the tissue, just snug enough to sit flat against the skin. If you don’t have a measuring tape, wrap a piece of string around the shaft, mark where it overlaps, then lay the string flat against a ruler.
The Stretched Flaccid Method
If measuring while erect isn’t practical, there’s an alternative that clinicians frequently use: the stretched flaccid length. Stand upright, grasp the head of the flaccid penis, and stretch it outward as far as it comfortably goes. Place a ruler along the top surface, pressed against the pubic bone, and measure to the tip. Meta-analyses have found that stretched flaccid length produces results nearly identical to erect length, which is why researchers consider it the gold standard for clinical settings where achieving a full erection would be awkward or unreliable.
That said, stretched length isn’t a perfect substitute. Some men find their erect length exceeds their stretched measurement by a small margin, and the reverse can also happen. It’s a strong approximation, not an exact match.
Factors That Affect Accuracy
Several things can throw off your measurement if you’re not aware of them. Room temperature matters: cold causes the blood vessels in the penis to constrict, which reduces flaccid size and can make it harder to achieve a full erection. Measure in a comfortably warm room. Anxiety and adrenaline have a similar constricting effect, so being stressed or rushed can give you a smaller reading. Recent ejaculation can also affect the firmness of your erection.
The biggest source of error, though, is simply how the measurement is taken. A 2025 study published in Sexual Medicine found that 73% of men overestimated their erect length when self-reporting, by an average of nearly 1 centimeter compared to clinical measurements. Most of that gap comes from not pressing the ruler into the pubic bone, measuring from the side or underside instead of the top, or measuring without a full erection. Sticking to the bone-pressed dorsal method eliminates most of these discrepancies.
How Your Measurement Compares
The largest meta-analysis on penis size, published in BJU International, compiled data from over 15,500 men. The averages it found: erect length of 13.12 cm (about 5.2 inches), erect girth of 11.66 cm (about 4.6 inches), and flaccid length of 9.16 cm (about 3.6 inches). These measurements were taken by clinicians, so they reflect bone-pressed technique without self-reporting bias.
A more recent analysis from Stanford Medicine, drawing on 75 studies and nearly 56,000 men measured between 1942 and 2021, found that average erect length has increased by about 24% over the past three decades, from roughly 4.8 inches to 6 inches. The researchers noted this trend appeared globally, though the reasons remain unclear. If you’re comparing yourself to averages, keep in mind that newer studies tend to report slightly longer figures than older ones.
Common Mistakes to Avoid
- Measuring from the side or underside. The top (dorsal) surface, from pubic bone to tip, is the standard. The underside is longer due to anatomy and will give an inflated number that doesn’t match any clinical reference point.
- Skipping the bone press. Without pressing into the pubic bone, your measurement is affected by body fat. Two men with identical penile tissue can get different readings depending on their weight. Bone-pressed measurement removes that variable.
- Pulling a flexible tape too tight for length. Soft measuring tapes can stretch, adding a few millimeters. Use a rigid ruler for length and save the tape for girth.
- Measuring when not fully erect. Even 80% erection can cost you several millimeters in both length and girth. If you can’t achieve a full erection, use the stretched flaccid method instead.
- Taking a single measurement. Erections vary in firmness from day to day. Measure on three separate occasions and average the results for the most reliable number.