Is your testicle size normal? Here is what the data says.

Somewhat, because the seminiferous tubules where sperm are made comprise approximately 80% of testicular volume, and studies do find a positive correlation between volume and sperm count. The link is loose enough to be useless as a personal prediction. Men with smaller testicles routinely have normal sperm counts, and a semen analysis measures fertility directly rather than by proxy.

A painless lump or swelling in one testicle is the commonest sign. Other signs include a feeling of heaviness in the scrotum, a dull ache in the lower abdomen or groin, sudden fluid build-up, or a change in the way a testicle feels. Pain shows up in only about 10 to 20% of cases at diagnosis, so waiting for it to hurt is the wrong plan. Cancer Research UK calls testicular cancer the most common cancer in young men, and its cure rate exceeds 98% when caught early, which is why anything new gets a doctor within 1 to 2 weeks. (Sources: Cancer Research UK; NHS; Testicular Cancer Society.)

Monthly, and ideally in or just after a warm shower when the scrotal skin is relaxed, which is what the Testicular Cancer Society recommends. Hold each testicle between your thumb and fingers and roll it gently, feeling for hard lumps, smooth bumps or any change in size or texture. The epididymis, a soft tube-like structure at the back of the testicle, is meant to be there and gets mistaken for a lump constantly. Book a GP or urologist appointment for anything unusual. Most lumps turn out to be cysts or varicoceles rather than cancer, and every new one still gets checked. (Sources: Testicular Cancer Society; NHS.)

Yes, at both ends of life. Size increases during puberty and reaches adult dimensions by age 16 to 18, holds through the 20s to 40s, and some studies find a gradual decline in volume after 50 as testosterone production falls, though that decline is modest in most men. Shrinkage you can actually see is a different matter. Testicular atrophy can come from varicocele, hormonal disorders, alcohol misuse or anabolic steroid use, and it needs medical assessment. (Sources: EAU; Prader orchidometer reference data.)

There is a correlation, though a weaker one than most people assume, because Leydig cells, the ones that produce testosterone, make up only about 10 to 20% of testicular volume. Clinically small testicles can point to lower testosterone, particularly in conditions such as Klinefelter syndrome or hypogonadism. Body fat, age, sleep, stress and genetics all move testosterone independently of size. A blood test is the only way to know. (Sources: EAU Guidelines on Male Hypogonadism.)

Anabolic steroid use is one of the commonest causes in younger men, since exogenous testosterone suppresses the body's own production. Varicocele, meaning enlarged veins in the scrotum, causes progressive atrophy, and hormonal imbalance, heavy alcohol consumption, certain medications, infections such as mumps orchitis, testicular torsion and ordinary ageing all do it too. A change in testicle size that you can notice is worth a urologist appointment. Many of the underlying causes are treatable. (Sources: EAU; NHS.)

You can estimate it with a ruler or a measuring tape. Take the length top to bottom and the width side to side on each testicle, then run the ellipsoid formula, volume = 0.71 x length x width x depth, all in cm, which gives a result in mL. Urologists use a Prader orchidometer, a string of calibrated beads, or an ultrasound when accuracy matters. Home numbers are good for tracking change over time and less precise than a clinical measurement, so measure the same way every time. (Sources: Sakamoto et al., 2007.)