What is the average testicle size when measured by ultrasound?

Will vary. Population based studies have found adult 95% of testes to be 4-8cm in length, & 18-25ml in volume. Ultrasound (us) & orchidometry studies estimate testis volume based on calculations & each method is subject to error. If you have any noticeable change in testicular size, texture, or symmetry, get evaluated without delay.