HEALTH · FREE TESTOSTERONE CALCULATOR (VERMEULEN EQUATION)

Free Testosterone Calculator (Vermeulen Equation)

Calculate free and bioavailable testosterone from total T, SHBG, and albumin using the validated Vermeulen equation (J Clin Endocrinol Metab 1999).

Lab Values

Sex hormone-binding globulin from blood test

Default 4.3 g/dL (Vermeulen standard)

02 Vermeulen Equation
Free Testosterone
103.8 pg/mL
2.08% of total T is free
Free testosterone103.8 pg/mL
Bioavailable testosterone252.06 ng/dL
Free fraction2.08%
Total T (nmol/L)17.34 nmol/L

Calculated using the Vermeulen equation (J Clin Endocrinol Metab 1999). Interpretation of results requires clinical context — consult your physician.

About This Calculator

Total testosterone measured by a blood test includes testosterone bound to sex hormone-binding globulin (SHBG), loosely bound to albumin, and a small unbound ("free") fraction. Only the free and albumin-bound (bioavailable) fractions are biologically active. This calculator uses the Vermeulen equation — the reference standard for computed free testosterone — to estimate these fractions from routine lab values.

How It Works

Enter your total testosterone (in ng/dL or nmol/L), SHBG (nmol/L), and albumin (g/dL, default 4.3 per the Vermeulen paper). The calculator applies the Vermeulen equilibrium equation to compute free testosterone in pg/mL and bioavailable testosterone in ng/dL, along with the free fraction percentage.

The Formula

freeT = totalT / (1 + Ka × [Alb] + Kb × [SHBG]) bioavailT = freeT + Ka × [Alb] × totalT / (1 + Ka × [Alb] + Kb × [SHBG])

totalT
total testosterone (molar concentration, nmol/L after conversion)
Ka
albumin association constant (3.6 × 10⁴ L/mol)
Kb
SHBG association constant (5.97 × 10⁸ L/mol)
[Alb]
albumin molar concentration (derived from g/dL, MW = 66,500 g/mol)
[SHBG]
SHBG molar concentration (nmol/L converted to mol/L)

Frequently Asked Questions

Why use the Vermeulen equation instead of measured free T?
Analog immunoassay kits for "measured" free T are notoriously inaccurate — their readings depend on immunoassay artifacts rather than true equilibrium. Equilibrium dialysis is the gold standard but expensive and rarely available. The Vermeulen equation, validated against equilibrium dialysis, is widely recommended by endocrinology guidelines (Endocrine Society, ISSAM) as the preferred calculated free T method.
What is a normal free testosterone range?
Free T reference ranges vary by laboratory and methodology. Typical adult male ranges are 50–210 pg/mL (free) and 130–680 ng/dL (bioavailable), but age, time of day, and the assay method all matter. Discuss your specific results with your physician — this calculator provides an estimate for educational purposes.
My albumin isn't on my lab report — what do I enter?
The default value of 4.3 g/dL is the Vermeulen paper's standard assumption and is appropriate for most adults without liver disease, kidney disease, or severe malnutrition. If you have a measured albumin value, enter it for a more accurate result.
Why does ng/dL vs nmol/L change the result slightly?
The unit conversion uses the factor 0.03467 (ng/dL → nmol/L). Minor floating-point rounding in the conversion may cause sub-1% differences between equivalent inputs expressed in different units. Both are well within the precision of the clinical measurement.