Most circulating testosterone is bound to SHBG (inactive) or loosely bound to albumin (weakly active) — only the free fraction is fully bioavailable. This calculator uses the Vermeulen equation, the same equilibrium-binding method referenced by major labs, to estimate free and bioavailable testosterone from three lab values.
Free & bioavailable testosterone calculator
A one-off calculation from lab values you enter — nothing here is saved or sent anywhere.
If albumin wasn't measured, 4.3 g/dL is a commonly used population-average default — using your own measured value is more accurate.
How to Use This Calculator
Three values from a standard hormone panel with SHBG and albumin included.
Enter your total testosteroneFrom your lab report, in either ng/dL (common in the US) or nmol/L (common internationally).
Enter your SHBGSex hormone-binding globulin, in nmol/L — this is the value that drives most of the difference between total and free testosterone.
Enter your albuminFrom the same panel if measured, or use the population-average default of 4.3 g/dL if it wasn't.
Read your estimated free and bioavailable testosteroneAlong with the free androgen index (FAI), a simpler but less reliable alternative ratio.
How the Math Works
The Vermeulen equation models testosterone's equilibrium binding to SHBG and albumin using the law of mass action — validated against equilibrium dialysis (the gold-standard direct measurement method) with strong correlation.
The equation. Free testosterone is found by solving a quadratic derived from the mass-action equilibrium between free, albumin-bound, and SHBG-bound testosterone: with N = 1 + Ka×[Albumin], a = N×Ks, b = N + Ks×([SHBG] − [Total T]), c = −[Total T] (all concentrations in mol/L), Free T = (−b + √(b² − 4ac)) ÷ (2a). 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):3666-3672.
The binding constants used. SHBG-testosterone association constant (Ks) = 1.0 × 10⁹ L/mol; albumin-testosterone association constant (Ka) = 3.6 × 10⁴ L/mol — the same constants used by Mayo Clinic Laboratories and the ISSAM (International Society for the Study of the Aging Male) reference calculator. Note some published implementations use a slightly different Ks (as low as 5.97 × 10⁸ L/mol), which is why results can differ slightly between calculators. Vermeulen et al. 1999; constants as implemented by Mayo Clinic Laboratories and ISSAM.
Bioavailable testosterone. Bioavailable T = free T + albumin-bound T, representing the fraction not tightly locked to SHBG and therefore considered more physiologically available than total T alone. Manni A, Pardridge WM, Cefalu W, et al. "Bioavailability of albumin-bound testosterone." J Clin Endocrinol Metab. 1985;61:705.
Free Androgen Index (FAI). FAI = (Total T ÷ SHBG) × 100, shown for reference as a much simpler but less reliable alternative — it tends to overestimate free testosterone at low SHBG levels and doesn't account for albumin binding at all. Free androgen index, standard clinical ratio; known limitations documented in comparative studies of calculated free testosterone methods.
Estimate only — not medical advice. This is a calculated estimate from an equilibrium-binding model, not a direct measurement — it can differ from your lab's own "free testosterone, direct" result or from equilibrium dialysis, and different online calculators may show slightly different results depending on which binding constants they use. This is not a diagnosis of low testosterone or any other condition. Interpret results with a licensed clinician alongside your full clinical picture and symptoms.
Frequently Asked Questions
Why isn't total testosterone enough on its own?
Total testosterone includes testosterone bound to SHBG, which is biologically inactive. Two people can have identical total testosterone but very different free testosterone if their SHBG levels differ — someone with high SHBG can have a "normal" total T but a low free T, and vice versa.
What's the difference between free and bioavailable testosterone?
Free testosterone is the small unbound fraction (typically 1-3%) available to tissues immediately. Bioavailable testosterone adds the albumin-bound fraction, which binds loosely enough to still be considered readily available — so bioavailable T is always higher than free T alone.
Why do I need my SHBG value for this calculation?
SHBG binds testosterone far more tightly than albumin does (roughly 28,000 times more tightly, per the association constants), so SHBG is the dominant factor determining how much testosterone stays free. Without a measured SHBG value, this calculation can't be performed meaningfully.
What if I don't know my albumin level?
The calculator defaults to 4.3 g/dL, a commonly cited population average for healthy adults. Albumin has a much smaller effect on the result than SHBG does, so this default introduces relatively little error for most people — but your own measured value is always more accurate if available.
Why might this calculator's result differ from my lab's reported "calculated free testosterone"?
Different labs and calculators sometimes use slightly different published association constants (particularly for Ks, the SHBG-binding constant) or different assumptions about albumin's molecular weight during unit conversion — small implementation differences that can shift results modestly.
Is the Vermeulen equation more accurate than a direct free testosterone assay?
Studies comparing methods generally find the Vermeulen calculation correlates well with equilibrium dialysis (the gold-standard direct method) and is often considered more reliable than the common direct analog immunoassay many commercial labs use — particularly when SHBG is abnormal. Equilibrium dialysis itself remains the reference standard.
What is the Free Androgen Index, and why is it shown as "less reliable"?
FAI is a simple ratio (total T divided by SHBG) that doesn't account for albumin binding at all, and research has shown it significantly overestimates free testosterone specifically when SHBG is low — which is exactly the scenario where getting an accurate free T estimate matters most.
Can this calculator diagnose low testosterone?
No. It estimates a lab value from other lab values — it doesn't evaluate your symptoms, medical history, or apply any diagnostic threshold. Diagnosing low testosterone (hypogonadism) requires clinical judgment from a licensed clinician, typically alongside repeat morning testing.
Does this tool save my lab values anywhere?
No — everything runs locally in your browser and nothing is stored or transmitted. To track lab values over time and see trend charts, see the full Peptide Plug app, which has a dedicated Lab Tracking feature.
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