Penis 12cm belongs to the normal range, the male penis after adulthood usually has a weak length and erect length, the normal length of a weak penis is generally the average Chinese in 4-6cm, the penis is fully erect, its length for Chinese men is generally between 11-16cm, some men and penis length can reach more than 20cm. So the penis is in the normal range if it is 12cm when fully erect. Common penis short generally refers to the penis is less than 3cm when weak, erect penis within 10cm, penis short is usually related to testicular atrophy, testicular hypoplasia, etc., if it seriously affects the quality of male sexual life, you can seek treatment.