What is free testosterone?

Free testosterone is the fraction of total testosterone not bound to proteins. Roughly 2–3% circulates unbound (free), while ~54% binds loosely to albumin and ~44% binds tightly to SHBG. Free + albumin-bound fractions are collectively called bioavailable testosterone — the portion accessible to tissues.

What is the Vermeulen equation?

The Vermeulen equation (Vermeulen et al., J Clin Endocrinol Metab 1999) is the most widely validated method for calculating free testosterone. It models the mass-action equilibria between testosterone, SHBG, and albumin using association constants: Ka(albumin) = 3.6 × 10⁴ L/mol and Ka(SHBG) = 5.97 × 10⁸ L/mol.

Why not just measure free testosterone directly?

Direct immunoassay methods for free testosterone are notoriously unreliable. Equilibrium dialysis is the gold standard but expensive and rarely available. The calculated Vermeulen method shows excellent correlation with equilibrium dialysis and is recommended by the Endocrine Society for clinical use.

What albumin value should I use if I don't have one?

A default of 4.3 g/dL is used when albumin is not measured. This is the population mean used in the original Vermeulen validation and is appropriate for most healthy adults. In liver disease or severe illness, use the actual measured value.

What are normal free testosterone ranges?

Normal ranges vary by laboratory and age. For adult men, free testosterone typically falls between 9–30 pg/mL (0.174–1.03 nmol/L). For adult women, 0.3–1.9 pg/mL (0.01–0.066 nmol/L). These are approximate — always compare against your laboratory's reference ranges.

Can this replace a physician's assessment?

No. This calculator provides an estimate based on the Vermeulen equation and should supplement, not replace, clinical evaluation. Testosterone interpretation requires consideration of symptoms, timing of sample collection (morning draw preferred), assay method, and individual clinical context.

Free Testosterone Calculator

Name: Free Testosterone Calculator
Availability: InStock
Author: Nham Vu

Enter total testosterone, SHBG, and albumin to calculate free and bioavailable testosterone using the validated Vermeulen equation.

Lab Values

Total Testosterone

Normal men: 8–29 nmol/L | Women: 0.5–2.6 nmol/L

SHBG (sex hormone-binding globulin)

nmol/L

Normal men: 10–57 nmol/L | Women: 18–144 nmol/L

Albumin (optional, default 4.3 g/dL)

g/dL

Normal: 3.5–5.0 g/dL. Leave blank to use 4.3 g/dL.

Enter your lab values and click Calculate.

Summary

Enter total testosterone, SHBG, and albumin to calculate free and bioavailable testosterone using the validated Vermeulen equation.

How it works

Enter your total testosterone level in nmol/L or ng/dL.
Enter your SHBG (sex hormone-binding globulin) value in nmol/L.
Enter your albumin level in g/dL (default 4.3 g/dL if not measured).
Click Calculate to see free and bioavailable testosterone.
Compare your results against the reference ranges shown.

Use cases

Interpret a testosterone lab panel when only total testosterone was reported.
Assess androgen status in men with symptoms of low testosterone despite normal total T.
Evaluate testosterone levels in women with PCOS or androgen excess.
Screen for testosterone deficiency when SHBG is elevated (e.g., liver disease, aging).
Monitor free testosterone trends during hormone replacement therapy.
Supplement direct free testosterone measurements, which are often inaccurate by equilibrium dialysis.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu