Free Androgen Index vs Free Testosterone
Last updated: June 2026
The free androgen index (FAI) and free testosterone answer the same question — how much of your testosterone is actually available to tissue — but they are not the same number: FAI is a unitless ratio (total testosterone ÷ SHBG × 100), while calculated free testosterone is a real concentration (ng/dL or pmol/L) worked out from total T, SHBG and albumin. You cannot compare an FAI of 50 against a free-T result in pmol/L — they sit on different scales, and in adult men the two often disagree.
Have your total T, SHBG and albumin from one blood draw? Skip the ratio and get the more accurate unbound number.
Free testosterone index calculator →TL;DR — key takeaways
- FAI is a ratio, free T is a concentration. FAI has no units; free testosterone is measured in ng/dL or pmol/L like any hormone.
- Different inputs. FAI uses only total T and SHBG. Calculated free T (the Vermeulen equation) adds albumin and binding constants, so it tracks the true unbound fraction far more closely.
- FAI was built for women. It was designed to screen hirsutism in females and is not validated for adult males, where it correlates poorly with measured free T.
- Don't compare the two numbers directly. An FAI of 60 and a free T of 60 pmol/L mean nothing to each other — they are different quantities.
A ratio versus a concentration
Most testosterone in blood is bound: tightly to sex hormone-binding globulin (SHBG) and loosely to albumin, leaving only a small unbound fraction — usually only about 1–3% of the total — free to enter cells and act. Both FAI and free testosterone try to capture that available slice, but they take very different routes.
FAI takes the shortcut: divide total testosterone by SHBG and multiply by 100. Because nmol/L cancels nmol/L, the result is dimensionless — a screening number, not a measured hormone. It treats SHBG as the only brake on availability and ignores albumin entirely. Calculated free testosterone takes the long road: it feeds total T, SHBG, and albumin into the Vermeulen equation, which models the binding equilibria with published association constants and returns an actual concentration you can quote in ng/dL or pmol/L.
That albumin step matters. Albumin carries a meaningful share of bioavailable testosterone, and leaving it out is exactly why FAI drifts away from reality — especially when SHBG is unusually high or low.
How each number is built
The diagram below shows the two pipelines side by side: the same starting labs, two different destinations.
Side-by-side: FAI vs free testosterone
| Property | Free androgen index | Calculated free testosterone |
|---|---|---|
| What it is | Screening ratio | Estimated concentration |
| Formula inputs | Total T, SHBG | Total T, SHBG, albumin |
| Uses albumin? | No | Yes |
| Units | None (dimensionless) | ng/dL or pmol/L |
| Reference method | — | Vermeulen equation |
| Validated for | Women (hirsutism) | Men and women |
| Best use | Quick flag to look closer | The number to act on |
Reference ranges for both are assay-specific and not standardised, so always read your result against your own laboratory's printed range rather than a number from the internet.
Worked examples
These run honest arithmetic on real lab pairings. FAI is exact (it is just division); free testosterone needs the full Vermeulen equation, which the calculator handles — here we show the unit and scale logic.
Example 1 — the FAI itself
Total T 22 nmol/L, SHBG 30 nmol/L. FAI = (22 ÷ 30) × 100 = 73.3. A mid-band ratio — but it is a 73.3 of nothing, not 73.3 ng/dL.
Example 2 — convert units before FAI
Lab reports total T as 600 ng/dL. Convert first: 600 ÷ 28.84 = 20.8 nmol/L. With SHBG 35: FAI = (20.8 ÷ 35) × 100 = 59.4. Skipping the conversion would give a meaningless 1714.
Example 3 — how small the free fraction is
If free testosterone is the usual ~2% of a 600 ng/dL total, that is about 12 ng/dL (≈ 0.42 nmol/L, or 420 pmol/L). The Vermeulen equation refines this using your actual SHBG and albumin — the 2% is only a rough anchor.
Example 4 — high SHBG drags FAI down
Total T 18 nmol/L, SHBG 70 nmol/L. FAI = (18 ÷ 70) × 100 = 25.7. Total T looks normal, but the high binding protein pulls the index low — the classic high-SHBG picture, and the case where calculated free T earns its keep.
Example 5 — low SHBG inflates FAI
Total T 15 nmol/L, SHBG 15 nmol/L. FAI = (15 ÷ 15) × 100 = 100 — impressively high for a below-average total T. At low SHBG, FAI overestimates the true free fraction, so calculated free T comes out far more modest.
