Quick answer: PSA density = serum PSA (ng/mL) ÷ prostate volume (cc). A PSA of 6.0 with a 45 cc prostate gives 0.13 ng/mL/cc. Below 0.15 is the figure most widely treated as reassuring; 0.15 and above raises concern. It is a decision aid, not a diagnosis — interpretation belongs with a urologist.
This is an educational tool, not medical advice. PSA density is one input among many into a complex clinical decision. Do not use this page to decide whether to seek care, accept or decline a biopsy, or interpret your own results. If your PSA is raised, discuss it with a doctor.
Why divide by volume at all
PSA is made by prostate tissue — both healthy and cancerous. That creates an obvious problem for interpretation: a bigger prostate makes more PSA simply by being bigger. Benign prostatic hyperplasia is extremely common with age, and a man can easily reach a 90 cc gland with no cancer at all.
So a raised PSA has two very different possible explanations, and the raw number cannot tell them apart. Dividing by volume can:
| Both men have PSA = 8.0 ng/mL | Patient A | Patient B |
|---|---|---|
| Prostate volume | 90 cc | 25 cc |
| Serum PSA | 8.0 ng/mL | 8.0 ng/mL |
| PSA density | 0.089 | 0.320 |
| Against the 0.15 threshold | Well below | More than double |
| Reasonable reading | Consistent with a large benign gland | High for that gland size |
Same blood result, very different pictures. That is the entire argument for PSA density in one table, and it is why the measure survives despite decades of newer biomarkers.
The formula and the volume behind it
Prostate volume (ellipsoid) = L × W × H × 0.52
The prostate is modelled as a prolate ellipsoid, and 0.52 is the rounded value of π/6 = 0.5236. Three perpendicular diameters in centimetres — typically transverse (width), anteroposterior (height) and craniocaudal (length) — go in; a volume in cubic centimetres comes out. Because 1 cc = 1 mL, the units cancel neatly into ng/mL/cc.
Worked example: a gland measuring 4.5 × 3.8 × 3.2 cm with a PSA of 6.0:
- Volume: 4.5 × 3.8 × 3.2 × 0.52 = 28.5 cc
- PSA density: 6.0 ÷ 28.5 = 0.21 ng/mL/cc
Note that the same PSA of 6.0 gave 0.13 against a 45 cc gland in the quick answer and 0.21 here. The denominator does the work, which is exactly why how you measure the volume matters as much as the blood test. The prostate volume calculator covers the measurement side in more depth.
Reading the number
The 0.15 ng/mL/cc cut-off comes from work by Benson and colleagues in the early 1990s and has proved durable. Commonly used bands:
| PSA density | Commonly described as | General interpretation |
|---|---|---|
| < 0.10 | Low | Strongly favours benign enlargement |
| 0.10 – 0.14 | Intermediate–low | Below the usual threshold; often reassuring in context |
| 0.15 – 0.19 | Threshold | At or above the classical cut-off; warrants further assessment |
| 0.20 – 0.29 | Raised | Higher likelihood of clinically significant disease |
| ≥ 0.30 | High | Substantially raised risk; usually prompts biopsy discussion |
These bands describe how the number is typically discussed in the literature. They are not diagnostic criteria and no single value confirms or excludes cancer. Local protocols vary, and some centres use 0.12 or 0.20 as their operating threshold depending on how it is combined with MRI.
Where it is genuinely useful in current practice is in combination with MRI. A low PSA density alongside a negative or low-suspicion MRI (PI-RADS 1–2) is one of the recognised routes to safely deferring a biopsy — the two pieces of information are more informative together than either alone. Conversely a high PSA density can raise concern even when imaging looks unremarkable.
PSA density at a glance
PSA density for common combinations. Shaded thinking: read down your PSA row, across to your gland size.
| PSA ↓ / Volume → | 20 cc | 30 cc | 40 cc | 60 cc | 80 cc | 100 cc |
|---|---|---|---|---|---|---|
| 2.0 ng/mL | 0.100 | 0.067 | 0.050 | 0.033 | 0.025 | 0.020 |
| 4.0 ng/mL | 0.200 | 0.133 | 0.100 | 0.067 | 0.050 | 0.040 |
| 6.0 ng/mL | 0.300 | 0.200 | 0.150 | 0.100 | 0.075 | 0.060 |
| 8.0 ng/mL | 0.400 | 0.267 | 0.200 | 0.133 | 0.100 | 0.080 |
| 10.0 ng/mL | 0.500 | 0.333 | 0.250 | 0.167 | 0.125 | 0.100 |
| 15.0 ng/mL | 0.750 | 0.500 | 0.375 | 0.250 | 0.188 | 0.150 |
The pattern worth noticing is the diagonal. A PSA of 4 in a 40 cc gland, 6 in a 60 cc gland, and 10 in a 100 cc gland all give exactly 0.10 — the same density from very different-looking blood results. That is the measure doing its job.
