Enter the total testosterone level and the sex hormone binding globulin level into the calculator to determine the Free Androgen Index. This calculator can also evaluate any of the variables given the others are known.
Free Androgen Index Formula
FAI = 100 × Total Testosterone (nmol/L) / SHBG (nmol/L)
- FAI = free androgen index, expressed as a percentage
- Total Testosterone = serum total testosterone in nmol/L
- SHBG = sex hormone-binding globulin in nmol/L
Both values must come from the same blood draw. If your testosterone is reported in ng/dL, divide by 28.84 to get nmol/L. If it is in ng/mL, multiply by 3.467. FAI is an estimate of bioavailable androgen and is most useful in women; it is less reliable at very high SHBG levels and in men with normal SHBG.
Rearranged forms used by the reverse mode:
Total Testosterone (nmol/L) = FAI × SHBG / 100 SHBG (nmol/L) = 100 × Total Testosterone (nmol/L) / FAI
Typical Reference Ranges
Cutoffs vary by lab and assay. Always compare your result to the reference interval printed on your own report.
| Group | Typical FAI (%) | Notes |
|---|---|---|
| Adult female | 0.5 – 5 | Values above 5 often flagged for androgen excess |
| Adult male | 30 – 150 | Wide spread; less clinically useful than in women |
| PCOS suspicion | > 5 | Often combined with clinical signs and other labs |
Testosterone Unit Conversions
| From | To | Multiply by |
|---|---|---|
| ng/dL | nmol/L | 0.0347 |
| ng/mL | nmol/L | 3.467 |
| nmol/L | ng/dL | 28.84 |
Worked Example
Total testosterone is 2.1 nmol/L and SHBG is 45 nmol/L.
FAI = 100 × 2.1 / 45 = 4.67%
That sits inside the common adult female range of 0.5 to 5, so the result would not typically be flagged for androgen excess.
FAQ
Why is testosterone converted to nmol/L? SHBG is reported in nmol/L. Both values must use the same units for the ratio to be dimensionless and the percentage to be meaningful.
Is FAI the same as free testosterone? No. FAI is a calculated ratio, not a measurement. Free testosterone is either measured directly or calculated using albumin (Vermeulen equation), which is more accurate.
When is FAI unreliable? At very low or very high SHBG, during pregnancy, with oral estrogens, and in men with normal SHBG. In those cases a calculated free testosterone is preferred.
What if my lab gives a different reference range? Use your lab's range. Assay methods differ enough that a single universal cutoff does not exist.
