The clinical standard for measuring penis length is the bone-pressed method, taken along the top of the shaft while pressing a rigid ruler firmly against the pubic bone. This technique is used in virtually every peer-reviewed study because it eliminates variation caused by body fat and gives the most consistent, comparable result. Here’s exactly how to do it, along with how to measure girth and what the numbers mean.
The Bone-Pressed Method
The layer of fat above the base of the penis (called the suprapubic fat pad) varies in thickness from person to person and changes with weight fluctuations. Measuring from the skin surface gives an inconsistent number. Pressing through that fat pad to the bone underneath gives you your actual shaft length, which is why researchers and urologists use this approach.
To measure bone-pressed erect length (BPEL):
- Get fully erect. Partial erections will give a shorter reading.
- Use a rigid ruler, not a tape measure. A flexible tape can follow curves and overestimate length.
- Place the ruler along the top of the penis (the side facing your abdomen when erect). This surface is called the dorsal side, and it’s the standard measurement surface in clinical studies.
- Hold the ruler perpendicular to your body and press the end firmly into the pubic bone, compressing the fat pad completely.
- Read the measurement at the tip of the glans (the head of the penis).
Measuring along the top rather than the bottom matters because the underside includes the curvature where the shaft meets the scrotum, which adds length that isn’t part of the shaft itself. If your penis curves noticeably to one side, measuring along the top still gives the most standardized result, though some men with significant curvature use a flexible tape along the outer curve and note it separately.
The Stretched Length Alternative
If measuring while erect isn’t practical, stretched penis length (SPL) is a widely accepted alternative. It correlates closely with erect length and is the method urologists use in clinical settings because it doesn’t require an erection.
To measure SPL, place one hand flat above the base of the penis to hold back the fat pad. With the other hand, grip the head of the penis between your thumb and forefinger and gently pull it straight outward, away from your body, until it’s fully extended but not uncomfortable. Measure from the pubic bone to the tip of the glans, just as you would with the erect method. Manchester University NHS Foundation Trust uses this technique to help patients estimate their functional length before implant surgery, which speaks to how reliable it is as a proxy for erect size.
How to Measure Girth
Girth is the circumference of the shaft, measured with a flexible tape measure or a strip of string that you then hold against a ruler. Wrap it around the thickest part of the shaft while erect. For most men this is somewhere around the mid-shaft, but the thickest point can be closer to the base or closer to the head. If the thickness varies noticeably along the shaft, measure at the widest point for the most useful number.
Make the tape snug but not tight enough to compress the tissue. You’re measuring the outside surface, not squeezing into it.
Getting a Consistent Reading
A few practical details make the difference between a reliable measurement and one that shifts every time you check:
- Measure more than once. Take three measurements on different days and average them. Erection quality varies with arousal level, time of day, temperature, and blood flow.
- Stand upright. Sitting or lying down can change the angle and compress tissue differently.
- Don’t measure immediately after exercise or cold exposure. Both temporarily affect blood flow and can reduce erect size slightly.
- Press firmly to the bone. The most common source of inconsistency is varying how much pressure you apply against the fat pad. Firm, consistent pressure each time is the goal.
What the Numbers Mean
A 2015 meta-analysis published in BJU International, drawing on measurements from over 15,500 men, found that the average erect penis length is 13.12 cm (about 5.16 inches). The average flaccid length is 9.16 cm (3.6 inches). These are bone-pressed measurements, so if you’re comparing your result to these averages, make sure you used the same technique.
There’s a wide range of normal. Most men fall within roughly an inch above or below the average. A micropenis, the clinical term for a penis significantly smaller than typical, is diagnosed only when the stretched length falls more than 2.5 standard deviations below the mean. For adults, that threshold is about 6.9 cm (2.67 inches) in stretched length. This is a specific medical diagnosis, not a casual label, and it applies to a very small percentage of men.
Why Body Weight Affects Visible Length
The fat pad above the base of the penis can hide a meaningful amount of shaft length. This is why the bone-pressed technique exists in the first place: it measures the full shaft regardless of how much fat sits on top. In men with significant weight in the lower abdomen, the visible portion of the penis can be substantially shorter than the actual shaft length underneath.
This also means that weight loss can increase visible (non-bone-pressed) length without any change to the penis itself. The shaft doesn’t grow, but more of it becomes exposed as the fat pad thins. It’s one of the most common reasons men notice a difference in apparent size over time, in either direction.