Free Testosterone Calculator

Why Calculate Free Testosterone?

Total testosterone alone does not show how much hormone is biologically active. Most testosterone binds to sex hormone-binding globulin (SHBG) or albumin. Free testosterone is the unbound fraction available to tissues. Bioavailable testosterone includes free testosterone plus albumin-bound testosterone that dissociates readily.

The Vermeulen Equation

This calculator uses the Vermeulen mass-action model. Enter total testosterone in ng/dL, SHBG in nmol/L, and albumin in g/dL. The equilibrium is solved with albumin and SHBG association constants to derive free and bioavailable fractions:

$$[T_{total}] = [FT] + K_a \times [Alb] \times [FT] + \frac{K_s \times [SHBG] \times [FT]}{1 + K_s \times [FT]}$$

Bioavailable testosterone is calculated as:

$$[Bio\text{-}T] = [FT] \times (1 + K_a \times [Alb])$$

Where $K_a$ is the albumin association constant and $K_s$ is the SHBG association constant. The calculator solves the quadratic form of this equilibrium for free testosterone concentration.

Reading the Results

Free testosterone is reported in pg/mL with percent of total. Bioavailable testosterone is reported in ng/dL with percent of total. Reference ranges vary by laboratory method, age, and sex. Compare results to your lab's assay-specific intervals and clinical context.