Free Testosterone Calculator
Calculate free and bioavailable testosterone levels using the Vermeulen formula. Enter total testosterone, SHBG, and albumin for accurate results with charts and clinical interpretation.
About This Calculator
The Free Testosterone Calculator estimates your free and bioavailable testosterone levels using the validated Vermeulen formula. This tool is designed for individuals and healthcare professionals who want to assess androgen status beyond total testosterone measurements. Free testosterone — the fraction not bound to transport proteins — is the biologically active form that enters cells and drives physiological effects.
The calculator uses the Vermeulen equation published in the Journal of Clinical Endocrinology & Metabolism (1999), which is the most widely adopted method for estimating free testosterone from total testosterone, sex hormone-binding globulin (SHBG), and albumin levels. The formula accounts for the binding affinities of SHBG and albumin, providing an accurate approximation of free and bioavailable fractions that correlates well with equilibrium dialysis measurements.
Regional Notes
India (IN): Testosterone reference ranges may vary across Indian laboratories. Total testosterone 8.7–33 nmol/L is the typical adult male range. Free testosterone by calculation is commonly used when direct free T testing via equilibrium dialysis is unavailable. Many Indian labs offer free testosterone by calculation using the Vermeulen formula directly.
United States (US): Free testosterone testing is widely available via direct methods (equilibrium dialysis or analog immunoassay) and calculated methods. The Endocrine Society recommends calculated free testosterone using the Vermeulen formula as a reliable alternative when direct measurement is not available. Reference ranges: free T 50–210 pg/mL (0.174–0.792 nmol/L) for adult males.
United Kingdom (UK): The UK guidelines for testosterone deficiency diagnosis emphasize calculated free testosterone using total testosterone and SHBG. NHS laboratories commonly report calculated free testosterone values. Testing should be performed between 7-10 AM due to circadian variation. Free T below 0.225 nmol/L may indicate hypogonadism in younger men.
Units note: Enter total testosterone in nmol/L. For conversion: 1 nmol/L = 28.85 ng/dL. This calculator assumes standard binding constants (Ka = 3.6 × 10⁴ M⁻¹ for albumin, Kshbg = 10⁹ M⁻¹ for SHBG) and molar mass of albumin = 66,500 g/mol.
Frequently Asked Questions
What is the difference between total and free testosterone?
Total testosterone measures all testosterone in your blood, including bound and unbound forms. Free testosterone is the small fraction (1-4%) not bound to proteins, which is biologically active and available for your cells to use. Free testosterone is often a better indicator of androgen status than total testosterone alone.
How is free testosterone calculated?
This calculator uses the Vermeulen formula, published in 1999 in the Journal of Clinical Endocrinology & Metabolism. It calculates free testosterone from total testosterone, SHBG (sex hormone-binding globulin), and albumin using their binding affinities. The formula accounts for the competitive binding dynamics between these transport proteins.
What are normal free testosterone levels?
Normal free testosterone levels for adult males range from 0.174 to 0.792 nmol/L (50-210 ng/L). For females, normal levels are typically below 0.0295 nmol/L (8.5 ng/L). Reference ranges may vary by laboratory and testing method.
What is bioavailable testosterone?
Bioavailable testosterone is the sum of free testosterone and albumin-bound testosterone. Albumin binds testosterone weakly, making it readily available to tissues. Together, free and albumin-bound fractions represent the testosterone that can enter cells and exert biological effects. SHBG-bound testosterone is not bioavailable.
When should I get a free testosterone test?
Free testosterone testing is recommended when total testosterone results are borderline or when symptoms of low testosterone persist despite normal total levels. Common reasons include low libido, erectile dysfunction, fatigue, infertility, or abnormal sexual development in boys. Testing should be done in the morning (7-10 AM) when levels are highest.
Can I rely on this calculator for diagnosis?
No, this calculator is for educational and reference purposes only. It provides an estimation based on the Vermeulen formula, which is a validated approximation used in clinical research. Results should not be used as a basis for any medical decision without consulting a healthcare professional.
How accurate is the Vermeulen formula?
The Vermeulen formula is the most widely validated method for calculating free testosterone from total testosterone, SHBG, and albumin. Studies show it correlates well with equilibrium dialysis (the gold standard measurement method), though it may slightly underestimate free testosterone in certain clinical conditions.
What factors affect SHBG and testosterone levels?
SHBG levels are influenced by age, obesity, liver disease, thyroid disorders, diabetes, and medications. Testosterone naturally declines with age after 30. High SHBG can reduce free testosterone even when total testosterone is normal, a condition often seen in aging men, while low SHBG (common in obesity and diabetes) can elevate free testosterone.