Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team
What is the free androgen index? FAI formula, ranges, and worked examples
The free androgen index (FAI) is a quick screening number that estimates how much of your testosterone is biologically available: FAI = total testosterone (nmol/L) / SHBG (nmol/L) x 100. It is a unitless ratio, not a measured hormone, so it only works when total testosterone is reported in nmol/L and SHBG in nmol/L from the same blood draw. This guide explains what FAI measures, works through 8 real-lab examples, and answers the questions people ask most.
Key takeaways
- FAI = total T divided by SHBG, times 100, with both in nmol/L. Higher SHBG drives FAI down even when total T is unchanged.
- US labs report total T in ng/dL; the formula needs nmol/L, so divide your ng/dL number by 28.84 first (e.g. 600 ng/dL ÷ 28.84 = 20.8 nmol/L). SHBG is already in nmol/L.
- Commonly cited adult-male reference bands sit roughly between 25 and 100, but ranges are lab-specific - always read against your own report.
- FAI is a screen, not a substitute for the more accurate calculated free testosterone (Vermeulen equation), which also uses albumin.
Want the more accurate unbound number instead of the ratio? Use the free testosterone index calculator to enter total T, SHBG and albumin and get calculated free T.
What the free androgen index actually estimates
Most testosterone in blood is bound - tightly to sex hormone-binding globulin (SHBG) and loosely to albumin - and only a small unbound fraction is free to act on tissue (StatPearls, Physiology of Testosterone). FAI tries to approximate that available fraction with one division: it treats SHBG as the brake on availability. When SHBG rises (with age, hyperthyroidism, or estrogen), FAI falls; when SHBG drops (with obesity, insulin resistance, or some steroid use), FAI rises - even if total testosterone never moved.
Because it ignores albumin and assumes a simple inverse relationship, FAI is a screening ratio rather than a true measurement. The Vermeulen calculation - the reference method for estimating free testosterone - uses total T, SHBG and albumin with binding constants, and tracks equilibrium-dialysis free T far better than a bare ratio (Vermeulen 1999). FAI is most useful as a fast flag that says "look closer," not as a final answer.
How this is calculated
The arithmetic has three honest steps. Step 1 - units. US labs report total testosterone in ng/dL; the formula needs nmol/L, so divide your ng/dL number by 28.84 (for example 600 ng/dL / 28.84 = 20.8 nmol/L). If your report is already in nmol/L, skip this step. SHBG is almost always already in nmol/L. Step 2 - divide. Total T / SHBG. Step 3 - scale. Multiply by 100 so the result lands in a readable two-to-three-digit band instead of a small decimal.
That is the whole formula: FAI = (total T / SHBG) x 100, unitless because nmol/L cancels nmol/L. The number is dimensionless, so it has no units after the "x 100" - a frequent source of confusion when people try to compare an FAI of 50 against a free-T result quoted in pmol/L. They are different scales and cannot be compared directly.
| Total T (ng/dL → nmol/L) | SHBG 20 | SHBG 35 | SHBG 50 | SHBG 70 |
|---|---|---|---|---|
| 346 → 12 | 60.0 | 34.3 | 24.0 | 17.1 |
| 519 → 18 | 90.0 | 51.4 | 36.0 | 25.7 |
| 692 → 24 | 120.0 | 68.6 | 48.0 | 34.3 |
| 865 → 30 | 150.0 | 85.7 | 60.0 | 42.9 |
Read down a column to see how the same SHBG produces different FAI as total T rises; read across a row to see how the same total testosterone gives a very different FAI depending on SHBG. The 692 ng/dL (24 nmol/L) row swings from 120 down to 34 purely on the binding protein.
Worked examples
Each block below runs the full formula on a different lab pairing so you can see exactly how SHBG and total T interact. All inputs are in nmol/L unless a conversion step is shown.
Example 1 - mid-range male
Total T 20, SHBG 30. FAI = (20 / 30) x 100 = 66.7. A typical mid-band result.
