Free Testosterone Calculator (Vermeulen)

Free and bioavailable testosterone fractions from the Vermeulen 1999 binding model. Results require method-specific reference intervals and clinical context.

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Result

Free testosterone: 0.298 nmol/L (85.8 pg/mL)
Interpretation: At or above the illustrative 0.25 nmol/L threshold; this does not establish normality
Free fraction: 1.98 %
Bioavailable testosterone: 6.97 nmol/L (46.5 %)
Total testosterone: 15.00 nmol/L (432 ng/dL)

Vermeulen 1999 model. For men, 0.225 and 0.25 nmol/L are illustrative thresholds, not universal reference limits. Diagnosis requires symptoms and repeated measurements; this calculation does not prescribe treatment. Reference ranges are population benchmarks from the cited sources. Laboratories use their own ranges, and interpretation depends on clinical context: discuss the result with your physician.

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How it is calculated

Only 1–3% of blood testosterone is free, about 40–50% is tightly bound to sex hormone-binding globulin (SHBG), and the rest loosely to albumin. The free and albumin-bound fractions ("bioavailable testosterone") are biologically active. Direct measurement of free testosterone (equilibrium dialysis) is expensive and scarce, and immunoassays are inaccurate, so ISSAM, the Endocrine Society and the EAU recommend the Vermeulen calculation (1999): it solves the binding equilibrium with association constants of 1×10⁹ L/mol for SHBG and 3.6×10⁴ L/mol for albumin. The method matters most when SHBG is high (age, hyperthyroidism, liver disease, estrogens) or low (obesity, insulin resistance, hypothyroidism), when total testosterone misleads.

Formula

N = Kalb × [Albumin] + 1; a = N × Kshbg; b = N + Kshbg × ([SHBG] − [T]) Free T = (−b + √(b² + 4·a·[T])) / (2·a) Bioavailable T = Free T × N Kshbg = 1×10⁹ L/mol; Kalb = 3.6×10⁴ L/mol; concentrations in mol/L; albumin g/L / 69,000 Conversion: T ng/dL × 0.0347 = nmol/L; free T nmol/L × 288.4 = pg/mL

Limits of the method

The calculation is valid when total testosterone is measured by an accurate method (LC-MS/MS or a calibrated immunoassay) in the morning between 7 and 11 a.m. fasting, twice several weeks apart. Abnormal albumin shifts the result; in pregnancy and on oral contraceptives SHBG changes sharply. Free testosterone references depend on method and age; the thresholds below apply to men — for women the calculator shows values without a category. A diagnosis of hypogonadism requires symptoms and an in-person work-up.

Sources of the method

This calculation is for reference only and is not medical advice, a diagnosis or a prescription. If you have a medical condition, are pregnant, take medication or have any doubts, discuss the result with a physician or a qualified nutrition professional.

How to calculate free testosterone

Test total testosterone and SHBG in the morning

Testosterone peaks between 7 and 10 a.m. and falls 20–30% by evening. Test fasting, outside acute illness, preferably by LC-MS/MS.

Add albumin

Use measured albumin in g/L. The initial 43 g/L is an example; substituting it for a measurement adds uncertainty.

Look at the free fraction when SHBG is atypical

When SHBG changes, total testosterone and its free fraction may suggest different interpretations. Consider both alongside symptoms, the assay method and repeated measurements.

Free testosterone worked examples

Worked example0.298 nmol/L

Total T 15 nmol/L, SHBG 35, albumin 43

Free testosterone is about 85.8 pg/mL (1.98%), with bioavailable testosterone 6.97 nmol/L. This numerical example does not confirm health or exclude deficiency.

The calculation does not establish the cause of symptoms.

Higher SHBG0.126 nmol/L

Total T 11 nmol/L, SHBG 72, albumin 43

With total testosterone 11 nmol/L and SHBG 72 nmol/L, the free fraction is 1.14%, about 36.3 pg/mL. Clinical significance depends on references and symptoms.

A single calculated result does not establish deficiency.

Lower SHBG0.237 nmol/L

Total T 9 nmol/L, SHBG 18, albumin 43

With total testosterone 9 nmol/L and SHBG 18 nmol/L, the free fraction is 2.63%, about 68.3 pg/mL. This alone neither confirms nor excludes hypogonadism.

Treatment choices require clinical assessment; this calculator does not compare treatments.

Why SHBG matters

SHBG concentration varies with age, health conditions and medicines. Total testosterone therefore does not always reflect the free fraction. Calculation is an additional tool when total testosterone is borderline or SHBG is altered, not a replacement for clinical assessment.

What to discuss with a clinician

Sleep, nutrition, health conditions, body weight and medicines can affect results. Bring laboratory reports with dates and units and a medication list to your consultation. This calculator does not determine the cause of an abnormality or select hormone therapy or supplements.

Guide values for discussion in adult men

Marker	Example guide value	Comment
Total testosterone	< 12 nmol/L (< 350 ng/dL) — EAU; < 9.2 nmol/L (< 264 ng/dL) — Endocrine Society	Two morning measurements; 8–12 nmol/L is a gray zone — calculate free
Free testosterone (calculated)	< 0.225 nmol/L (< 65 pg/mL)	An additional estimate when SHBG is altered, not a diagnosis on its own
Bioavailable testosterone	< 2.5–3.0 nmol/L	References depend on method; use as a supplement
SHBG	18–54 nmol/L — reference	Higher SHBG may raise total T while lowering its free fraction; lower SHBG may lower total T

Reference intervals depend on population, laboratory and method. These values do not define universal normality and do not apply to women or adolescents. A diagnosis cannot be established from a single result.

Frequently asked questions

What is a normal free testosterone in men?

There is no single interval for all methods. Compare the result with references for calculated free testosterone; direct immunoassay intervals may not be comparable. A clinician considers symptoms and repeat measurements.

Why not just measure free testosterone in the lab?

Equilibrium dialysis is a laboratory method for measuring the free fraction. Calculation is useful when such measurement is unavailable, but depends on total testosterone, SHBG and albumin assay accuracy and model assumptions. Methods are not fully interchangeable.

How does SHBG affect the result?

SHBG reversibly binds testosterone. At the same total testosterone, higher SHBG generally yields a smaller calculated free fraction. This does not make total testosterone useless: the measurements are considered together.

Is the calculator suitable for women?

The model returns calculated values but assigns no female category here. At low concentrations, assay accuracy and female reference intervals for the method are particularly important. A clinician should interpret the result.

When should testosterone be tested?

In the morning between 7 and 11 a.m. fasting, not after a night shift, acute illness or intense training the day before. A deficiency diagnosis needs two measurements several weeks apart.

Can diet raise testosterone?

Nutrition and energy availability can affect hormone measurements. No food or supplement universally normalizes testosterone. The cause of an abnormal result and any need for treatment require clinical assessment.

Testosterone is linked to body composition, sleep and metabolism:

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What next

Formula breakdowns, biomarker reference ranges and nutrition articles with cited sources are in the NutriFit knowledge base.

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