Free & bioavailable testosterone calculator

How this calculator works

Testosterone in blood exists in three states: bound tightly to sex hormone-binding globulin (SHBG), bound loosely to albumin, and unbound (free). Only the free fraction, plus the albumin-bound fraction that dissociates quickly enough to act on tissue, is considered biologically available. Total testosterone alone does not show this split.

Vermeulen's 1999 equation solves for free testosterone using the law of mass action: given total testosterone, SHBG, albumin, and the known binding affinities of SHBG (association constant 1 × 10⁹ L/mol) and albumin (apparent association constant 3.6 × 10⁴ L/mol) for testosterone, it calculates the unbound concentration that satisfies the binding equilibrium. Bioavailable testosterone adds back the albumin-bound fraction.

What you'll need

A recent blood draw reporting total testosterone and SHBG, both usually included on a standard hormone panel. Albumin comes from a basic metabolic panel or comprehensive metabolic panel; if you don't have a recent value, the calculator can use a typical average for a healthy adult.

Limitations

This is a calculated estimate, not a direct measurement. It assumes the standard binding affinities from Vermeulen's original paper, which were derived from adult male serum and may be less accurate in unusual clinical states that alter SHBG or albumin binding behavior, such as severe liver or thyroid disease. Equilibrium dialysis remains the reference method, but it is rarely available outside research labs. This tool does not apply a "low," "normal" or "optimal" label to the result, since reference ranges vary by lab and by the assay used for the original total testosterone measurement.

Frequently asked questions

What is the difference between total, free and bioavailable testosterone?

Total testosterone is everything in the blood: testosterone bound tightly to SHBG, testosterone loosely bound to albumin, and a small unbound fraction. Free testosterone is only the unbound fraction. Bioavailable testosterone is free testosterone plus the albumin-bound portion, since that portion releases easily enough to reach tissue.

Why does SHBG matter if total testosterone looks normal?

SHBG binds testosterone tightly, so a high SHBG level can pull free testosterone down even when total testosterone sits in a normal range. This calculator exists because total testosterone alone can miss that pattern.

What is the Vermeulen method?

It is a calculation published by Vermeulen and colleagues in 1999 that estimates free and bioavailable testosterone from total testosterone, SHBG and albumin using the known binding affinities of each protein for testosterone, rather than requiring a separate direct assay.

Is a calculated free testosterone as accurate as a direct lab measurement?

Equilibrium dialysis, the direct reference method, is more accurate but expensive and rarely run outside research settings. The Vermeulen calculation is the most widely validated estimate and correlates well with equilibrium dialysis in most published comparisons, which is why it is used clinically and in research.

What albumin value should I use if I do not have a lab result?

Vermeulen's original paper used a default of 4.3 g/dL when albumin was not separately measured, since albumin varies little in healthy adults. Use that default only as a rough estimate, and use your own lab value whenever you have one.