Example 2 - high SHBG masking normal total T
Total T 18, SHBG 60. FAI = (18 / 60) x 100 = 30.0. Total T looks fine but high SHBG pushes FAI to the low end - the classic "high-SHBG hypogonadism" pattern.
Example 3 - low SHBG inflating FAI
Total T 14, SHBG 14. FAI = (14 / 14) x 100 = 100.0. Below-average total T but low SHBG (common with obesity/insulin resistance) yields a high index.
Example 4 - ng/dL conversion first
Lab reports total T as 750 ng/dL, SHBG 40. Convert: 750 / 28.84 = 26.0 nmol/L. FAI = (26.0 / 40) x 100 = 65.0. Skipping the conversion would have given a meaningless 1875.
Example 5 - same total T, two SHBGs
Total T 24 with SHBG 20 gives (24 / 20) x 100 = 120.0; the same 24 with SHBG 50 gives (24 / 50) x 100 = 48.0. Identical testosterone, very different index.
Example 6 - low total T, high SHBG
Total T 9, SHBG 55. FAI = (9 / 55) x 100 = 16.4. Both signals point the same way, so the low index is consistent with the low total.
Example 7 - female-range screen (PCOS context)
Total T 2.4, SHBG 25. FAI = (2.4 / 25) x 100 = 9.6. In women, FAI above roughly 5 is often used as a hyperandrogenism flag, but cut-offs are assay-specific.
Example 8 - cross-check against calculated free T
Total T 20, SHBG 30, albumin 43 g/L. FAI = 66.7, while the Vermeulen calculated free T is about 0.40 nmol/L (~400 pmol/L). The two numbers are on different scales - use FAI to screen, calculated free T to quantify.
FAI vs calculated free testosterone: which to trust
FAI is fast and needs only two inputs, which is why labs print it automatically. But it has real limits. It assumes a fixed inverse relationship to SHBG, ignores albumin entirely, and its reference ranges are poorly standardised - reviewers note that free-testosterone surrogates can quietly reintroduce age effects and lack well-established cut-offs, especially in older men whose SHBG rises with age (Asian Journal of Andrology 2014). When SHBG is at an extreme, calculated free testosterone tracks the true unbound fraction more faithfully than FAI.
Practically: use FAI to triage a report at a glance, then use calculated free T (the Vermeulen equation, with albumin) for anything you act on. And never read either number in isolation - the Endocrine Society advises interpreting testosterone results alongside symptoms and confirming low results with a repeat morning sample before any diagnosis (Bhasin 2018).
So, what is the free androgen index (FAI)?
The free androgen index is a unitless ratio that estimates biological testosterone availability using a single formula: FAI = total testosterone (nmol/L) divided by SHBG (nmol/L), multiplied by 100. Both inputs must be in nmol/L — if your lab reports total T in ng/dL, divide by 28.84 first. Use FAI as a fast triage screen, then confirm with the more accurate free testosterone index calculator when you need a number to act on.
FAQs
What is the free androgen index (FAI)?
What is a normal free androgen index?
Why is my FAI low when my total testosterone is normal?
Is FAI the same as free testosterone?
Do I have to convert units before calculating FAI?
Sources
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999. PubMed PMID: 10523012.
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018. PubMed PMID: 29562364.
- Nassar GN, Leslie SW. Physiology, Testosterone. StatPearls. NCBI Bookshelf NBK526128.
- Ng Tang Fui M, Dupuis P, Grossmann M. Lowered testosterone in male obesity: mechanisms, morbidity and management. Asian J Androl. 2014. PMC3955331.
- CDC. Safe Injection Practices to Prevent Transmission of Infections to Patients. CDC injection safety guidance.
This guide is for general educational purposes only and does not constitute medical advice. FAI reference ranges are lab-specific; always interpret results with your prescriber and your laboratory's printed ranges.