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Free Testosterone Calculator

Calculate free and bioavailable testosterone from total T, SHBG, and albumin using the validated Vermeulen formula — the same equation used by the ISSAM calculator and cited in Endocrine Society guidelines. Built and reviewed by Dr. Nikola Topalovic, MD PhD.

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Use a fasted, before-9am draw for a clean read. SHBG and albumin should be from the same panel.

UK / EU / AU labs usually report nmol/L. US labs report ng/dL.

Typical adult-male range: 18–55 nmol/L.

Default 43 g/L (clinical mean). Vermeulen results are insensitive to small albumin changes.

Enter total T and SHBG above to see your calculated free testosterone.

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What free testosterone actually tells you

Total testosterone is a poor stand-alone marker. Roughly 98% of circulating testosterone is bound — tightly to SHBG (sex hormone binding globulin) and loosely to albumin. Only the unbound (free) fraction crosses cell membranes and binds the androgen receptor.

That's why two men with identical total T can have very different symptoms. If your SHBG is high — common in lean men, with thyroid issues, or on certain medications — your free T can be deep in the symptomatic range while your total T looks reassuringly normal on a standard panel.

The Vermeulen formula (Vermeulen et al., 1999) solves a quadratic for free T from total T, SHBG, and albumin using published binding constants. It correlates closely with the equilibrium-dialysis gold standard and is the method cited by the Endocrine Society's male hypogonadism guidelines and the ISSAM online calculator.

Typical reference ranges

MarkerTypical adult-male rangeNotes
Total testosterone10–35 nmol/L (288–1009 ng/dL)Wide range — context matters
SHBG18–55 nmol/LHigh in lean men; low in metabolic syndrome
Free T (Vermeulen)200–600 pmol/L (5.8–17.3 ng/dL)Below ~180–220 pmol/L often symptomatic
Bioavailable T3.5–13 nmol/LFree + albumin-bound fraction

Ranges are reference ranges for healthy adult men. Your lab may report slightly different cutoffs. Clinical interpretation always needs the full panel and symptom context — not a single number.

When to retest

Hormone markers swing. Before acting on a low or borderline free T number, confirm with a second draw: fasted, before 9am, no training that morning, no acute illness in the prior week. A single result is a snapshot — a repeat in 2–4 weeks on the same conditions is the floor for any clinical decision.

If both reads are low, the next step is a full hormone workup: total T, free T, SHBG, sensitive estradiol, LH, FSH, prolactin, DHEA-S, plus thyroid (TSH, free T4), fasting insulin, HbA1c, and a metabolic / lipid panel. That's what tells you whether the issue is primary (testicular), secondary (pituitary), or driven by something fixable upstream — sleep, body comp, training load, medications, or metabolic dysfunction.

FAQs

What is free testosterone?
Free testosterone is the fraction of total testosterone not bound to SHBG or albumin. It's the portion the body can actually use — and it explains why men with normal total T can still have low-T symptoms when SHBG is elevated.
What formula does this calculator use?
The Vermeulen equation — the most widely-validated method in clinical endocrinology, used by the ISSAM free testosterone calculator and cited in Endocrine Society guidelines. It solves a quadratic for free T from total T, SHBG, and albumin using published binding constants.
What's a normal free testosterone level?
Vermeulen-calculated free T in healthy adult men typically falls between 200–600 pmol/L (5.8–17.3 ng/dL). Below 180–220 pmol/L is generally considered low and often correlates with symptoms even when total T looks normal.
Why does SHBG matter so much?
SHBG binds testosterone tightly. High SHBG (common in lean men, thyroid issues, or with some medications) drops bioavailable T even when total T is fine. Low SHBG (common in metabolic syndrome and insulin resistance) raises free T but is itself a red flag for metabolic dysfunction.
Should I rely on calculated free T or measured free T?
Direct free T assays (analog, RIA) are unreliable. Equilibrium dialysis is the gold standard but rarely available. Vermeulen-calculated free T from total T + SHBG + albumin correlates strongly with equilibrium dialysis and is the recommended clinical method.
I got a low number — what should I do?
A single calculation isn't a diagnosis. Free T varies with time of day, recent illness, training load, and sleep. Confirm with a full hormone panel (total T, free T, SHBG, sensitive estradiol, LH, FSH, prolactin) drawn fasted before 9am, then have it interpreted by a clinician familiar with men's health.

Calculator suggesting low free T? Get it reviewed.

Send your numbers. Dr. Nikola Topalovic, MD PhD replies within 24h with what to do next — TRT, enclomiphene, or none of the above.

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Medical disclaimer. This calculator is an educational tool, not a medical diagnosis. Vermeulen-calculated free T is one input among many. Decisions about testosterone replacement, fertility, or any other clinical pathway require a full workup and a qualified clinician. References: Vermeulen A, et al. J Clin Endocrinol Metab 1999;84(10):3666–3672. Bhasin S, et al. Endocrine Society Clinical Practice Guideline 2018.

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