PSA density among the other PSA derivatives
| Measure | Calculation | Adjusts for | Needs |
|---|---|---|---|
| PSA density | PSA ÷ volume | Gland size | One PSA + imaging |
| PSA transition zone density | PSA ÷ TZ volume | The zone BPH actually affects | Zonal volume on imaging |
| Free : total PSA ratio | free ÷ total × 100% | Molecular form; lower ratio is less reassuring | A free PSA assay |
| PSA velocity | Δ PSA ÷ Δ years | Rate of change | ≥ 3 readings over 18–24 months |
| PSA doubling time | ln(2) ÷ growth rate | Rate, on a log scale | Serial readings |
| Age-specific PSA | reference by decade | Age | Nothing extra |
| PSA mass | PSA × plasma volume | Body size / haemodilution | Height and weight |
The last row is the one that surprises people. PSA is measured as a concentration, so a larger man with a greater plasma volume dilutes the same amount of PSA into more fluid and reads lower. PSA mass — total PSA circulating, rather than its concentration — corrects for that. It has never entered routine practice, but it explains part of why PSA runs lower in men with obesity.
Common mistakes & pro tips
- Mixing up which volume was used. Whole-gland volume and transition-zone volume give very different densities. TZ density uses a different, higher threshold — do not compare the two.
- Comparing an MRI density to an ultrasound density. MRI-derived volumes tend to run smaller, which pushes density up. Track the same modality over time.
- Using a stale volume with a fresh PSA. Prostates grow. A volume from four years ago paired with today's PSA overstates the density.
- Forgetting what raises PSA transiently. Recent ejaculation, cycling, catheterisation, digital rectal examination, prostatitis and urinary infection can all raise PSA independently of cancer. A density computed on an inflamed prostate is not interpretable.
- Forgetting what lowers it. 5-alpha reductase inhibitors such as finasteride and dutasteride roughly halve PSA after 6–12 months, and also shrink the gland — both terms move, so the density does not simply halve.
- Pro tip: record the three diameters, not just the volume. It lets anyone recompute with a different coefficient later and see how sensitive the answer is.
- Pro tip: treat 0.15 as a soft boundary. 0.14 and 0.16 are not meaningfully different. The measurement error on prostate volume alone is comfortably larger than that gap.
How to use this calculator
- Choose how you know the volume — enter it directly from a report, or give the three ultrasound diameters.
- Enter the total PSA in ng/mL.
- Pick a coefficient if measuring — 0.52 is the standard prolate ellipsoid value.
- Optionally add free PSA, height and weight for the free:total ratio and PSA mass.
Frequently asked questions
How do you calculate PSA density?
Divide the serum PSA in ng/mL by the prostate volume in cc. A PSA of 6.0 ng/mL with a 45 cc prostate gives a PSA density of 6.0 ÷ 45, which is 0.13 ng/mL/cc. The prostate volume normally comes from transrectal ultrasound or MRI, or is calculated from three measured diameters using the ellipsoid formula.
What is a normal PSA density?
A PSA density below 0.15 ng/mL/cc is the figure most widely used as reassuring, and below 0.10 is considered low risk in most published series. Values at or above 0.15 are associated with a higher likelihood of clinically significant prostate cancer. The 0.15 cut-off dates from work by Benson and colleagues in the early 1990s and remains in common use, but it is a decision aid rather than a diagnostic line.
Why use PSA density instead of PSA alone?
Because benign prostatic hyperplasia raises PSA simply by adding more prostate tissue. A man with a 90 cc gland and a PSA of 8 has a PSA density of 0.09, whereas a man with a 25 cc gland and the same PSA of 8 has a density of 0.32. The same blood result means very different things in those two men, and dividing by volume is what separates them.
How is prostate volume measured for PSA density?
Most commonly with the prolate ellipsoid formula: length × width × height × 0.52, using three perpendicular diameters in centimetres from transrectal ultrasound or MRI. The coefficient 0.52 approximates π/6. MRI-derived volumes tend to be slightly smaller and more reproducible than ultrasound ones, and because volume sits in the denominator, the measurement method meaningfully changes the density.
What is the difference between PSA density and PSA velocity?
PSA density adjusts a single PSA result for the size of the gland producing it. PSA velocity measures how fast PSA is rising over time, usually in ng/mL per year, and needs at least three readings over 18–24 months. Density answers whether this PSA is high for this prostate; velocity answers whether it is changing faster than expected. They are complementary and are often considered together.
Does PSA density replace a biopsy decision?
No. PSA density is one input among several, alongside digital rectal examination, MRI findings and PI-RADS score, free:total PSA ratio, family history, ethnicity, age and patient preference. Contemporary practice increasingly combines PSA density with MRI: a low PSA density together with a negative or low-suspicion MRI is one of the recognised routes to safely avoiding an immediate biopsy. That decision belongs with a urologist.
How is PSA doubling time calculated?
PSA doubling time is the number of months it takes PSA to double, assuming steady exponential growth. With two readings, use doubling time = ln(2) × months between tests ÷ ln(later PSA ÷ earlier PSA). A rise from 4.0 to 6.0 ng/mL over 12 months gives 0.693 × 12 ÷ ln(1.5) = about 20.5 months. Guidelines usually fit at least three readings over several months, so treat a two-point figure as a rough estimate and discuss it with your doctor.
References & further reading
- Benson, M. C. et al. (1992), "Prostate specific antigen density: a means of distinguishing benign prostatic hypertrophy and prostate cancer", Journal of Urology — the origin of the 0.15 threshold.
- European Association of Urology guidelines on prostate cancer — PSA density in the risk-stratified diagnostic pathway alongside MRI.
- NICE guideline NG131, Prostate cancer: diagnosis and management (UK).
- PI-RADS v2.1 — MRI reporting system commonly combined with PSA density.
Educational use only. This calculator performs a published arithmetic calculation. It is not a diagnostic tool and does not constitute medical advice. See our accuracy policy.