Free Testosterone Calculator
Most testosterone in your blood is bound to proteins — only the unbound ('free') fraction is biologically active. A calculated free testosterone, derived from total testosterone, SHBG, and albumin via the Vermeulen equation, is the standard way to estimate it without a specialized assay. Enter your three lab values below to get your free T and percent free T.
Morning draw is the standard
Sex hormone-binding globulin, from the same panel
If unknown, enter 4.3 — the population average; its effect is small
Enter your values above to see your results.
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How it works
The Vermeulen equation
The calculator solves the equilibrium between free, albumin-bound, and SHBG-bound testosterone using published association constants (SHBG ~1×10⁹ L/mol, albumin ~3.6×10⁴ L/mol). Because albumin binding is weak and unsaturable, an assumed albumin of 4.3 g/dL introduces only a small error when your actual value is unknown.
Why free T and not just total T?
SHBG can vary two- to three-fold between individuals (age, thyroid status, liver, insulin resistance, estrogens). Two men with the same total testosterone can have very different free fractions — which is why guidelines suggest calculated free T when total T is borderline or SHBG is suspected abnormal.
Limitations
Calculated free T assumes the published binding constants and a standard two-protein model. Equilibrium dialysis remains the reference method, and direct analog 'free T' assays are known to be inaccurate. One value is a snapshot — morning draws and repeat measurements give the truer picture.
Frequently asked questions
How do I calculate free testosterone from total testosterone and SHBG?
Enter your total testosterone, SHBG, and albumin above — the calculator applies the Vermeulen equation, which models how testosterone distributes between free, albumin-bound, and SHBG-bound states. If you don't know your albumin, 4.3 g/dL is a safe default.
What is a normal free testosterone level?
Calculated free T in adult men commonly falls around 9–30 ng/dL (roughly 1.5–3% of total T), but reference intervals vary by lab, age, and method — always compare against the range on your own report. A single value is a screening signal, not a diagnosis.
My total testosterone is in nmol/L — how do I convert?
Divide nmol/L by 0.0347 to get ng/dL (e.g. 17.3 nmol/L ≈ 500 ng/dL), or multiply ng/dL by 0.0347 to go the other way. SHBG is already reported in nmol/L on most panels, so no conversion is needed there.
Why does SHBG matter so much?
SHBG binds testosterone tightly — the more SHBG you have, the smaller your free fraction, even with the same total T. SHBG rises with age, thyroid excess, and estrogens, and falls with insulin resistance and obesity, which is why free T can differ so much between people with identical totals.
Is a calculated free testosterone a diagnosis of low T?
No. Diagnosis requires consistent symptoms plus repeated morning measurements, interpreted by a qualified clinician. Calculated free T is a well-validated estimate, but it is screening information, not a diagnosis.
Sources
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab, 1999.
- Bhasin S, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab, 2018.
This calculator is for informational purposes only and is not medical advice, diagnosis, or treatment. Reference ranges are general population guidelines — always discuss your lab results with a qualified healthcare professional.