Scientific Studies on Average Penis Size: What Research Shows

Scientific studies on average penis size report an average erect length of approximately 13–14 cm (5.1–5.5 inches) in several major research reviews. However, estimates vary according to the studies included, how measurements were taken, and which participants were studied. A widely cited 2015 systematic review reported a mean erect length of 13.12 cm, while a 2023 systematic review and meta analysis reported 13.93 cm. Neither figure should be treated as a universal measurement for every population or individual.

What Do Scientific Studies Say About Average Penis Size?

Research provides useful reference values for understanding average penis length, erect measurements, and circumference. The strongest evidence comes from studies that use clearly described measurement methods and from systematic reviews that combine findings from multiple studies.

Two important reviews illustrate the range of published estimates:

• A 2015 review by Veale and colleagues combined professionally measured data to construct reference charts for flaccid length, stretched length, erect length, and circumference.
• A 2023 review by Belladelli and colleagues examined 75 studies involving 55,761 men and calculated pooled estimates for flaccid, stretched, and erect length.

Their reported erect length means were 13.12 cm and 13.93 cm, respectively. The difference does not automatically mean that one study is wrong. Each review used its own set of eligible studies, samples, and statistical methods.

The main point is that scientific research provides reference estimates, not one exact figure that describes every person.

Major Scientific Studies on Average Penis Size

The following reviews help explain how researchers have measured penis size and why different publications can report different averages.

Sources: Veale et al. (2015), Belladelli et al. (2023), and Mostafaei et al. (2025).

These figures are useful when read in context. They should not be combined into a single average without considering each review’s methodology and included evidence.

Understanding Average Penis Length, Stretched Length, and Erect Length

One reason scientific studies can appear to disagree is that they do not always measure the same state.

Flaccid length

Flaccid length is measured when the penis is not erect. This measurement can vary with conditions such as temperature and physiological state, so it does not directly predict erect length for every individual.

Stretched length

Stretched length is measured by extending the flaccid penis according to a defined protocol. Some studies use it as a clinical reference measure, but it is not identical to erect length.

Erect length

Erect length is measured during an erection. The method used to obtain the erection and conduct the measurement can differ between studies, which makes methodology important when comparing results.

The 2015 Veale review reported the following results:

The sample sizes differ because not every participant in the underlying research contributed data for every measurement. In particular, the review included fewer erect length measurements than flaccid or stretched length measurements.

Why this distinction matters: A number described as an average penis length is incomplete unless the measurement state is clear. Comparisons should specify whether the value refers to flaccid, stretched, or erect length.

For more information on measurement states, see erect vs. flaccid measurement.

What Research Says About Penis Girth

Length and circumference are separate measurements. Circumference, often referred to as girth, describes the distance around the penis at the measurement location and state specified by the study.

In the 2015 Veale review, mean circumference was reported as:

The 2025 systematic review and meta analysis reported a pooled mean erect circumference of 11.91 cm among a measurement specific sample of 5,168 participants.

These estimates should be interpreted as findings from different research collections rather than exact competing measurements of the same group. The population, inclusion criteria, measurement protocols, and statistical approach may affect the reported results.

This page summarizes the broader scientific evidence. Readers interested specifically in circumference methods and girth measurements should consult the site’s related measurement resources, including accurate penis measurement.

How Researchers Measure Penis Size

A scientific result is only as interpretable as the method used to produce it. Researchers may differ in the measuring instrument, participant position, anatomical reference points, measurement state, examination conditions, and who performs the measurement.

A 2021 systematic review by Cakir and colleagues examined the methodology used in 70 studies of penile length. It found substantial variation in measurement techniques and reporting practices, supporting the need for clearer and more standardized protocols.

Important methodological considerations include:

• Measurement state: Whether the penis is flaccid, stretched, or erect.
• Measurement technique: The anatomical landmarks and procedure used to determine length or circumference.
• Instrument: The ruler or other measurement tool used.
• Examiner: Whether measurements are performed by a healthcare professional or recorded by the participant.
• Conditions: Factors such as temperature and the conditions under which the measurement is obtained.
• Participant selection: The characteristics of the people included and the reasons they participated.

The review highlighted the importance of precise, standardized procedures for producing comparable clinical and research measurements.

For practical guidance, see how to measure penis correctly. You may also find common penis measurement mistakes useful when interpreting self measured results.

Source: Cakir et al. (2021), Penile Length Measurement: Methodological Challenges and Recommendations, a Systematic Review.

Why Do Scientific Studies Report Different Averages?

Different published estimates do not necessarily indicate that research is unreliable. They may reflect differences in the evidence each study includes.

1. Different participant samples

A review may include men assessed in clinical settings, volunteers, or populations recruited for particular medical reasons. Results from these groups may not represent every population equally.

2. Different measurement protocols

Differences in measurement state, technique, instruments, and examination conditions can affect the resulting statistics. Researchers may also report different types of measurements.

3. Different inclusion criteria

One review may exclude certain clinical conditions or types of measurement, while another may use a different set of eligibility requirements. The studies included therefore influence the pooled estimate.

4. Different statistical methods

A review may calculate a weighted mean, use a pooled estimate from a meta analysis, or report a confidence interval to describe uncertainty. These values do not have identical meanings.

5. Geographic and population variation

Some reviews have reported differences between research populations or regions. Such findings require careful interpretation because the included studies may differ in their samples and methodologies. They should not be treated as definitive rankings of individuals or as precise estimates for every country.

The 2023 review also reported an increase in measured erect length across studies published over several decades. That finding is an observed trend across the available research, not proof of a specific cause or evidence that an individual’s measurements will change in the same way. The authors identified methodological and population related limitations that matter when interpreting the result.

