Free testosterone calculator (Vermeulen)
Total testosterone does not tell you how much of the hormone is available to tissues, because part of it is bound to SHBG and albumin. The Vermeulen formula estimates the free fraction. You need total testosterone, SHBG and albumin from the same test.
Calculations run in your browser. We do not send or store anything.
Result
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How we calculate it
The Vermeulen formula (1999) is a binding equilibrium model: testosterone binds to SHBG (constant 10⁹ L/mol) and more weakly to albumin (3.6 × 10⁴ L/mol), and the free fraction is obtained by solving a quadratic equation. We assume albumin of 43 g/L if you do not know it. Testosterone in ng/dL is converted to nmol/L by dividing by 28.84.
The thresholds are indicative: total testosterone below 8 nmol/L, 8–12 nmol/L, and free testosterone below 0.225 nmol/L (EAU). The Endocrine Society guidelines (2018) base the diagnosis on symptoms and on at least two measurements taken in the morning, fasting.
Limitations
- This is a calculated value, not a measured one. It differs from a measurement by equilibrium dialysis, and direct “free testosterone” assays from analyzers can be unreliable.
- Diagnosing testosterone deficiency requires symptoms and repeated morning fasting measurements. A single result, illness, lack of sleep and some medications change the level.
- This tool is not meant for deciding whether to start hormone treatment. That decision belongs to a doctor.
Sources
Read more
- Testosterone Test: What Time to Have Blood Drawn and Why You Must Repeat the Result
- Low Testosterone After 40 — Normal Aging or a Problem Worth Treating?
- Low Testosterone: 12 Symptoms Men Often Don't Connect to Their Hormones
- Total vs. Free Testosterone — Which Result Actually Matters?
- SHBG (Sex Hormone-Binding Globulin)
- How to Interpret Testosterone, SHBG, and Albumin Together?
- Why Can Free Testosterone Be Low Despite Normal Total Testosterone?