To measure your penis accurately, you need a rigid ruler for length and a flexible tape (or a piece of string) for girth. The method that matches what researchers use in clinical studies is called the bone-pressed measurement: you press the ruler firmly against the pubic bone at the base of the penis and measure to the tip. Here’s how to do both measurements correctly and what the numbers mean.
How to Measure Length
Stand upright with a full erection. Place a rigid ruler or straight-edge measuring tool along the top of your penis, starting where the shaft meets your abdomen. Press the end of the ruler firmly against the pubic bone. This compresses the fat pad that sits above the base of your penis, giving you a consistent measurement regardless of body weight. Measure in a straight line from the pubic bone to the very tip of the glans (the head). That number is your bone-pressed erect length.
Pressing into the pubic bone matters more than you might think. Research on measurement methods has shown that discrepancies in recorded size are most common in overweight patients, precisely because the fat pad can hide a significant portion of the shaft. Without pressing firmly, two people with the same actual penile length could get very different numbers depending on their body composition.
If your penis has a noticeable curve, measure along the top surface following the curve rather than trying to force a straight line. A rigid ruler won’t conform to a curve well, so a flexible measuring tape laid along the top of the shaft works better in this case.
How to Measure Girth
Girth is the circumference of the shaft. With a full erection, wrap a soft measuring tape snugly around the thickest part of the shaft, typically just below the head. Read the number where the tape meets itself. Don’t pull the tape tight enough to compress the tissue, but don’t leave it loose either. It should sit flat against the skin.
If you don’t have a flexible measuring tape, use a piece of string or a thin strip of paper. Wrap it around the thickest part, mark or pinch where it overlaps, then lay it flat against a ruler to get the measurement. This two-step method is just as accurate.
Getting a Consistent Erection
Your measurement is only as reliable as your erection. Clinicians use a four-point erection hardness scale, and you want to be at the top: completely hard and fully rigid. A partial erection, even one that feels firm, can undercount your length by a meaningful amount. If you’re not fully erect, the tissue hasn’t expanded to its maximum, and both length and girth will read shorter than your actual size.
Don’t rush the process. Measure when you’re naturally and fully aroused rather than trying to maintain an erection while fumbling with a ruler. Taking a few attempts on different occasions will give you a more reliable average than a single measurement.
Why Flaccid Measurements Aren’t Reliable
Flaccid size changes constantly. In warm environments, blood vessels dilate and smooth muscles relax, which temporarily increases blood flow to the penis and makes it appear larger. Cold temperatures do the opposite: blood flow decreases and muscles contract, making the penis noticeably smaller. Neither state reflects your actual functional size.
Clinical research confirms this variability. A large multi-observer study found that flaccid measurements were only moderately accurate in predicting erect length, with significant variation between different observers measuring the same person. If you’re measuring for any practical reason, such as choosing a condom size, the erect measurement is the one that matters.
Some clinicians use a stretched flaccid measurement as a proxy for erect length. This involves gently stretching the flaccid penis to its maximum length and measuring from the pubic bone to the tip. While roughly 60% of clinical studies use this method for convenience, it’s an approximation. Your erect measurement is the most accurate number you can get at home.
What the Average Numbers Look Like
A meta-analysis published in BJU International compiled data from 17 studies covering over 15,500 men, all measured by healthcare professionals using standardized techniques. The averages:
- Erect length: 13.12 cm (5.16 inches)
- Erect circumference: 11.66 cm (4.59 inches)
- Flaccid length: 9.16 cm (3.61 inches)
- Flaccid stretched length: 13.24 cm (5.21 inches)
These numbers represent the middle of the distribution. Most men fall within about an inch above or below the average in either direction. It’s also worth noting that the stretched flaccid length (13.24 cm) closely matched the erect length (13.12 cm) across this dataset, which supports the idea that a stretched measurement is a reasonable stand-in when an erect measurement isn’t practical.
Measuring Curvature
Some degree of curvature during an erection is normal. If you want to quantify a curve, whether out of curiosity or because you’ve noticed a change, the clinical tool for this is called a goniometer, which measures the angle of deviation. At home, the simplest approach is to photograph your erect penis from directly above (for left-right curves) or from the side (for up-down curves) against a flat background, then estimate the angle from the image.
Curves of less than about 30 degrees rarely cause functional problems. If a curve has developed recently, increased over time, or is accompanied by pain or a hard lump under the skin, that pattern is consistent with Peyronie’s disease, a condition where scar tissue forms inside the penis and pulls it in one direction. Clinicians typically evaluate this with an in-office erection and a goniometer to get a precise angle.
Common Mistakes That Skew Results
Measuring from the side or underside of the penis instead of the top will give an inaccurate number, since the top surface from base to tip is the standardized reference point used in research. Measuring without pressing into the pubic bone will undercount your length, sometimes by a centimeter or more if you carry extra weight in that area. Using a ruler for girth instead of a flexible tape introduces error because you can’t wrap a rigid tool around a curved surface.
Room temperature also plays a subtle role. If you’ve just stepped out of a cold shower, your baseline blood flow is reduced and even your erection may not reach its full potential. A comfortable, warm room gives the most consistent conditions. Taking measurements on two or three separate occasions and averaging them will smooth out any day-to-day variation in erection quality or arousal level.