PSA Density Calculator: Prostate Volume From 3 Measurements and PSAD
Work out PSA density (PSAD) from your PSA and prostate volume, or from three MRI or ultrasound diameters, and compare it with the 0.10, 0.15 and 0.20 cut-offs.
How it works
Prostate volume (mL) = length × width × height (cm) × 0.52 (ellipsoid; or π/6 = 0.5236; bullet formula 5π/24 ≈ 0.6545) · PSA density = PSA (ng/mL) ÷ prostate volume (mL), in ng/mL/cc · Cut-offs: 0.15 classic threshold; under 0.10 low; 0.20 used in some MRI pathways
PSA (prostate-specific antigen) comes from both normal and cancerous prostate tissue, so a large benign prostate can raise PSA on its own. PSA density corrects for this by dividing the PSA level by the size of the gland. The same PSA of 7 ng/mL means more in a 30 mL prostate (density 0.23) than in an 80 mL one (0.09). The volume usually comes from an MRI or transrectal ultrasound report. If you only have the three diameters, the calculator applies the ellipsoid formula, length × width × height × 0.52, which PI-RADS (the standard for reporting prostate MRI) uses. You can switch to π/6 (0.5236, almost identical) or the bullet formula (5π/24 ≈ 0.65), which gives a volume about 26% larger. The ellipsoid formula tends to underestimate the true volume slightly, which nudges the density up. A PSA density of 0.15 ng/mL/cc or more is the classic threshold for raised suspicion. Below 0.10, clinically significant cancer is unlikely when the MRI is also negative, and some MRI pathways use 0.20 to decide on biopsy for an equivocal (PI-RADS 3) lesion. PSA density supports a decision; it does not diagnose cancer. Unlike the IPSS score, which rates urinary symptoms, this calculator looks only at the PSA level relative to gland size.
Worked example
An MRI report gives prostate measurements of 4.0 cm (length) × 5.0 cm (width) × 4.0 cm (height). Their product is 80 cm³, and 80 × 0.52 = 41.6 mL. With a PSA of 7.0 ng/mL, PSA density = 7.0 ÷ 41.6 = 0.168 ng/mL/cc, which is above the 0.15 cut-off (band 0.15 to 0.19). The PSA that would give exactly 0.15 in this gland is 0.15 × 41.6 = 6.24 ng/mL. With the bullet formula the volume would be 52.4 mL and the density 0.134, below 0.15, so the result sits close to the threshold.
Sources: Radiology at Hand: Prostate volume and PSA density calculator (0.52 ellipsoid, 0.65 bullet, 0.10 and 0.15 thresholds) · Diagnostic and Interventional Radiology: Prostate volume in mpMRI under PI-RADS v2 and v2.1 (ellipsoid W × H × L × 0.52) · Omni Calculator: PSA density (π/6 ellipsoid and worked example 0.7 ng/mL in 23.5 mL) · Omni Calculator: Prostate volume (ellipsoid π/6 and bullet 5π/24 formulas) · OC Academy: MRI prostate volume assessment and PSA density cut-offs 0.10, 0.15, 0.20
Frequently asked questions
What is a normal PSA density?
There is no single normal value, but a PSA density under 0.15 ng/mL/cc is generally reassuring and under 0.10 is considered low, especially after a negative MRI. Readings of 0.15 or more are commonly used as a reason for further tests.
How do I calculate PSA density?
Divide your PSA (ng/mL) by your prostate volume (mL). For example, a PSA of 0.7 ng/mL in a 23.5 mL prostate gives 0.7 ÷ 23.5 = 0.03. If you only have three diameters in cm, multiply them together and by 0.52 to get the volume first.
How is prostate volume measured?
On MRI or transrectal ultrasound the radiologist measures the gland’s largest length, width and height at right angles to each other and uses the ellipsoid formula (× 0.52). Some reports use computer segmentation instead, which is more precise for large or irregular glands.
Why is my PSA high but my PSA density low?
A large prostate, most often from benign prostatic hyperplasia (BPH), makes more PSA simply because there is more tissue. Dividing by the volume shows that the PSA per mL of tissue is normal, which lowers the concern. Your doctor still decides whether follow-up is needed.
Does PSA density replace a biopsy or MRI?
No. It is one input that urologists combine with the MRI (PI-RADS) score, digital rectal exam, age, family history and PSA trend when deciding whether a biopsy is worthwhile.
Related calculators
For information only, not a diagnosis. PSA density is one factor among several; discuss your results and any biopsy decision with a urologist.