Example 6 — same total T, two SHBGs
Total T 20 nmol/L at SHBG 25 → FAI 80; the same 20 at SHBG 50 → FAI 40. The ratio halves. Free T also falls, but not by exactly half, because albumin-bound testosterone buffers part of the change — a buffer FAI cannot see.
Example 7 — the scales don't match
An FAI of 50 and a calculated free T of 50 pmol/L are unrelated. Free T 0.45 nmol/L = 450 pmol/L ≈ 13 ng/dL; the FAI of 50 is a pure ratio. Comparing the raw digits is a category error.
FAI by total testosterone and SHBG (ng/dL inputs)
Because so many labs print total T in ng/dL, this chart converts to nmol/L first, then computes FAI across three SHBG levels. Values are exact arithmetic; reference bands still belong to your own lab.
| Total T | = nmol/L | SHBG 30 | SHBG 45 | SHBG 60 |
|---|---|---|---|---|
| 400 ng/dL | 13.9 | 46.2 | 30.8 | 23.1 |
| 550 ng/dL | 19.1 | 63.6 | 42.4 | 31.8 |
| 700 ng/dL | 24.3 | 80.9 | 53.9 | 40.4 |
| 900 ng/dL | 31.2 | 104.0 | 69.4 | 52.0 |
Read across any row: the same total testosterone produces a wildly different FAI as SHBG moves. That sensitivity is FAI's whole point as a screen — and also why a single ratio should never be the last word.
Which one should you trust?
For men, calculated free testosterone. FAI was originally devised to assess androgen excess in women, and its mathematical derivation rests on an assumption that does not hold in adult males; in men it correlates only weakly with directly measured free testosterone. When four calculated free-T equations were compared against FAI in men, FAI tracked the reference poorly and overestimated free testosterone at low SHBG. Major clinical guidance therefore leans on equilibrium dialysis or a validated calculated free testosterone, not on FAI, when total testosterone is borderline or SHBG is abnormal.
FAI is not useless — it is a fast, cheap flag that says “look closer.” But the number you and a clinician actually act on should be a concentration, ideally calculated free T from the same draw.
How this is calculated
FAI uses one division and one multiplication: confirm both total T and SHBG are in nmol/L (divide ng/dL by 28.84 if needed), divide, then ×100. There is no albumin term and no binding constant — that simplicity is its strength as a screen and its weakness as a measurement. Calculated free testosterone instead solves the binding equilibria with the Vermeulen association constants for SHBG and albumin, which is why it returns a real concentration. Our calculator runs that maths in your browser and shows the formula behind the result. None of this is medical advice; it is the arithmetic behind numbers your clinician interprets in context.
Frequently asked questions
Is the free androgen index the same as free testosterone?
No. FAI is a unitless ratio of total testosterone to SHBG; free testosterone is an actual concentration in ng/dL or pmol/L that also accounts for albumin. They estimate the same biology but are different quantities on different scales.
Why is my FAI high but my free testosterone normal?
Usually low SHBG. FAI divides by SHBG, so a low binding protein inflates the ratio; calculated free T, which factors in albumin, often comes out much less dramatic. FAI is known to overestimate free testosterone when SHBG is low.
Can I convert an FAI into a free testosterone value?
Not directly — there is no fixed conversion factor, because FAI ignores albumin and the relationship is non-linear. To get free testosterone you need total T, SHBG and albumin run through the Vermeulen equation.
Should men use FAI at all?
As a quick screen, sometimes; as the number to act on, no. FAI was validated for women and performs poorly in adult males, so calculated free testosterone or equilibrium dialysis is preferred when it matters.
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)
- Kapoor P, Luttrell BM, Williams D. The free androgen index is not valid for adult males (J Steroid Biochem Mol Biol, 1993)
- Ho CKM, et al. Calculated free testosterone in men: comparison of four equations and with free androgen index (Ann Clin Biochem, 2006)
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline (J Clin Endocrinol Metab, 2018)
- Hammond GL. Diverse Roles for Sex Hormone-Binding Globulin in Reproduction (Biol Reprod, 2011)
- Nassar GN, Leslie SW. Physiology, Testosterone (StatPearls, NCBI Bookshelf)
This guide is for general educational purposes only and does not constitute medical advice. Always follow your prescriber’s specific instructions and consult a qualified clinician before changing any protocol.