Educational tool — not a diagnosis. This calculator applies a published formula to numbers you enter. It cannot see your scan, your symptoms or your history, and prostate volume on its own does not diagnose or exclude anything.
Discuss any result with the clinician who ordered your scan. Do not use this page to make decisions about tests, treatment or medication.
Quick answer: volume = length × width × height × 0.52, with the three perpendicular diameters in centimetres. A 4.5 × 3.8 × 3.2 cm prostate is about 28.5 cm³. PSA density is PSA ÷ volume, and 0.15 ng/mL/cm³ is the threshold most often quoted.
The formula and where 0.52 comes from
The prostate is modelled as a prolate ellipsoid — an egg shape. The exact volume of an ellipsoid is (4/3)πabc using its three semi-axes. Since ultrasound reports give full diameters rather than half-diameters, each semi-axis is half the measured value, and (4/3)π × (1/2)³ = π/6 = 0.5236. Most literature rounds this to 0.52.
The prolate ellipsoid method is long-established in urological ultrasound and is the approach used by transrectal and transabdominal protocols alike. The coefficient is pure geometry rather than an empirical fit, which is why it is identical across sources.
Worked example — a report giving 4.5 cm (length) × 3.8 cm (width) × 3.2 cm (height):
- Multiply the diameters: 4.5 × 3.8 × 3.2 = 54.72
- Apply the coefficient: 54.72 × 0.5236 = 28.65 cm³
- Since 1 cm³ = 1 mL, that is 28.65 mL — and about 28.65 g, as prostate tissue is close to the density of water
Typical volume ranges
| Volume | Commonly described as |
|---|---|
| Under 20 cm³ | Small |
| 20–30 cm³ | Within the usual adult range |
| 30–40 cm³ | Mildly enlarged |
| 40–80 cm³ | Moderately enlarged |
| Over 80 cm³ | Markedly enlarged |
These descriptors are conventions, not diagnoses. Prostate volume rises gradually with age in most men, and the relationship between size and symptoms is weak — a man with a 70 cm³ prostate may have no trouble at all, while another with 35 cm³ has significant urinary symptoms. What matters clinically is the combination of size, symptoms, flow measurements and other findings.
PSA density: why volume matters alongside PSA
A larger prostate contains more glandular tissue, and more tissue produces more PSA. So a raised PSA in a large gland may be entirely explained by size, while the same PSA in a small gland is more notable. PSA density is the attempt to account for that.
Worked example: a PSA of 5.0 ng/mL in a 28.65 cm³ prostate gives 5.0 ÷ 28.65 = 0.175. In a 60 cm³ prostate the same PSA gives 0.083 — less than half.
| PSA density | Commonly described as |
|---|---|
| Below 0.10 | Low |
| 0.10–0.15 | Intermediate |
| 0.15 and above | The threshold most often cited as warranting closer attention |
The 0.15 figure appears widely in the literature and in several guidelines, but thresholds differ between guidelines, and PSA density is only one factor among many — alongside PSA trend over time, examination findings, MRI results, family history and age. It is not a test that rules anything in or out on its own.
How accurate is this?
Two limitations are worth understanding.
Shape. The formula assumes a smooth ellipsoid. Real prostates deviate — particularly when there is a prominent median lobe protruding into the bladder, which the three standard diameters do not capture well. In irregular glands the ellipsoid method tends to overestimate.
Compounding measurement error. Because three numbers are multiplied, errors multiply too. If each diameter is measured 10% too large, the volume comes out roughly 33% too large (1.1³ = 1.33). This is why the same patient can get meaningfully different volumes from different operators or on different days, and why a change between scans should be interpreted cautiously.
Where precision genuinely matters, planimetry or 3-D reconstruction from MRI is more accurate. The ellipsoid method persists because it is fast, needs no special equipment, and is good enough for most purposes.
Frequently asked questions
How is prostate volume calculated?
With the prolate ellipsoid formula: volume = length × width × height × 0.52, using three perpendicular diameters in centimetres. A prostate measuring 4.5 × 3.8 × 3.2 cm gives 54.7 × 0.52 ≈ 28.5 cm³. Because 1 cm³ equals 1 mL, the result is often written in millilitres or grams interchangeably.
What is a normal prostate volume?
A young adult prostate is typically around 20 to 25 cm³. Volume increases naturally with age, and a figure above roughly 30 cm³ is commonly described as enlarged. Enlargement alone is not a disease — many men with large prostates have no symptoms, and some with modest enlargement have significant ones.
What is PSA density?
PSA density is the serum PSA divided by the prostate volume, expressed in ng/mL per cm³. It exists because a large prostate produces more PSA simply by being large. A commonly cited threshold is 0.15, above which the raised PSA is less likely to be explained by size alone — but thresholds vary between guidelines and are only ever one input among many.
Why is 0.52 used in the formula?
It is π/6, which equals 0.5236. That is the exact coefficient for the volume of a prolate ellipsoid expressed in terms of its three full diameters rather than its semi-axes. Most sources round it to 0.52, and the difference from the exact value is well under one percent.
How accurate is the ellipsoid method?
It is a good approximation for a regularly shaped gland and is standard practice because it needs only three routine measurements. It tends to overestimate when the prostate is irregular or has a prominent median lobe. Since three measurements are multiplied together, small measurement errors compound — a 10 percent error in each diameter produces roughly a 33 percent error in volume.
Is prostate volume in grams the same as millilitres?
They are used interchangeably in practice. Prostate tissue has a density very close to 1 g/cm³, so a 30 cm³ prostate weighs about 30 grams. Reports may use cm³, mL or g and mean effectively the same number.
References
- Prolate ellipsoid volume formula — standard geometric derivation, V = (π/6) × d₁ × d₂ × d₃.
- Terris MK, Stamey TA. Determination of prostate volume by transrectal ultrasound. Journal of Urology, 1991.
- Benson MC, et al. Prostate specific antigen density: a means of distinguishing benign prostatic hypertrophy and prostate cancer. Journal of Urology, 1992.
- National and international urological guidelines discuss PSA density thresholds; values differ between guidelines and are applied alongside other findings.