Free Testosterone Calculator: Transparent Estimate
Estimate the unbound testosterone fraction using total testosterone, SHBG and albumin. It is not a measured laboratory result or a diagnostic test.
Method, assumptions and limits
We solve the Vermeulen mass-action quadratic. Constants: albumin binding 3.6 × 10⁴ L/mol; SHBG binding 1 × 10⁹ L/mol; albumin molar mass 69,000 g/mol; testosterone molar mass 288.42 g/mol. Total ng/dL converts to nmol/L by ×10/288.42; free nmol/L converts to pg/mL by ×288.42.
Sources: Vermeulen et al. (1999) and the University Hospital Ghent / ISSAM calculator. Assay methods, binding assumptions and some clinical circumstances affect applicability. No diagnostic reference interval or low/normal/high category is assigned.
Correction October 19, 2026: replaced the earlier incorrect iteration and unit conversion. Earlier results should not be used. This implementation has mathematical checks and external sample comparisons, not independent clinical validation. Ask your clinician to interpret results.
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Testosterone testing costs · TRT first-year costs · TRT insurance questions
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Method and interface checked October 19, 2026. Next scheduled review: January 19, 2027. Written and maintained by The Health Architect, a non-clinician researcher. Research and correction standards.