Most of the testosterone in blood is bound to carrier proteins and isn't immediately usable by tissues. This calculator applies the Vermeulen equation — the calculated-free-testosterone method most clinical guidelines prefer — to estimate the free and bioavailable fractions from a standard total testosterone, SHBG, and albumin panel.
How the Vermeulen equation works
Testosterone in blood exists in three states: about 1–3% circulates completely free, roughly 40–50% is loosely bound to albumin, and the rest is tightly bound to sex hormone-binding globulin (SHBG). Because SHBG binds testosterone with far higher affinity than albumin does, and because SHBG itself is present in limited, saturable quantities, the relationship between total testosterone and free testosterone isn't a fixed ratio — it depends on how much SHBG is present.
The Vermeulen equation solves this with the law of mass action: it sets up the binding equilibria between testosterone, SHBG, and albumin using their known association constants, and solves the resulting quadratic equation for the free testosterone concentration. This calculated method was validated against the gold-standard equilibrium dialysis technique and is now the calculated-free-T approach most widely used in clinical practice and research, since direct free-testosterone immunoassays are known to be unreliable — especially when SHBG is abnormal.
Inputs and what they mean
Total testosterone is the standard lab measurement, entered in ng/dL (the conventional US unit) or nmol/L. SHBG is entered in nmol/L, the standard reporting unit, and must be greater than zero. Albumin is optional — if you don't have a recent value, the calculator uses the standard default of 4.3 g/dL, which is what the original equation and most online calculators assume when albumin isn't available. Entering your actual albumin (from a comprehensive metabolic panel) improves accuracy, particularly if it's abnormal.
The result includes the Free Androgen Index (FAI) as a secondary cross-check — a simpler, older ratio (total T ÷ SHBG × 100) that some labs still report but that ignores albumin binding entirely, making it less precise than the full Vermeulen calculation.
Limits and edge cases
A calculated free testosterone value is a screening and monitoring tool, not a standalone diagnosis. Diagnosing hypogonadism requires clinical correlation: consistent symptoms (fatigue, low libido, erectile dysfunction, loss of muscle mass, etc.) plus a confirmed low result on a repeat morning fasting sample — testosterone follows a strong diurnal rhythm and peaks in the early morning — evaluated alongside LH, FSH, and the full clinical picture by a qualified clinician.
No universally validated, age-stratified reference interval exists for the Vermeulen calculated method; published ranges vary meaningfully by lab, assay, and population studied. This calculator's reference-range tab presents illustrative, commonly cited figures rather than a single diagnostic cutoff — always compare your result to your own lab's reported reference range first. Extreme or physiologically implausible inputs (very low albumin from severe liver disease or nephrotic syndrome, for example) can also reduce the accuracy of any calculated-free-T formula, including this one.