The normal amount of residual urine in men is no more than 20 ml. The urine that remains in the bladder after urination is called residual urine. The normal value of residual urine for men is no more than 20 ml. Exceeding the normal value of residual urine indicates that the bladder function is impaired, and the residual urine stored in the bladder is prone to bacterial growth and infection. It can be measured by direct or indirect methods. 1. Direct measurement method, that is, through the catheterization method to determine the residual urine. According to the catheterization routine, after urination, insert a catheter into the bladder through the urethra, and draw out the urine into a graduated measuring cup or a graduated urinal, and the urine drawn out is the residual urine. 2. Indirect measurement, i.e., measurement by imaging such as ultrasound, CT and intravenous pyelography, etc. Indirect measurement is non-invasive, avoids the infection that may be caused by catheterization and is easy to repeat many times. If the residual urine in the bladder exceeds 20ml by measurement, it is recommended to consult a doctor in time and actively treat the disease to avoid delaying the treatment.