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Free testosterone calculator

Enter total testosterone, SHBG, and albumin to calculate free and bioavailable testosterone using the Vermeulen equation — the formula used in clinical practice.

By OptiPin Editorial·Last updated
Sex hormone binding globulin, from the same panel.
Optional — defaults to 4.3 g/dL if left blank.
Enter your values above
Fill in total testosterone and SHBG to see your free and bioavailable testosterone.
Educational use only. This calculator estimates free testosterone using the Vermeulen equation and fixed binding constants. It is not a diagnosis and does not replace a lab free-testosterone measurement. Reference ranges vary by laboratory, assay, age, and sex. Always interpret results with your clinician.
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How to use this calculator

1
Enter total testosterone
From your blood panel, in ng/dL or nmol/L.
2
Enter SHBG
In nmol/L. SHBG is the main driver of how much testosterone is free.
3
Enter albumin (optional)
Use your measured albumin, or leave the 4.3 g/dL population default.
4
Read your results
Free testosterone, bioavailable testosterone, and percent free are calculated with the Vermeulen equation.

About free & bioavailable testosterone

Why free testosterone matters. Most circulating testosterone is bound to SHBG and is not biologically active. Free testosterone (~1–3% of total) and the loosely albumin-bound fraction make up bioavailable testosterone — the portion available to tissues. When SHBG is high or low, total testosterone can look normal while free testosterone is not.

The Vermeulen equation. This tool uses the Vermeulen et al. (1999) law-of-mass-action method with association constants KSHBG = 1.0×10⁹ L/mol and Kalbumin = 3.6×10⁴ L/mol. Calculated free testosterone tracks closely with equilibrium dialysis, the reference method.

Typical adult-male reference range. Free testosterone is roughly 5–21 ng/dL (≈174–729 pmol/L) in adult men, though ranges vary widely by lab and age. Ranges differ for women and are not shown here.