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Free Androgen Index Calculator

Enter total testosterone and SHBG in nmol/L to calculate the Free Androgen Index.

How it is calculated

The tool divides total testosterone by SHBG and multiplies by 100: FAI = 100 × T ÷ SHBG. T is total testosterone and SHBG is sex hormone-binding globulin, and both must be in nmol/L, so the units cancel and the index has no unit. The result is rounded to two decimals. SHBG must be greater than zero, otherwise there is no result. The tool does not interpret the number, it shows no reference range and makes no allowance for age or sex. If your lab report gives testosterone in ng/dL, convert it to nmol/L first (divide by about 28.8); an unconverted value gives a wrong index.

Worked example

A blood test reports total testosterone of 2.1 nmol/L and SHBG of 45 nmol/L. Divide: 2.1 ÷ 45 = 0.04667. Multiply by 100: 4.67. That is the Free Androgen Index. If SHBG had been 30 nmol/L with the same testosterone, the index would be 100 × 2.1 ÷ 30 = 7.00, which shows how a lower SHBG raises the index even when total testosterone does not change.

Try this example →

Common mistakes

  • Entering testosterone in ng/dL or SHBG in µg/mL without converting both to nmol/L; the index is only valid with both in nmol/L.
  • Entering free testosterone instead of total testosterone; the formula needs total testosterone.
  • Comparing the result with a reference range from a different laboratory or a different population; ranges depend on the assay, age and sex, and the tool gives none.
  • Forgetting the factor of 100 when comparing with a lab-printed ratio that has not been multiplied by 100, so the numbers differ by 100 times.
  • Treating the FAI as a diagnosis; it is only a ratio, and results need a clinician's interpretation.

Sources

About this calculator

Most testosterone in the blood is bound to sex hormone-binding globulin (SHBG) and is not biologically active; only the small unbound fraction acts on tissue. Rather than measuring free testosterone directly — a technically difficult assay — clinicians use the Free Androgen Index as a proxy: FAI = (total testosterone ÷ SHBG) × 100, with both values in nmol/L, giving a simple ratio that tracks how much androgen is actually available.

This calculator takes total testosterone and SHBG in nmol/L and returns the FAI. Endocrinologists and gynecologists order it as part of the hormonal workup for suspected polycystic ovary syndrome (PCOS), hirsutism, or male hypogonadism, since an elevated FAI in women or a depressed FAI in men often points to an androgen imbalance even when total testosterone alone looks borderline.

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