Free Testosterone Calculator — with Bioavailable Levels

Written by Great Medico

Updated September 25, 2026 •2 min read

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Total testosterone does not tell the whole binding story. When SHBG or albumin changes, the fraction of testosterone that is free can change even if the total concentration looks similar. This calculator estimates free and bioavailable testosterone from those three laboratory values.

What this calculator does

Enter albumin in g/dL, SHBG in nmol/L and total testosterone in ng/dL. The calculator returns calculated free testosterone in nmol/L, the free fraction, calculated bioavailable testosterone and its fraction. It is a calculation from binding assumptions, not a direct laboratory measurement.

How to use it

Use laboratory values from the same clinical evaluation when possible and check units carefully. Testosterone interpretation is especially sensitive to assay method and sampling context, so do not substitute a value from a different unit system without correct conversion.

How the calculation works

The engine converts the inputs to molar units and solves a Vermeulen-style mass-action binding model using fixed association constants for testosterone binding to albumin and SHBG. It estimates the unbound concentration, then adds the albumin-bound fraction to calculate bioavailable testosterone.

Example

With albumin 4.3 g/dL, SHBG 30 nmol/L and total testosterone 500 ng/dL, the demonstration result is about 0.376 nmol/L free testosterone, a free fraction near 2.17%, and calculated bioavailable testosterone around 9.13 nmol/L.

How to interpret the result

Calculated free testosterone can be useful when SHBG alters the relationship between total testosterone and androgen exposure, but the result must be compared with an assay- and population-appropriate reference range and the clinical question. It does not by itself diagnose hypogonadism or justify testosterone treatment.

Limitations and notes

All calculated free-testosterone equations are approximations. Results can differ by formula, assay quality, binding constants and abnormal protein states. Equilibrium dialysis is a reference measurement approach, while calculated methods remain practical proxies. Discuss unexpected or borderline results with a clinician familiar with the laboratory method used.