How to Measure Testicles: Methods and What Size Means

Testicular volume is measured in clinical settings using either a handheld comparison tool called a Prader orchidometer or, more accurately, an ultrasound scan. In healthy adult men, each testis typically measures somewhere between about 12 and 25 mL, though averages vary across populations and depend heavily on which method is used. The measurement matters more than most men realize: testicular size is one of the strongest physical indicators of sperm production capacity and hormonal output, and tracking changes over time can flag problems well before symptoms appear.

The Prader Orchidometer and How It Works

The most common bedside method is the Prader orchidometer, a string of 12 numbered wooden or plastic beads shaped like ellipsoids, each representing a known volume from 1 mL to 25 mL. A clinician holds each bead against the testis through the scrotal skin, finds the bead that feels closest in size, and records the corresponding number. The whole process takes under a minute and requires no equipment beyond the beads themselves, which is why it remains the default in urology and endocrinology exams around the world.

The problem is that orchidometry consistently overestimates true testicular volume. In one study of men being evaluated for infertility, orchidometer readings ran about 5 mL higher than ultrasound measurements on both sides, and the gap was largest for smaller testes in the 5 to 15 mL range.¹ In newborns and infants, the overestimation is even more dramatic: one study found the orchidometer inflated volume by nearly three-fold compared to ultrasound.² Part of the issue is that the beads include surrounding scrotal tissue and the epididymis in the comparison, not just the testis itself. Another part is human judgment: different examiners disagree on which bead matches, especially in the mid-range sizes where the volume jumps between beads are larger.

Why Ultrasound Is Considered the Gold Standard

Ultrasound gives the examiner three precise linear measurements of the testis (length, width, and height), then plugs those into a formula to estimate volume. The approach strips away scrotal tissue, isolates the actual gland, and produces measurements that correlate far more tightly with the true volume of removed testes measured by water displacement.³

Not all ultrasound formulas perform equally, though. The most commonly cited formula multiplies length × width × height × 0.52, treating the testis as a perfect ellipsoid. That formula tends to underestimate the real volume by roughly a third. A better option is the Lambert formula, which uses the same three measurements but multiplies by 0.71 instead. In head-to-head comparisons against water-displacement volumes of surgically removed testes, the Lambert formula came within about 6 to 7 percent of the true volume, making it the most accurate formula in routine clinical use.⁴ Despite this, many radiology departments still default to the 0.52 formula, which means the specific numbers on your ultrasound report may undercount by a few milliliters unless the Lambert formula was used.

For pediatricians evaluating pubertal progress, one group of researchers showed that even a simple ruler measurement of testicular width through the scrotum, combined with the right calculation, can produce volumes that match ultrasound values and track well across different developmental stages.⁵ That is useful in resource-limited settings where ultrasound is not readily available.

What Counts as Normal in an Adult

Normal testicular volume depends on who you study and how you measure. In a large European reference study using ultrasound and the ellipsoid formula, the average volume in healthy, fertile men was about 17 mL per testis, with the lower limit set at roughly 12 mL on the right and 11 mL on the left.⁶ Other populations show different averages. A study of young Korean men found mean orchidometer volumes of about 18 mL on each side, while ultrasound-based measurements in Somali men averaged around 10 to 11 mL per testis.⁷⁸ These differences reflect genuine population-level variation in body composition, genetics, and measurement techniques, so there is no single universal cutoff that cleanly separates “normal” from “small.”

That said, most clinicians treat ultrasound-derived volumes below about 12 mL as a trigger for further evaluation. The European Academy of Andrology has formally proposed that threshold based on the fertile-men reference data, calling volumes below it “testicular hypotrophy.”⁶

How Size Changes Over a Lifetime

Testicular volume follows a distinctive curve. Growth accelerates during puberty, peaks around age 30, holds relatively steady through middle age, and then begins to decline after 60.⁹ The late-life decline is not just about shrinkage; it reflects a genuine shift in testicular function, with lower hormone production and reduced sperm output, though spermatogenesis does not stop entirely.¹⁰ One older study found that roughly four in ten men over 60 had a measurable decrease in testicular volume, and nearly eight in ten had decreased testicular length.¹¹

Body weight matters too. In obese men, testicular volume tends to be inversely related to body mass index, independent of testosterone levels. For each additional unit of BMI, testicular volume dropped by close to 0.4 mL in one cross-sectional analysis.¹² That same study raised an important clinical point: because obesity independently reduces testicular size, a small testis in an obese man may not signal low testosterone the way it would in a lean man. Clinicians should be cautious about using size alone as a marker for androgen deficiency in that setting.

Why Size Matters for Fertility

The bulk of the testis is made up of seminiferous tubules, the coiled structures where sperm are produced. Larger testes simply contain more of this tissue, which translates into higher daily sperm output. Research in both animals and humans confirms that testicular volume is one of the strongest physical predictors of sperm production. In infertile men, volume correlates with sperm concentration, progressive motility, and total sperm count per ejaculate.¹³¹⁴

Testicular volume also tracks with hormone levels. A positive relationship exists between volume and total testosterone, which makes intuitive sense: the Leydig cells that produce testosterone sit between the seminiferous tubules, and a larger testis generally has more of them.¹³ When spermatogenesis is impaired and the seminiferous tubules shrink, the proportion of the testis occupied by Leydig cells actually goes up even though the overall gland is smaller. In severe cases, Leydig cells form microscopic clusters called micronodules, which are associated with a high ratio of luteinizing hormone to testosterone, a hormonal pattern that signals the pituitary gland is working overtime to compensate for failing testes.¹⁵

The bottom line for men undergoing fertility evaluations: testicular size is one of the first things a reproductive urologist assesses, and it correlates with both the sperm-making and hormone-making sides of testicular function.¹⁶

Conditions That Cause Small Testes

The most common genetic cause of small, firm testes is Klinefelter syndrome, a condition in which a man carries an extra X chromosome (47,XXY). It affects roughly one in 600 live male births and is the leading genetic cause of both primary hypogonadism and male infertility.¹⁷ The hallmark features in adults include small testes, sparse body hair, tall stature, and absent or severely reduced sperm production.¹⁸ Beyond the reproductive effects, men with Klinefelter syndrome face higher rates of metabolic problems like type 2 diabetes, changes in body composition with more fat and less muscle, and reduced bone density.¹⁹ The condition is frequently missed: many men are not diagnosed until they present with infertility in adulthood.

