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Calculator · 01

Free & Bioavailable Testosterone Calculator

Most lab reports hand you one number: total testosterone. Only a small fraction of it is active. This tool estimates your free and bioavailable testosterone from three values on a standard panel, using the Vermeulen equation clinicians and researchers rely on.1

This is not a diagnosis. A calculation is not a test result. A normal total can still hide a low free fraction, and the only way to confirm anything is a lab-grade (LC-MS/MS) panel read by a clinician.2
Your lab values
The headline number on your blood test.

Sex hormone binding globulin. It controls how much testosterone is bound versus free.

If it’s not on your panel, leave the default of 4.3 g/dL.

About this calculation

This uses the Vermeulen equation (Vermeulen, Verdonck & Kaufman, 1999)1, the standard method for estimating free testosterone from total testosterone, SHBG and albumin. It models how testosterone binds to SHBG and albumin, then solves for the free fraction.

Constants used: SHBG association constant Kₜ = 1.0 × 10⁹ L/mol, albumin association constant Kₐ = 3.6 × 10⁴ L/mol, testosterone molar mass 288.4 g/mol, albumin molar mass 69,000 g/mol. Default albumin is 4.3 g/dL when not entered.

A calculated free testosterone is an estimate. It tracks measured (equilibrium dialysis) values well across most men but can differ at extreme SHBG levels. Use it to understand your numbers, not to diagnose yourself.

Sources

  1. Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666–72. PubMed ↩
  2. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715–44. DOI ↩