Testicular Volume Calculation Formula
Use the 0.71 formula (Length × Width × Height × 0.71) for calculating testicular volume, as this is the most accurate method validated against actual testicular volume by water displacement. 1
Evidence Supporting the 0.71 Formula
The 0.71 Lambert equation has been directly validated as the most accurate formula in multiple studies:
In a study comparing ultrasound formulas against actual testicular volume measured by water displacement after orchiectomy, the L × W × H × 0.71 formula had the smallest mean difference from actual volume, overestimating by only 0.80 cm³ (7.42%). 1 This was significantly more accurate than the 0.52 Hansen formula variants.
The correlation between the 0.71 formula and actual testicular volume was extremely strong (r = 0.965, P <0.0001), demonstrating excellent accuracy across a range of testicular sizes. 1
For your specific measurements (4.2cm × 2.5cm × 2.1cm), the 0.71 formula yields: 4.2 × 2.5 × 2.1 × 0.71 = 15.6 mL
Why Not the 0.5 Formula?
The 0.52 Hansen formula variants (L × W² × 0.52 or L × W × H × 0.52) consistently overestimate testicular volume compared to actual measurements:
The prolate ellipsoid formula (L × W × H × 0.52) showed larger deviations from actual testicular volume than the 0.71 formula. 1
One study proposed modifying the Hansen constant to 0.59 for smaller testes specifically, but this was only validated for testes under 8.5 mL. 2 Your measurement of 15.6 mL falls outside this range.
Clinical Context and Guidelines
The European Association of Urology guidelines specifically state that ultrasound testicular volume should be calculated using standard formulas, with the 0.71 coefficient being the accepted standard in clinical practice. 3
Multiple validation studies across different testicular size ranges (from 2.5 mL to 23 mL) consistently demonstrate the 0.71 formula's superior accuracy. 1
The 0.71 formula maintains accuracy whether measuring normal-sized adult testes or smaller atrophic testes. 2, 1
Important Clinical Considerations
For a testicular volume of 15.6 mL (calculated with 0.71):
This falls below the normal adult range, as total testicular volume (both testes combined) should typically be ≥30 mL by ultrasound for normal spermatogenesis. 4
A single testis volume of 15.6 mL suggests potential testicular dysfunction if bilateral measurements are similar, warranting semen analysis and hormonal evaluation (FSH, testosterone). 3, 5
Scrotal ultrasound is indicated when precise volume measurement is needed, particularly when physical examination is difficult or when assessing for non-homogeneous testicular architecture that may indicate impaired spermatogenesis. 3