To measure penis size accurately, you need a rigid ruler for length and a flexible measuring tape (or a piece of string plus a ruler) for girth. The technique matters more than the tools, though, because small differences in where you place the ruler or how aroused you are can shift your measurement by a centimeter or more. Here’s exactly how to do it.
How to Measure Length
Stand upright and hold the penis straight out, perpendicular to your body. Place a rigid ruler along the top of the shaft, pressing the end firmly against the pubic bone. This “bone-pressed” method is the same one used in clinical studies, and it gives the most consistent result because it accounts for the fat pad above the pubic bone that can hide part of the shaft. Read the measurement at the tip of the head.
A flexible measuring tape isn’t ideal for length because it can stretch slightly and follow the curve of the shaft, inflating the number. If you do have a noticeable curve, a rigid ruler held along the top still gives you the standardized measurement. If you want to know your “functional” curved length, lay a piece of non-stretchy string along the curve, mark where the tip ends, then measure the string flat against a ruler.
How to Measure Girth
Girth is the circumference, the distance around the shaft. Wrap a soft measuring tape snugly around the thickest part of the shaft, which is usually just below the head. Read the number where the tape overlaps itself. “Snug” means the tape touches the skin all the way around without compressing it.
If you don’t have a measuring tape, wrap a piece of string or a strip of paper around the shaft in the same way. Pinch or mark the spot where the end meets the wrapped portion, then lay the string flat against a ruler. Be careful with stretchy measuring tapes. Pulling too firmly can give you a reading that’s artificially large.
Conditions That Affect Your Measurement
The penis changes size constantly depending on temperature, blood flow, arousal, stress, and even time of day. Cold air causes tissues to contract noticeably, so measuring right after a cold shower or in a chilly room will give you a shorter, smaller result than your typical size. A warm, comfortable room and a relaxed state produce the most representative measurement.
For erect measurements, you need a full erection, not a partial one. Semi-erect readings can be a centimeter or more shorter than your actual erect size. If you’re anxious or distracted, it can be difficult to reach full rigidity, which is one of the reasons clinical studies often report both erect and “flaccid stretched” measurements as a workaround.
Erect vs. Flaccid: Why Both Matter
A flaccid measurement is not a reliable predictor of erect size. Research published in the Journal of Urology found that neither age nor flaccid length accurately predicted how large the penis would be when erect. In that study, the average flaccid length was 8.85 cm (about 3.5 inches) and the average erect length was 12.89 cm (about 5.1 inches), a gain of roughly 4 cm. But the range of that gain varied widely from person to person.
This is the “grower vs. shower” phenomenon. Some men see very little change between flaccid and erect states, while others nearly double in length. The study found that the best single predictor of erect length wasn’t the resting flaccid measurement but the “stretched flaccid” length, where the penis is gently pulled straight out and measured from base to tip while still soft. That stretched number tends to closely match the erect number.
What’s Average
The largest review of penis size data, published by researchers at King’s College London, combined measurements from over 15,000 men across multiple countries. 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 circumference: 9.31 cm (3.66 inches)
These are means, so roughly half of men fall above and half below each number. Most men cluster within about an inch of the average in either direction. If your measurement falls anywhere in that range, you’re statistically typical.
Common Mistakes to Avoid
Measuring from the side or underside of the shaft adds length that isn’t captured in standard measurements, making comparison to averages meaningless. Always measure along the top. Pressing the ruler into the fat pad is correct for the bone-pressed method, but pressing from the side simply shifts the starting point and inflates the number.
Another common error is measuring while partially erect and rounding up, or measuring multiple times and only keeping the largest number. Your most accurate reading comes from a single measurement at full erection in a warm, relaxed setting. If you want to confirm it, measure on two or three separate days and take the average.
Finally, comparing your result to numbers you see online outside of peer-reviewed studies is unreliable. Self-reported surveys consistently produce averages that are significantly higher than clinician-measured data, because people tend to round up or use inconsistent techniques. The King’s College London review used only measurements taken or verified by health professionals, which is why it remains the most cited reference point.