Free Androgen Index (FAI) Calculator

Free Androgen Index (FAI) Calculator for Endocrinology. The index is not the percentage of testosterone that is free. A lower denominator can raise the index without demonstrating a corresponding change in biologically available testosterone. No universal male or female diagnostic bands are assigned.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

Free androgen index = 100 x total testosterone / SHBG, with both concentrations in nmol/L. US testosterone is multiplied by 0.0347 before division. SHBG remains in nmol/L in both modes; it is not relabeled as an unconverted mass concentration.

When to use this calculator

Use as a clearly defined scaled ratio within an appropriate laboratory and clinical assessment. The 2025 Society for Endocrinology guideline discusses its limitations in androgen-excess evaluation, particularly when SHBG is low. The number alone must not determine treatment.

Inputs used

  • Total testosterone in nmol/L or ng/dL
  • SHBG in nmol/L

Clinical interpretation

The index is not the percentage of testosterone that is free. A lower denominator can raise the index without demonstrating a corresponding change in biologically available testosterone. No universal male or female diagnostic bands are assigned.

Worked example

Testosterone 2.3 nmol/L and SHBG 22 nmol/L give 230 / 22 = 10.455. With the same testosterone and SHBG 30, the index is 7.667. The first example triggers a low-SHBG caution; exactly 30 does not trigger the below-30 message.

Limitations and safety notes

Low SHBG, assay variability and population differences limit inference; the guideline specifically cautions about SHBG below 30 nmol/L. Reference intervals depend on method and population. A zero denominator is rejected. This ratio does not replace a suitable calculated or measured free-testosterone assessment.

Frequently asked questions

Does an FAI of 10 mean 10% is free?

No. Multiplication by 100 creates an index scale; it does not calculate a binding fraction.

Can I enter SHBG in mg/dL?

No. The input is nmol/L. A laboratory result in another unit requires an appropriate analyte-specific conversion.

References

  • Society for Endocrinology 2025 primary androgen-excess guideline section 6.3; FAI is unreliable at low SHBG, especially below 30 nmol/L, with assay/population-specific interpretation. https://doi.org/10.1111/cen.15265

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 9, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

Related reviewed calculators