Prostate Volume from Ultrasound Calculator
Prostate Volume from Ultrasound Calculator
Gland volume from the three dimensions on an ultrasound report, with both published coefficients shown — the prolate ellipsoid π/6 and the bullet 5π/24 — because they disagree by a quarter and the published bias runs in opposite directions in small and large glands.
Prostate Volume from Ultrasound
Three dimensions → mLA transrectal ultrasound reporting 4.1 cm craniocaudal × 4.7 cm transverse × 4.0 cm anteroposterior, read with the prolate ellipsoid coefficient
The formula, and which coefficient to use
Prolate ellipsoid k = π/6 = 0.5236
Bullet k = 5π/24 = 0.6545, which is exactly 5/4 of π/6
Equivalent gland weight (g) = volume × 1.02
- π/6 and 5π/24
- the two published coefficients. They are NOT approximations of each other: the bullet coefficient is exactly 25 per cent larger, and a gland is either 40 mL or 50 mL depending on which was used. The rounded 0.52 and 0.65 in circulation are both low by the same 0.69 per cent, because 0.52/(π/6) and 0.65/(5π/24) are the same ratio; this page uses π/6 and 5π/24 exactly rather than the roundings
- the three dimensions are interchangeable
- the formula is a symmetric product, so a transposed pair cannot change the answer. That is worth knowing because it is the one transposition in this category that is harmless — the stone pages, the pressure-flow page and the voiding-efficiency page all have inputs that are not interchangeable
- two of the three inputs are biased
- against 153 prostatectomy specimens, transrectal ultrasound overestimated the length by 10.8 per cent and the height by 11.1 per cent, with the width agreeing. Three multiplied dimensions, two of them about a tenth too large, is roughly a quarter too large before the coefficient is chosen
- 1.02 g per cc
- the prostate density that study measured by water displacement on eight glands, and used to convert specimen weight into the reference volume. It is why a gland weight in grams and a gland volume in millilitres are usually quoted as if they were the same number — they are within two per cent
- what this page does not do
- it does not compute a PSA density. That is the PSA density calculator, which takes this volume as its denominator and carries the published density thresholds; this page exists because that page does not measure a gland. This page reports a figure and what the published sources attach to it. It does not make a clinical decision and cannot.
Worked example
A transrectal ultrasound reporting 4.1 cm craniocaudal × 4.7 cm transverse × 4.0 cm anteroposterior, read with the prolate ellipsoid coefficient
4.1 × 4.7 × 4.0 = 77.08 cm³
77.08 × π/6 = 77.08 × 0.5236 = 40.4 mL
The bullet coefficient gives 77.08 × 0.6545 = 50.4 mL for the same three measurements — 25 per cent more, and shown under the headline above
At 40.4 mL this gland is in the study's medium group, where the ellipsoid coefficient overestimated by about 14 per cent and the bullet coefficient underestimated by about 8 per cent. The two answers bracket the truth; neither is it
If the same three numbers had come from a digital rectal examination rather than an ultrasound, they would be an underestimate — the EAU notes that the examination underestimates volume, increasingly so above 30 mL
The two coefficients against 153 prostatectomy specimens
| Gland size group | Prolate ellipsoid π/6 | Bullet 5π/24 | Study’s fitted 0.66 |
|---|---|---|---|
| All 153 cases | Underestimated by 8.80 cc (18 per cent) | Overestimated by 1.97 cc (4.6 per cent) | Overestimated by 2.76 cc (5.7 per cent) |
| Under 30 cc (n = 13, mean 26.7 cc) | Over by 12.6 per cent | Over by 32.2 per cent | Over by 34.2 per cent |
| 30 to 60 cc (mean 42.7 cc) | Over by 14 per cent | Under by 8.3 per cent | Under by 10 per cent |
| Above 60 cc (n = 37, mean 76.0 cc) | Under by 30 per cent | Under by 8 per cent | Under by 6.6 per cent |
| Regression against the specimen (R², outliers removed) | 0.55 | 0.60 | 0.64 |
What the volume is used for, and what it is read against
| Use | Where the arithmetic lives |
|---|---|
| PSA density — total PSA divided by this volume | PSA density calculator, which takes the volume as an input |
| Following a gland over time, or against treatment | This page, with the same coefficient both times. Changing coefficient between two scans produces a 25 per cent change in volume and no change in the gland |
| Comparing with a digital rectal estimate | The EAU states that transrectal ultrasound is superior to transabdominal measurement and more accurate than the digital examination, which underestimates volume, increasingly above 30 mL |
| Comparing with a PSA-based estimate | The EAU notes that total and free PSA predict the ultrasound volume to within ±20 per cent in more than 90 per cent of cases — which is a statement about agreement, not a reason to stop measuring |
| The same geometry on another organ | thyroid nodule volume calculator, which applies the same prolate ellipsoid coefficient to a thyroid nodule |
Two published coefficients, 25 per cent apart
A prostate volume is almost never measured. It is computed from three orthogonal diameters on an ultrasound report and a coefficient, and the coefficient is where the trouble is. Two are in print. The prolate ellipsoid coefficient is π/6, about 0.52, and is the conventional one. The bullet coefficient is 5π/24, about 0.65, and is exactly five quarters of it. The same three measurements therefore yield either 40 mL or 50 mL depending on which was used, and reports rarely say.