How to Interpret Average Penis Size Statistics

Scientific studies often report several kinds of statistics. Understanding the distinction helps prevent a common mistake: treating an average as a boundary between normal and abnormal.

• Mean: The arithmetic average of the measurements included in a sample.
• Standard deviation: A statistical description of how dispersed measurements are around the mean.
• Confidence interval: A range that expresses uncertainty around an estimate under the statistical method used.
• Percentile: A position within a distribution, indicating how a measurement compares with the reference population represented by that distribution.

A mean describes a central value; it does not tell you the full range of measurements. Similarly, a confidence interval around a pooled mean is not a range that contains a specified proportion of individual measurements.

The 2015 Veale review used its pooled means and standard deviations to construct nomograms, including simulated observations from a normal distribution. Those reference charts should be understood in light of the underlying data and the method used to create them.

For a separate explanation of distributions and relative position, visit the Penis Size Percentile guide.

Does an Average Define Normal Erect Penis Size?

No single mean can determine whether a particular individual’s measurement is medically normal. Research averages describe groups studied under particular conditions; they are not, by themselves, diagnostic thresholds.

It is also important to separate a statistical comparison from a health assessment. A measurement above or below a reported mean does not automatically establish a medical problem. Anyone concerned about a physical change, pain, function, or another health issue should seek appropriate clinical advice rather than rely on a population average alone.

Scientific studies are most useful when they provide context for understanding measurements without turning statistical findings into rigid standards for individuals.

How to Use Research Findings Responsibly

When reading an article or a chart about average penis size, check the following:

1. Identify the measurement: Confirm whether the result concerns flaccid length, stretched length, erect length, or circumference.
2. Check the source: Prefer peer reviewed studies and systematic reviews that describe their methods.
3. Inspect the sample size: Make sure the sample corresponds to the particular measurement being discussed.
4. Read the limitations: Note differences in participant selection, measurement protocols, and statistical methods.
5. Avoid overgeneralization: Do not assume that a pooled research estimate applies identically to every individual or population.

This scientific review focuses on broad averages and research methodology. Dedicated country, age, height, and girth articles should address those narrower comparisons rather than having their entire scope repeated here.

If you want to understand how a measurement relates to a reference distribution, use the Penis Size Percentile guide. For an interactive option, the Penis Size Calculator is a separate resource; its result should be interpreted according to the tool’s stated methodology.

Conclusion

Scientific studies on average penis size provide useful reference points, but the findings depend on which participants were measured, how the measurements were collected, and how researchers combined the data. Major reviews report average erect lengths around 13–14 cm, yet their estimates should not be treated as interchangeable or universally applicable.

A careful interpretation considers the measurement type, sample size, study method, and statistical uncertainty. These details are more informative than relying on one number without context.

References

1. Veale, D. et al. (2015). Am I normal? A systematic review and construction of nomograms for flaccid and erect penis length and circumference in up to 15,521 men. BJU International.
2. Belladelli, F. et al. (2023). Worldwide Temporal Trends in Penile Length: A Systematic Review and Meta-Analysis. World Journal of Men’s Health.
3. Mostafaei, H. et al. (2025). A Systematic Review and Meta-Analysis of Penis Length and Circumference According to WHO Regions: Who has the Biggest One?. Urology Research and Practice.
4. Cakir, O. O. et al. (2021). Penile Length Measurement: Methodological Challenges and Recommendations, a Systematic Review. The Journal of Sexual Medicine.

Frequently Asked Questions

1. What is the average erect penis length reported in scientific studies?

Major reviews have reported mean erect lengths of 13.12 cm in a 2015 systematic review and 13.93 cm in a 2023 meta analysis. The figures differ because the reviews included different research and methods, so neither should be treated as a universal value.

2. What did the 2015 systematic review find?

The review by Veale and colleagues reported a mean erect length of 13.12 cm and a mean erect circumference of 11.66 cm. The measurement specific samples were 692 for erect length and 381 for erect circumference.

3. Why do studies report different average penis sizes?

Studies may differ in participant selection, measurement protocols, examination conditions, instruments, and statistical methods. A systematic review can also include a different collection of studies from another review.

4. Is average penis length the same as average erect penis size?

Not necessarily. “Penis length” can refer to flaccid, stretched, or erect measurements. A study must be checked to see which measurement state it reports.

5. What is the difference between stretched and erect length?

Stretched length is measured by extending the flaccid penis according to a specific protocol. Erect length is measured during an erection. Although stretched length is used in some clinical contexts, the two measurements are not identical.

6. Do scientific studies measure penis girth as well as length?

Yes. Some studies measure circumference, which is commonly called girth, in addition to length. However, fewer participants may have circumference data, and the measurement state must be identified before comparing results.

7. What does the standard deviation mean in penis size research?

Standard deviation describes the spread of measurements around the mean in the sample. It is not the same as a confidence interval and does not, on its own, define a medically normal range.

8. Are professionally measured results more useful than self reported measurements?

Professional measurement using a standardized protocol can improve consistency and comparability. However, the quality of the study also depends on participant selection, the method used, and the way the data are analyzed.

9. Does an average define normal erect penis size?

No. An average is a statistical summary of a particular sample, not a universal medical threshold. A measurement above or below the average does not by itself establish whether someone has a health problem.

10. How can I compare my measurement with scientific reference data?

First, make sure your measurement method and state match the reference data you are using. You can then consult the Penis Size Percentile guide to understand distributions. Any result should be interpreted with the limitations of its reference data in mind.

Written by: SooperTools Editorial Team

Last updated: October 24, 2026