PSA Doubling Time Calculator (log-slope of two or more PSA results)

Understanding the PSA doubling time calculator — concept and principle

PSA doubling time (PSADT) is how long the prostate-specific antigen (PSA) level would take to double if it kept rising at the same exponential rate. This calculator fits all your dated PSA results and also gives PSA velocity. It is mainly used after surgery or radiotherapy for prostate cancer, when PSA starts to rise again.

Fit a line to log PSA: with the default inputs the result is 16.4 months (doubling time · rising).

How it is calculated

Fit a straight line to ln(PSA) against time by least squares; slope b is per day.

PSADT (months) = ln 2 ÷ b ÷ 30.44

PSA velocity (ng/mL per year) = slope of a straight line through the raw PSA values × 365.25

Enter one "YYYY-MM-DD value" pair per line, 2 to 30 results spanning at least 4 weeks.
If PSA is flat or falling there is no doubling time.

The principle

When a tumour grows exponentially, PSA rises by the same percentage in each equal time step, so its logarithm rises in a straight line. The slope of that line is the growth rate, and ln 2 divided by the growth rate is the time needed to double. Using every result through regression, rather than only the first and last, makes the estimate less sensitive to one unusual value. PSA velocity instead measures the absolute rise per year.

Worked example

Default inputs: 1 May 2025 2.1, 1 Nov 2025 2.8, 1 May 2026 3.6, 1 Nov 2026 4.5 ng/mL.

Days from the first result: 0, 184, 365, 549 → time span 18 months; results used 4.
Regression slope of ln(PSA) = 0.001388 per day.
PSADT = 0.6931 ÷ 0.001388 = 499.3 days ÷ 30.44 = 16.4 months; trend Rising.
PSA velocity = regression slope of PSA × 365.25 = 1.6 ng/mL per year.

How to read the result

After treatment, a shorter doubling time suggests faster-growing disease; doubling times under roughly 3–12 months are often regarded as concerning, while long doubling times suggest slow progression. Doctors combine PSADT with the time since treatment, Gleason grade and imaging to decide on further treatment. Before any diagnosis, PSA rises can have benign causes.

Common mistakes

Mixing results from different laboratories or assays.
Using values only a few days apart, where measurement noise dominates.
Including a value taken soon after ejaculation, cycling, infection or a catheter.

Keep in mind: This is not a diagnosis. Interpretation of PSA trends belongs to your urologist or oncologist.

Frequently asked questions

What is a good PSA doubling time?

Longer is better; very long doubling times usually reflect slow-growing or no active disease.

How many PSA results do I need?

At least two, but three or more over 3 months or longer give a more reliable estimate.

What is the difference between velocity and doubling time?

Velocity is the absolute rise per year; doubling time reflects the percentage rise.

Sources: Least-squares regression of ln(PSA) on time, doubling time = ln 2 ÷ slope (standard log-linear method used by published PSADT calculators)

Checked 9 Nov 2026. Standard method: least-squares regression of ln(PSA) on time, doubling time = ln 2 ÷ slope (the log-linear approach used by published PSADT calculators); PSA velocity as the linear regression slope of PSA on time.

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