Abstract
Most men believe that the average length of an erect penis is greater than 6 inches (15.24 cm). This belief is due, in part, to several often-cited studies that relied on self-reported measurements, with means of about 6.2 inches (15.75 cm) for heterosexual men and even greater for gay men. These studies suffered from both volunteer bias and social desirability bias. In this review, the combined mean for 10 studies in which researchers took measurements of erect penises was 5.36 inches (13.61 cm; n = 1,629). For 21 studies in which researchers measured stretched penises, the mean was approximately 5.11 inches (12.98 cm; n = 13,719). Based on these studies, the average length of an erect penis is between 5.1 and 5.5 inches (12.95–13.97 cm), but after taking volunteer bias into account, it is probably toward the lower end of this range. Studies show that a majority of men wish they were larger, with some choosing penile lengthening surgery. These surgeries are considered by the American Urological Association to be risky. Most men seeking surgery have normal sized penises. Counseling with factual information about penis size might be effective in alleviating concerns for the majority of men who worry about having a small penis.
Studies find that many men have concerns that their penis is not large enough and that they are smaller relative to other men (Johnston, McLellan, & McKinlay, 2014; Lee, 1996; Lever, Frederick, & Peplau, 2006; Morrison, Bearden, Ellis, & Harriman, 2005; Tiggemann, Martins, & Churchett, 2008). They equate penis size with sexual competence and masculinity (Morrison et al., 2005; Tiggemann et al., 2008; Wylie & Eardley, 2007). As a result, 45–68.3% of men wish they had a larger penis (Lever et al., 2006; Tiggemann et al., 2008). Most men believe that the average erect penis is over 6 inches (15.24 cm) in length, and for many their ideal penis length is considerably longer than that (Johnston et al., 2014).
This paper reviews all known studies of measurements of erect or stretched penis length. The review includes 10 studies that relied on self-reported measurements, 11 studies in which researchers measured erect penises, and 22 studies in which researchers measured stretched penises (see Table 1). Only studies of abnormalities of the penis, or of children, were excluded. The purpose of not excluding other studies is two-fold: (1) to point out the methodological flaws in many studies that contributed to men’s false beliefs that the average-size erect penis is 6+ inches (15.24+ cm) in length, and (2) based on better conducted studies, to estimate within a small range of values the actual mean length of an erect penis. By including flawed studies, therapists may better address false beliefs by clients based on those studies. The primary sources for the review were Medline and Social Sciences with Full Text (1975 to present), using “penis” as the key search word.
Conclusion
With one exception (Herbenick et al., 2014), the studies of erect penis length that relied on self-reported measurements had serious flaws, most notably volunteer bias and social desirability bias. Some of these studies have no doubt contributed to men’s insecurities about penis size and should be dismissed and ignored. In 22 studies, researchers measured stretched penises, thus eliminating social desirability bias, but this technique tends to under-report erect penis size (e.g., Habous et al., 2015; Şengezer et al., 2002). In 11 studies, researchers measured erect penises. However, one used an unconventional technique (Sparling, 1997) and this and another study had obvious volunteer bias (Ansell Research, 2001; Sparling, 1997). Based on the other nine studies, the actual average length of an erect penis is probably between 5.1 and 5.5 inches (12.95 to 13.97 cm).
Recall that among the studies in which researchers measured erect penis length, one of the largest reported means was 5.71 inches (14.5 cm) (da Ros et al., 1994). In that study, only 12% of men had an erect penis longer than 6.3 inches (16.0 cm). In the study in which men self-reported erect penis lengths in order to receive correctly-sized condoms (but there still may have been some over-reporting due to social desirability), only 17% of men had penises that were longer than 6.3 inches (16.0 cm) (Herbenick et al., 2014). Mean erect penis lengths in these two studies were at the high end of the many studies in which measurements could be trusted as accurate. Thus, it is likely that even fewer than 12–17% of men have a penis that measures greater than 6.3 (16.0 cm) inches when erect.
In a previous review of studies of penis size, Veale et al. (2015) cautioned that because many of these studies relied on volunteers there is still the possibility of volunteer bias. That is, men with larger penises might have been more likely to volunteer to be measured than men with smaller penises. If true, the estimated average erect penis length of 5.1 to 5.5 inches (12.95 to 13.97 cm) is likely to be toward the lower end of this range.
For a review of flaccid penis length and circumference, see Veale et al. (2015).