The study that settled which is better measured both against 153 fresh prostatectomy specimens, using the specimen’s weight divided by a density of 1.02 g per cc as the reference. Over the whole cohort the ellipsoid coefficient underestimated by 8.80 cc, about 18 per cent, and the bullet coefficient overestimated by 1.97 cc, about 4.6 per cent. But the cohort average conceals the useful finding: the direction of the error flips with gland size. In the 37 glands above 60 cc the ellipsoid coefficient was 30 per cent low; in the 13 glands below 30 cc it was 12.6 per cent high, and the bullet coefficient was 32.2 per cent high. Neither coefficient is right across the range, which is why this page shows both answers at once rather than choosing for you.
The inputs are biased too, and in a known direction. In the same cohort, transrectal ultrasound overestimated the gland’s length by 10.8 per cent and its height by 11.1 per cent against the ruler measurements made on the specimen, while the width agreed. Three multiplied dimensions, two of them about a tenth too large, is already a quarter too large before any coefficient is applied — and 11.7 per cent of the cohort had to be discarded as outliers before the regressions would behave.
The practical rules follow from that. Record the coefficient with the volume, because a volume without one cannot be compared with a previous volume. Use the same coefficient and the same modality on both scans when following a gland. And treat the number as having a confidence interval of tens of per cent rather than of per cent — which matters most where it is used as a denominator, as it is in the PSA density calculator. The formula is symmetric in its three dimensions, so it does not matter which measurement goes in which field; the same is not true of any other page in this category.
Frequently asked questions
Should I use 0.52 or 0.65?
Both are published and this page shows both. Against 153 prostatectomy specimens the bullet coefficient (5π/24, about 0.65) was closer overall and clearly closer above 60 cc, where the prolate ellipsoid coefficient was 30 per cent low; below 30 cc the ellipsoid coefficient was the better of the two. The conventional choice remains π/6, which is what most reports and most of the literature use, so the important thing is to record which one produced your number.
Does it matter which dimension I put in which field?
No. The formula is the product of the three diameters and a constant, so it is symmetric: any ordering gives the same answer. What matters is that the three are orthogonal and come from the same study.
Why is my ultrasound volume different from the weight the pathologist reported?
Partly density and partly measurement. The study behind this page measured prostate density as 1.02 g per cc, so a volume in mL and a weight in grams are within about two per cent of each other and the implied weight is shown under the headline. The larger part of the discrepancy is the ultrasound: it overestimated the length by 10.8 per cent and the height by 11.1 per cent against the specimen.
Can I use this to work out a PSA density?
Take the volume to the PSA density calculator, which divides total PSA by it and carries the published density thresholds. This page deliberately does not duplicate that arithmetic. The PSA velocity calculator, the PSA doubling time calculator, the free-to-total PSA ratio calculator and the PSA unit converter are the other pages built on a PSA result.
Is a transabdominal measurement good enough?
The EAU states that transrectal ultrasound is superior to transabdominal volume measurement, and that both are more accurate than a digital rectal examination, which underestimates volume increasingly above 30 mL. If your three dimensions came from a transabdominal study, the arithmetic here is the same and the uncertainty is larger.
What this page will not tell me
Every threshold, range and performance figure on this page is the published figure from the source named beside it, and each one depends on the population, the method and the equipment it was derived in. Where your own report, laboratory or local guideline gives a different figure, that figure governs. A score, an index, a measured volume or an attenuation value is not a diagnosis, and a proportion measured in a cohort is not a probability for one patient. This page reports a figure and what the published sources attach to it. It does not make a clinical decision and cannot.
Related calculators
References
- Aprikian, et al. Improving ultrasound-based prostate volume estimation. BMC Urol. 2019;19:68 — 153 radical prostatectomy specimens against pre-operative transrectal ultrasound dimensions.
- European Association of Urology. Guidelines on the Management of Non-neurogenic Male Lower Urinary Tract Symptoms, 2026 edition — Diagnostic Evaluation. uroweb.org.
- Prostate volume measured by magnetic resonance imaging is not a predictor of lower urinary tract symptoms. J Family Med Prim Care. 2019 Jan.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/