What is the Vermeulen equation?

The Vermeulen equation (Vermeulen et al., 1999) estimates free testosterone from total testosterone, SHBG, and albumin using the law of mass action. Calculated free testosterone correlates well with the gold-standard equilibrium dialysis method and is the most widely used formula in clinical practice.

What is bioavailable testosterone?

Bioavailable testosterone is the fraction not tightly bound to SHBG - it includes free testosterone plus the loosely albumin-bound portion. It represents the testosterone readily available to tissues.

Is this a substitute for a lab free testosterone test?

No. This is an educational estimate. Calculated free testosterone depends on accurate SHBG and albumin values and uses fixed binding constants. Reference ranges vary by lab, age, and sex. Always interpret results with your clinician.

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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 June 6, 2026

Total testosterone

SHBG (nmol/L)

Sex hormone binding globulin, from the same panel.

Albumin

Optional - defaults to 4.3 g/dL if left blank.

Show free T in

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

Enter total testosterone

From your blood panel, in ng/dL or nmol/L.

Enter SHBG

In nmol/L. SHBG is the main driver of how much testosterone is free.

Enter albumin (optional)

Use your measured albumin, or leave the 4.3 g/dL population default.

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. → free vs total testosterone explains the mechanism, what shifts SHBG, and the thresholds that matter.

The Vermeulen equation. This tool uses the Vermeulen et al. (1999) law-of-mass-action method with association constants K_SHBG = 1.0×10⁹ L/mol and K_albumin = 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.