Free Testosterone Calculator

Estimate your calculated free and bioavailable testosterone from total testosterone, SHBG, and albumin using the Vermeulen 1999 equation, the most validated of the simple calculation methods. Most testosterone in blood is bound and not biologically active, so the free fraction can tell a different story than the total. Informational reference using a published formula, not a diagnostic test.

Total testosterone

nmol/L

SHBG (sex hormone-binding globulin)

nmol/L

Albumin (optional: defaults to 43 g/L)

g/L

Calculated Free Testosterone

This is a calculated estimate from the Vermeulen equation, not a directly measured free testosterone. Reference ranges differ markedly by sex, age, and laboratory, and are not standardised for the calculated method, so this tool reports the value without categorising it for women. Discuss your result with a healthcare provider.

Tracking your hormones? See your testosterone and SHBG trends over time in the Health3 app.

Why Free Testosterone Differs From Total

Most of the testosterone circulating in your blood is not free to act. Roughly two-thirds is bound tightly to sex hormone-binding globulin (SHBG), and most of the rest is bound loosely to albumin. Only a small percentage (typically 1–3% in men) is truly free. The "bioavailable" fraction is the free testosterone plus the loosely albumin-bound testosterone, since the albumin-bound portion can dissociate and become available to tissues.

Because conditions that change SHBG, such as ageing, obesity, thyroid disease, and liver disease, shift the balance between bound and free testosterone, the total can look normal while the free fraction is low, or vice versa. Calculating free testosterone from total testosterone, SHBG, and albumin gives a more complete picture when SHBG is abnormal. The most validated simple method is the Vermeulen equation (Vermeulen et al., 1999, J Clin Endocrinol Metab 84:3666–3672), which solves the law-of-mass-action binding equilibria.

Vermeulen equation (1999): a quadratic derived from the binding equilibria of testosterone with SHBG and albumin.
Association constants: SHBG Kₙ = 1×10⁹ L/mol, albumin Kₜ = 3.6×10⁴ L/mol. Albumin is converted from g/L to mol/L using a molar mass of 69,000 g/mol; when albumin is not entered, 43 g/L (4.3 g/dL) is assumed. Free testosterone (mol/L) = [−b + √(b² + 4·a·T)] ÷ 2a, then converted to nmol/L and ng/dL.

Published Reference Context (Men)

There are no universally standardised age-specific reference intervals for the calculated method. The orientation below reflects ranges reported in the literature for adult men; female interpretation requires different ranges and clinical framing and is not categorised here.

Calculated free testosterone (men)	nmol/L	ng/dL
Below commonly reported range	Below ~0.225	Below ~6.5
Within commonly reported range	~0.225 – 0.785	~6.5 – 22.6
Above commonly reported range	Above ~0.785	Above ~22.6

Conversion used: free testosterone ng/dL = nmol/L × 28.84.

Limitations

It is a calculation, not a measurement. The gold standard is equilibrium dialysis; the Vermeulen estimate correlates well with it but is not identical.
Assay variability. Total testosterone and SHBG assays differ between labs, and errors propagate into the calculated free value.
Reference ranges are not standardised. Cutoffs vary by sex, age, time of day of the draw, and laboratory. Morning samples are standard for testosterone.

Explore Related Topics

Total Testosterone Free Testosterone (biomarker) DHEAS Hormonal Balance

Frequently Asked Questions

What is the Vermeulen equation?

The Vermeulen equation, published by Vermeulen and colleagues in 1999, estimates free and bioavailable testosterone from total testosterone, SHBG, and albumin by solving the law-of-mass-action binding equilibria. It is the most validated of the simple calculation methods and correlates well with equilibrium dialysis, the gold standard. It uses association constants of 1x10^9 L/mol for SHBG and 3.6x10^4 L/mol for albumin.

What is the difference between free and bioavailable testosterone?

Free testosterone is the small fraction (about 1 to 3% in men) that is completely unbound. Bioavailable testosterone is the free fraction plus the loosely albumin-bound testosterone, because the albumin-bound portion can dissociate and reach tissues. SHBG-bound testosterone is held too tightly to be readily available. This calculator reports both.

Do I need to enter albumin?

No. Albumin has a relatively small effect on the result, so when it is not entered the calculator assumes a normal 43 g/L (4.3 g/dL), following the standard Vermeulen convention. If you have a measured albumin, entering it makes the estimate slightly more precise, which can matter in liver disease or other states with abnormal albumin.

Why is calculated free testosterone better than total in some cases?

Conditions that change SHBG, such as ageing, obesity, thyroid disease, and liver disease, shift the balance between bound and free testosterone. When SHBG is abnormal, total testosterone can look normal while the active free fraction is low, or the reverse. Calculating free testosterone from total, SHBG, and albumin gives a fuller picture in those situations.

What is a normal free testosterone level?

For adult men, literature ranges for calculated free testosterone are roughly 0.225 to 0.785 nmol/L (about 6.5 to 22.6 ng/dL), but there is no universally standardised reference interval for the calculated method, and values vary by age, time of day, and laboratory. Female interpretation requires different ranges. These are reference points, not a diagnosis; discuss your result with your clinician.

Medical Disclaimer: This tool estimates free and bioavailable testosterone using the Vermeulen equation (Vermeulen A, Verdonck L, Kaufman JM. J Clin Endocrinol Metab. 1999;84(10):3666–3672). The calculated value is an estimate, not a direct measurement (equilibrium dialysis is the gold standard), and reference ranges are not standardised across labs, ages, or sexes. This tool is for informational and educational use only and does not diagnose any condition. Always consult a qualified healthcare provider to interpret your hormone results.

Track Your Hormone Markers Over Time

Health3 logs your total testosterone, SHBG, albumin, and 50+ other biomarkers, shows trends, and keeps your lab reports in one place.