Free Testosterone Calculator (Vermeulen Equation)

How it is calculated

Free T solves Kt·Kat-adjusted binding: FT = (−b + √(b² + 4a·T)) ÷ 2a, with a = Kt(1 + Kat·Alb), b = 1 + Kat·Alb + Kt(SHBG − T); Kt = 1 × 10⁹ L/mol, Kat = 3.6 × 10⁴ L/mol.

What your result means

• Calculated free testosterone depends on all three inputs, so a change in SHBG alone can move the result even when total testosterone is unchanged.
• Results differ between methods, calculated or measured, so compare like with like over time.
• Your own lab's printed range and your clinician's read of your history come first; this calculator is educational and is not a diagnosis.

Test the inputs

The Hormone Complete (Him) panel measures the values this calculator uses. Related: testosterone total, shbg, albumin.

Common questions

Why calculate free testosterone instead of measuring it?

Direct free testosterone assays vary considerably between methods; the Vermeulen calculation is a widely used, consistent alternative.

What does SHBG do?

SHBG binds testosterone tightly, so higher SHBG generally means a smaller free fraction.

Can I enter albumin in g/L?

Enter albumin in g/dL as most US reports print it; divide a g/L value by 10 first.

References

1. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab, 1999.
2. Implementation of Automated Calculation of Free and Bioavailable Testosterone in Epic Beaker Laboratory Information System. J Pathol Inform, 2017.