Free Testosterone Calculator

Free testosterone is the small fraction (roughly 2–3%) of total testosterone that is not bound to any protein and is immediately available to cells. It is considered the most biologically active form, making it a more accurate indicator of androgenic activity than total testosterone alone.

Free testosterone is entirely unbound, while bioavailable testosterone includes both the free fraction and the portion weakly bound to albumin. The albumin-bound fraction can easily dissociate and become available to tissues, so both are biologically relevant. SHBG-bound testosterone, by contrast, is biologically inactive.

This calculator uses the Vermeulen equation (published in JCEM, 1999), which is widely regarded as the gold standard for estimating free testosterone from total testosterone, SHBG, and albumin without requiring an equilibrium dialysis measurement.

Normal free testosterone ranges vary by age and sex. For adult males, typical ranges are approximately 5–21 ng/dL (174–729 pmol/L). For adult females, normal levels are much lower, around 0.1–0.85 ng/dL. Always compare results with your laboratory's reference ranges and consult a healthcare professional.

If you don't have a measured albumin value, you can use the standard default of 4.3 g/dL, which is the population mean used in most clinical equations. Albumin levels are relatively stable and do not vary greatly between healthy adults.

Testosterone can be reported in ng/dL, ng/mL, or nmol/L depending on the laboratory. The calculator converts your input to nmol/L internally before applying the Vermeulen equation, so selecting the correct units for your lab report is essential for an accurate result.

Yes. The Vermeulen equation applies to both males and females. Testosterone levels in women are significantly lower, so ensure you enter the correct value and units. As always, results should be interpreted alongside a clinical consultation rather than in isolation.

No. This calculator is intended for informational and educational purposes only. The results are mathematical estimates and do not replace a diagnosis or treatment recommendation from a qualified healthcare professional. Always consult your doctor before making any health-related decisions.