The formula for calculating residual urine volume is to measure the liquid dark area of urine in the bladder after emptying the bladder under ultrasound guidance, measure the anterior and posterior diameters, the upper and lower diameters, and the left and right diameters, multiply these three indicators together, and then multiply by 0.52, and the calculated value is the residual urine volume. The residual urine volume may be seen in men with prostatic hyperplasia and neurogenic bladder, prostate cancer, bladder neck contracture, or obstruction at the bladder outlet. The amount of residual urine volume determines whether a catheter should be left in place or whether surgery is required, and it is an indicator to assess the effectiveness of preoperative or postoperative treatment.