thumb|300px|A schematic of an orchidometer
An orchidometer (or orchiometer) is a medical instrument used to measure the volume of the testicles.
Instrument
right|thumb|178x178px|Tanner scale for males with corresponding orchidometer bead and volume.
The orchidometer was introduced in 1966 by Swiss pediatric endocrinologist Andrea Prader of the University of Zurich. It consists of a string of twelve numbered wooden or plastic beads of increasing size from about 1 to 25 millilitres. Doctors sometimes informally refer to them as "Prader's balls", "the medical worry beads", or the "endocrine rosary".
The beads are compared with the testicles of the patient, and the volume is read off the bead which matches most closely in size. Prepubertal sizes are 1–3 ml, pubertal sizes are considered 4 ml and up and adult sizes are 15-25 ml.