Anabolic steroid use is another well-known cause of testicular shrinkage. When you flood the body with external testosterone or related compounds, the brain’s signaling to the testes shuts down, causing them to atrophy. Current steroid users in one study had an average testicular volume of only about 12 mL, compared to roughly 22 mL in non-users. Former users who had stopped showed partial recovery, averaging about 17 mL, but that was still about 5 mL smaller than controls, and some former users had persistently low testosterone levels years after quitting.²⁰ The suppression of the body’s hormonal axis is a well-documented consequence of steroid use and does not always fully reverse.²¹

Varicoceles and Catch-Up Growth

A varicocele, an enlargement of veins within the scrotum, is one of the most common reasons an adolescent or young man has one testis that is noticeably smaller than the other. The affected side, usually the left, gets less blood flow or overheats, and the testis may lag in growth. In adolescents who undergo surgical repair, roughly 70 percent achieve “catch-up growth,” meaning the smaller testis reaches a volume within 15 to 20 percent of the other side within a couple of years.²² Age at surgery and the grade of the varicocele do not appear to predict who will catch up and who will not.²³

Interestingly, some adolescent varicoceles resolve on their own. A recent study followed 40 adolescents managed without surgery and found that 80 percent eventually reached a testicular volume differential below 15 percent, with the average age of catch-up around 15 years.²⁴ This is one of the reasons that the decision to operate on an adolescent varicocele is not always straightforward: if the size difference is moderate and stable, watchful waiting with serial ultrasounds is a reasonable approach.

Undescended Testes and Long-Term Size

When a testis fails to descend into the scrotum during fetal development (cryptorchidism), it raises long-term concerns about both size and function. A large cross-sectional study of more than 6,000 men from the general population found that a history of cryptorchidism was associated with a total testicular volume about 3.5 mL lower than men without that history, along with roughly 28 percent lower sperm concentration and modestly reduced testosterone relative to luteinizing hormone levels.²⁵

That sounds grim, but context helps. For men who had only one undescended testis (unilateral cryptorchidism), one older study found that even when the affected testis was small at the time of surgical correction, paternity rates and adult testicular volumes were not meaningfully worse than average.²⁶ And for acquired undescended testes, the kind that descend normally at birth but later retract, most come back down spontaneously during puberty and grow to a normal size afterward.²⁷

Is Asymmetry Normal?

Yes, and it is extremely common. In a study of 345 healthy adolescent boys, about 59 percent had a left testis that was smaller than the right. The average asymmetry index was just under 3 percent, which most men would never notice. Around 15 percent of the boys had asymmetry exceeding 20 percent, and roughly 13 percent had a volume difference greater than 2 mL favoring the right side.²⁸ Mild asymmetry, in other words, is the rule rather than the exception. It only warrants investigation when it is new, progressive, or accompanied by pain, because those features could suggest a varicocele, torsion, or a testicular mass.

When Testes Are Unusually Large

While most clinical attention goes to small testes, unusually large ones can also signal a problem. The condition called macroorchidism, where testicular volume well exceeds the normal range, is a recognized feature of fragile X syndrome, the most common inherited cause of intellectual disability. In fragile X, macroorchidism is tied to reduced levels of a specific protein (FMRP) and tends to become apparent after puberty.²⁹ In the absence of a genetic syndrome, an enlarging testis could also indicate a tumor or fluid collection (hydrocele), both of which require imaging.

Testicular Size Across Species

If you have ever wondered whether human testicular size is large or small by primate standards, comparative biology offers an interesting lens. Across primate species, testis size is shaped by mating systems: species where females mate with multiple males tend to have larger testes, because sperm competition drives investment in production capacity. A study across many primate species found a trade-off between sexual ornaments and testes size. Species with elaborate visual displays (like facial hair growth, bright coloration, or exaggerated body features used to attract mates) tended to have smaller testes, while species with larger canine teeth, used as weapons in direct male-to-male competition, actually had larger testes.³⁰ Humans fall somewhere in the middle of the primate spectrum: larger testes than gorillas, which have a single-male mating system, but much smaller than chimpanzees, which have a multi-male system with intense sperm competition.

Self-Examination and When to See a Doctor

You do not need a Prader orchidometer to monitor your own health. Monthly self-examination is straightforward: after a warm shower, when the scrotal skin is relaxed, gently roll each testis between your thumb and fingers. You are checking for lumps, hard areas, swelling, or a change in size from one month to the next. The goal is not to measure volume precisely but to develop a baseline familiarity with what feels normal for you, so that any change is obvious.

See a doctor if you notice a new lump or firmness, sudden pain or swelling, a progressive size difference between the two sides, or a noticeable shrinkage that was not there before. An ultrasound can quickly distinguish a benign cyst or hydrocele from something that needs more urgent attention. And if you are being evaluated for fertility, insist on an ultrasound measurement of testicular volume rather than orchidometry alone, since the ultrasound gives a more accurate picture and the measurement directly informs clinical decision-making about whether further hormonal or semen testing is warranted.