Free androgen index calculator
A calculated estimate of how much testosterone is free to act, found by dividing total testosterone by SHBG. Type your results in the units on your report and it works out FAI as you type, in your browser.
AI-drafted. Awaiting verificationLast reviewed 2026-10-03Sex Hormones & Fertility
Type your results above, or fill in the example, to see FAI.
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Educational information, not medical advice. Blood Test Tracker doesn’t diagnose or treat, and it doesn’t replace your clinician. Talk with your clinician before changing medications, supplements or care.
How it’s worked out
The formula
FAI = total testosterone (nmol/L) ÷ SHBG (nmol/L) × 100. Both values must be in nmol/L and from the same blood draw. To convert total testosterone from ng/dL to nmol/L, divide by 28.84.
Fasting before the blood test: recommended. FAI is worked out from a total testosterone and an SHBG result from the same blood draw. Testosterone guidance asks for a fasting sample taken in the early morning (about 7 to 10 a.m.), and the same applies here.
A worked example
Example numbers, not anyone’s real results, run through the same code as the calculator above.
| Testosterone | 15 nmol/L |
|---|---|
| SHBG | 40 nmol/L |
| FAI | 37.5 index (no unit) |
What FAI tells you
The free androgen index (FAI) is a calculation, not a direct measurement. Most testosterone in blood is held by a protein called sex hormone-binding globulin (SHBG), and only the rest is free to act. FAI divides total testosterone by SHBG, with both in nmol/L, and multiplies by 100. A lab or app works it out from two results from the same blood draw.
FAI is used mostly in women, to help look into signs of extra androgens such as acne, extra facial or body hair, or irregular periods. The 2023 international PCOS guideline says free testosterone can be estimated by the FAI. In one study, FAI and calculated free testosterone were among the best predictors of polycystic ovary syndrome (PCOS). FAI is less reliable in men. It can overestimate free testosterone when SHBG is low, and other calculations that also use albumin are often preferred.
Read the full page on FAI, with what a high or low result can mean.
Usual ranges
The ranges the result above is placed on. Your own lab’s printed range comes first if it gives one.
- Women, 18+, premenopausal0.6–4.4 index (no unit)
Source: Clinica Chimica Acta
Sources
The equation
- Sex steroids and sex steroid-binding globulin levels amongst middle-aged and elderly men and women from general population, European Journal of Clinical Investigation (2022)
- Testosterone, free testosterone, and free androgen index in women: Reference intervals, biological variation, and diagnostic value in polycystic ovary syndrome, Clinica Chimica Acta (2015)
3 more sources for the ranges and the explanation
- Calculated free testosterone in men: comparison of four equations and with free androgen index, Annals of Clinical Biochemistry (2006)
- Recommendations From the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome, Journal of Clinical Endocrinology & Metabolism (2023)
- Plasma steroid-binding proteins: primary gatekeepers of steroid hormone action, Journal of Endocrinology (2016)
AI-drafted. Awaiting verification. Last reviewed 2026-10-03. The calculator is version 1.0.0 of our code for this equation, tested against published examples. How we build and check these pages.