The most accurate way to measure penis length is while erect, using a rigid ruler pressed firmly against the pubic bone at the base. This method, called bone-pressed erect length, is the standard used in clinical research and gives the most consistent, reproducible result. Girth is measured separately using a flexible tape or string wrapped around the shaft.
Why Measurement Method Matters
Self-measurement sounds simple, but small differences in technique can easily add or subtract half an inch or more. Measuring along the underside instead of the top, using a flexible tape that curves with the body, or skipping the press against the pubic bone all introduce error. Clinical studies have found that even trained observers produce inconsistent results when measuring in the flaccid state, so standardizing your approach is key to getting a number you can trust.
A large multicenter study published in BJU International found that flaccid measurements are “both unreliable and observer dependent,” with clinically significant variation between different people measuring the same penis. Erect measurements are far more consistent. If you want a number that actually means something, measure while fully erect.
How to Measure Length
Use a rigid ruler, not a soft tape measure. Flexible tapes bend along the contour of your body and tend to give inflated readings. A straight, stiff ruler keeps the measurement honest.
Stand upright or stand at a slight angle where you can see clearly. Place the ruler along the top of the penis (the side facing your belly when erect) and press the end of the ruler firmly into the pubic bone. The fat pad above the base can hide a meaningful amount of length, and pressing through it is how researchers standardize across different body types. Read the measurement at the tip of the glans. That number is your bone-pressed erect length.
If your penis has a noticeable curve, a rigid ruler will still give the most standardized result for clinical comparison, though the outer edge of the curve will technically be longer than what the ruler reads. For a pronounced curve, you can use a flexible tape laid along the top surface from pubic bone to tip, following the curve. Just be aware that most published averages are based on straight-line ruler measurements.
How to Measure Girth
Girth is circumference, not diameter. Use a soft fabric measuring tape or a piece of non-stretchy string. Wrap it snugly around the thickest part of the shaft, which for most men is just below the head. Don’t pull it tight enough to compress the tissue, but don’t leave slack either.
If you’re using string, pinch the point where the end meets the wrapped portion, then lay the string flat against a ruler to read the measurement. Some men vary in thickness along the shaft. Researchers have used different locations (base, mid-shaft, just below the head), but the most commonly cited studies measure at the thickest point. If you want to compare your number to published averages, measure at the widest section.
What About Flaccid or Stretched Measurements?
Some sources suggest measuring stretched flaccid length as a stand-in for erect length. The idea is that you pull the flaccid penis to its maximum stretch and measure from pubic bone to tip. While some studies have found a rough correlation between stretched flaccid and erect length, research that has measured both simultaneously shows significant variability. Stretched length is not a reliable proxy for erect length in an individual person, even if the averages line up across large groups.
If you can measure while erect, do that. It’s the gold standard and the basis for all widely cited size data.
Getting a Consistent Result
A few practical factors affect accuracy. Arousal level matters: a partial erection will measure shorter than a full one. Room temperature can affect blood flow, so a cold room may make it harder to reach full erection. Time of day, stress, and recent physical activity can all play a role. None of these effects are dramatic, but they can shift your result by enough to be noticeable.
For the most reliable number, measure on three separate occasions when you’re fully aroused and comfortable, then take the average. Single measurements have more noise. Three readings smooths that out and gives you a number you can be confident in.
How Your Numbers Compare
The most widely cited dataset on penis size comes from a 2015 systematic review that pooled data from over 15,500 men across multiple countries. The averages from that analysis: erect length was 13.12 cm (about 5.16 inches) with a standard deviation of 1.66 cm, and erect girth was 11.66 cm (about 4.59 inches) with a standard deviation of 1.10 cm.
Standard deviation tells you how spread out the population is. About two-thirds of men fall within one standard deviation of the mean. For length, that puts most men between roughly 4.5 and 5.8 inches erect. For girth, most fall between about 4.15 and 5.0 inches. Being outside that range in either direction is normal too, it just becomes less common the further you go.
These numbers come from studies using bone-pressed measurement by clinicians, so if you follow the method above, your result is directly comparable. If you measure without pressing to the pubic bone, your number will likely be shorter than these averages, not because you’re smaller but because the technique is different.