A micropenis is defined by a stretched penile length (SPL) significantly smaller than average for age, specifically at least 2.5 standard deviations below the mean.
- In adult males, a micropenis is typically defined as a stretched penile length of 3.67 inches (9.3 cm) or less
- Some sources also note that an erect penis shorter than 7 cm (about 2.75 inches) or a flaccid penis less than 4 cm (about 1.57 inches) can be considered a micropenis
- For newborns, a micropenis is defined as a stretched penile length less than 0.75 inches (1.9 cm)
In summary, an adult micropenis is generally a stretched length of 3.67 inches (9.3 cm) or less, which is markedly smaller than the average adult stretched penile length of about 5.25 inches (13.3 cm)
