Prostate volume & PSA density
Calculate prostate volume from three ultrasound or MRI diameters using the ellipsoid formula, and derive PSA density. Includes thresholds and how to read the result.
Inputs
Volume
24.7mL
Weight
26.0g
PSA density
0.24ng/mL²
Volume 24.7 mL · PSA density 0.24 ng/mL²
A volume of 24.7 mL falls within the range expected for an adult prostate without appreciable benign enlargement (roughly up to 25 mL). Estimated weight 26.0 g.
0.24 ng/mL² is clearly raised. In the EAU pathway a PI-RADS 3 lesion with a density from roughly 0.20 upward is generally biopsied rather than followed. State the density explicitly in your report.
The ellipsoid formula assumes a symmetric gland. With a prominent median lobe, an asymmetric contour or after TURP the estimate drifts from planimetry; in those cases record how you measured.
How to read the result
Two numbers come out, and they answer different questions. Volume describes the gland; density asks whether the PSA is explained by that gland or by something else.
The formula
Volume (mL) = craniocaudal × transverse × anteroposterior × π/6
π/6 ≈ 0.524, the coefficient for a prolate ellipsoid. Enter all three diameters in centimetres.
PSA density (ng/mL²) = serum PSA ÷ prostate volume.
Gland weight in grams is close to the volume in millilitres, since prostatic tissue sits near 1.05 g/mL.
When to use it
- Reporting a prostate MRI or transrectal ultrasound, where volume belongs in every structured report.
- Deciding whether a PI-RADS 3 lesion warrants biopsy, where PSA density carries real weight.
- Assessing benign enlargement, where volume steers the choice between transurethral and enucleating techniques.
- Following a gland over time, where a consistent measuring method matters more than the absolute number.
⚠️ Pitfalls
- The ellipsoid formula assumes symmetry. A prominent median lobe or an irregular contour makes it drift from planimetry, which remains the reference standard.
- Measure the transverse and anteroposterior diameters on the same axial image and the craniocaudal on sagittal; mixing planes is the commonest source of disagreement between readers.
- MRI-derived volumes tend to run slightly lower than transrectal ultrasound. Comparing across modalities without saying so makes a stable gland look like it shrank.
- Density is only as good as the PSA it uses. A PSA drawn after instrumentation, or during prostatitis, produces a density that means nothing.
Frequently asked questions
What is the formula for prostate volume?
Volume equals the craniocaudal, transverse and anteroposterior diameters multiplied together and then by π/6, roughly 0.524. With diameters in centimetres the result is in millilitres.
What is a normal prostate volume?
Around 25 mL or less is typical for an adult gland without benign enlargement. Volume rises with age, and a value between 25 and 40 mL is common and often asymptomatic.
What does PSA density add over PSA alone?
It corrects the PSA for the amount of tissue producing it. A large benign gland can generate a raised PSA on its own, and dividing by volume separates that from a small gland producing the same PSA, which is far more concerning.
Which PSA density threshold should I use?
0.15 ng/mL² is the classic cutoff for raised suspicion. In the current EAU pathway a value around 0.20 ng/mL² is what typically tips a PI-RADS 3 lesion toward biopsy rather than surveillance.
Do MRI and ultrasound give the same volume?
Not exactly. MRI measurements tend to be slightly lower than transrectal ultrasound. For follow-up, stay with one modality and one measuring method, and state which you used.
References
📄Benson MC, et al. Prostate specific antigen density: a means of distinguishing benign prostatic hypertrophy and prostate cancer. J Urol. 1992;147(3 Pt 2):815–816.
📄Terris MK, Stamey TA. Determination of prostate volume by transrectal ultrasound. J Urol. 1991;145(5):984–987.
📄Turkbey B, et al. Prostate Imaging Reporting and Data System Version 2.1. Eur Urol. 2019;76(3):340–351.
📄EAU Guidelines on Prostate Cancer, current edition.↗This calculator supports clinical judgement and does not replace it. Always check the result against your own measurements and the guideline in force.
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