The normal size of the penis is (13.0 ± 1.3) centimeters when erect and (7.1 ± 1.5) centimeters when weak. Penis size is measured by holding the head of the penis as handily as possible and straightening it so that the length of the penis is comparable to the length of the penis when it is fully erect, and then measuring the length from the tip of the head to the pubic symphysis, i.e., the size of the penis, with a ruler. Penis size is expressed as the length and circumference of the penis when it is flaccid and when it is erect. The normal penis length for a full-term newborn is (3.5 ± 0.7) centimeters and diameter (1.1 ± 0.2) centimeters. The average length in adult foreign males is (13.3±1.6) centimeters. Domestic adult male penis length at erection was (13.0±1.3) cm with a circumference of (7.8±0.7) cm. When weak, the length is (7.1±1.5) centimeters and the circumference is (12.2±1.2) centimeters. If the penis size is abnormal, you should go to the hospital to find out the cause and standardize the treatment.