Percent free PSA calculator
The share of your PSA that is not attached to other proteins. With a mildly raised total PSA, a lower share goes with a higher chance a biopsy finds cancer. Type your results in the units on your report and it works out % Free PSA as you type, in your browser.
AI-verified against cited sources. Awaiting expert reviewLast reviewed 2026-10-03Cancer Markers
Type your results above, or fill in the example, to see % Free PSA.
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Educational information, not medical advice. Blood Test Tracker doesn’t diagnose or treat, and it doesn’t replace your clinician. Talk with your clinician before changing medications, supplements or care.
How it’s worked out
The formula
% free PSA = free PSA (ng/mL) ÷ total PSA (ng/mL) × 100. Both values come from the same blood sample, ideally measured with the same maker's assays. Some labs print the result as a ratio (0.15) instead of a percent (15%).
When the equation works
Outside these limits the calculator says so instead of giving a number.
- Free PSA can’t be more than total PSA (above 100%).
A worked example
Example numbers, not anyone’s real results, run through the same code as the calculator above.
| Free PSA | 1 ng/mL |
|---|---|
| PSA | 5 ng/mL |
| % Free PSA | 20.0 % |
What % Free PSA tells you
PSA in your blood comes in two main forms: attached to other proteins (bound) and floating free. Percent free PSA is the free part divided by total PSA, times 100. Labs calculate it for you when both tests run on the same blood sample. Some labs print it as a ratio such as 0.15 instead of 15%. It is not a stand-alone test for cancer. It is meant for men 50 and older whose total PSA is mildly raised (Mayo reports it for total PSA of 4.0 to 10.0 ng/mL) and whose rectal exam was not suspicious.
When total PSA is only mildly raised, there are many possible causes besides cancer, and a biopsy often finds nothing. Percent free PSA gives an extra clue. A lower percentage goes with a higher chance that a biopsy will find prostate cancer, and a higher percentage goes with a lower chance. In Mayo's table, cancer was found in 56 of 100 men whose percent free PSA was 10% or lower, and in 8 of 100 men whose percent was over 25%. Your clinician may use it to help decide between watching, an MRI or a biopsy. It does not diagnose cancer. The AUA guideline calls percent free PSA perhaps the most widely available add-on test.
Read the full page on % Free PSA, with what a high or low result can mean.
Sources
The equation
- PSAFT - Overview: Prostate-Specific Antigen (PSA), Total and Free, Serum, Mayo Clinic Laboratories
- Prostate Specific Antigen, Free Percentage (Includes Free PSA and Total PSA) (test 0080206), ARUP Laboratories
3 more sources for the ranges and the explanation
- P2PHI - Overview: [-2]Pro Prostate Specific Antigen with Prostate Health Index, Serum, Mayo Clinic Laboratories
- Use of the percentage of free prostate-specific antigen to enhance differentiation of prostate cancer from benign prostatic disease: a prospective multicenter clinical trial, JAMA (1998)
- Early Detection of Prostate Cancer: AUA/SUO Guideline (2023, amended 2026), American Urological Association (2026)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. The calculator is version 1.0.0 of our code for this equation, tested against published examples. How we build and check these pages.