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Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team

How do you calculate the free testosterone index? Eight worked lab examples

To calculate the free testosterone index (FTI), also called the free androgen index or FAI, divide total testosterone by SHBG (both in nmol/L) and multiply by 100. For a US report of total testosterone 600 ng/dL (20.8 nmol/L) and SHBG 35 nmol/L that is (20.8 ÷ 35) × 100 = 59.4. This guide walks through the formula, explains the unit conversion trap, works through eight lab scenarios, and answers the questions people ask most.

Key takeaways

  • FAI = (total testosterone ÷ SHBG) × 100, with both values in nmol/L.
  • US labs report total T in ng/dL — divide by 28.84 to convert to nmol/L before applying the formula (e.g. 600 ng/dL ÷ 28.84 = 20.8 nmol/L).
  • The index is unitless. The ×100 just turns a small fraction into a readable number.
  • Albumin is not used by the FAI — it belongs to the concentration-based Vermeulen estimate.
  • Skip the arithmetic with the free testosterone index calculator.

The one-line formula and the unit trap

The free testosterone index is a single ratio: total testosterone divided by sex hormone-binding globulin (SHBG), then multiplied by 100. The trap that catches most people is units. SHBG is reported in nanomoles per liter (nmol/L) almost everywhere, but total testosterone is reported in nanograms per deciliter (ng/dL) on US lab reports and in nmol/L elsewhere. You cannot divide ng/dL by nmol/L and get anything meaningful — convert total testosterone to nmol/L first.

Testosterone's molecular weight is 288.4 g/mol, so to switch scales you use 28.84: ng/dL → nmol/L is "divide by 28.84" and nmol/L → ng/dL is "multiply by 28.84". The examples below start from the lab report — US ng/dL panels are converted to nmol/L, metric panels are already there. For the units themselves see testosterone units explained: nmol/L vs ng/dL.

How this is calculated

There is no hidden step: one optional unit conversion, one division, one multiplication by 100. Because the operations are linear you can sanity-check the result before reaching for a calculator — doubling SHBG halves the index, doubling total testosterone doubles it. Nothing in the FAI uses albumin, free-T assays, or binding constants; those belong to the Vermeulen 1999 calculated free testosterone, covered further down.

Worked example 1 — the full conversion

US report: total testosterone 600 ng/dL, SHBG 35 nmol/L.

Convert total T: 600 ÷ 28.84 = 20.8 nmol/L.

FAI = (20.8 ÷ 35) × 100 = 0.594 × 100 = 59.4

Worked example 2 — already metric

UK report: total testosterone 15.0 nmol/L, SHBG 30 nmol/L. No conversion needed.

FAI = (15.0 ÷ 30) × 100 = 0.500 × 100 = 50.0

Same total testosterone, different SHBG

The index exists because total testosterone alone does not tell you how much hormone is potentially available to tissues. SHBG binds testosterone tightly and holds it out of circulation, so two people with identical total testosterone can land at very different indices. The table below fixes total testosterone at 600 ng/dL (20.8 nmol/L) and only changes SHBG, so you can see the denominator do the work.

Total TSHBG (nmol/L)CalculationFAI
600 ng/dL15(20.8 ÷ 15) × 100138.7
600 ng/dL20(20.8 ÷ 20) × 100104.0
600 ng/dL35(20.8 ÷ 35) × 10059.4
600 ng/dL50(20.8 ÷ 50) × 10041.6
600 ng/dL70(20.8 ÷ 70) × 10029.7

Identical total testosterone, yet the index falls from 139 to 30 as SHBG climbs. High SHBG mathematically lowers the index; low SHBG raises it. This is why a "normal" total testosterone paired with very high SHBG can still produce a low calculated index. The binding proteins themselves are covered in SHBG and albumin explained.

Free androgen index falling as SHBG rises Bar chart showing the free androgen index dropping from 139 to 30 as SHBG increases from 15 to 70 nmol/L at a fixed total testosterone of 600 ng/dL (20.8 nmol/L). FAI 139 15 104 20 59 35 42 50 30 70 SHBG (nmol/L) — total testosterone fixed at 600 ng/dL
Free androgen index at a fixed total testosterone of 600 ng/dL (20.8 nmol/L), as SHBG rises from 15 to 70 nmol/L. The index is driven almost entirely by the denominator.

More worked examples across real lab values

The scenarios below mix US and metric reports, low and high SHBG, and male and female-range numbers, so you can find one close to your own panel. Each shows the conversion (where needed), the division, and the final index.

Worked example 3 — low SHBG, US units

Total testosterone 720 ng/dL → 720 ÷ 28.84 = 24.97 nmol/L. SHBG 18 nmol/L.

FAI = (24.97 ÷ 18) × 100 = 138.7 — low SHBG pushes the index high.

Worked example 4 — high SHBG, normal total T

Total testosterone 519 ng/dL (18.0 nmol/L), SHBG 72 nmol/L.

FAI = (18.0 ÷ 72) × 100 = 25.0 — a healthy total T can still give a low index when SHBG is high.

Worked example 5 — female-range panel

Total testosterone 52 ng/dL (1.8 nmol/L), SHBG 45 nmol/L.

FAI = (1.8 ÷ 45) × 100 = 4.0 — below the ~5 threshold often cited for biochemical hyperandrogenism in women.

Worked example 6 — female-range, low SHBG

Total testosterone 75 ng/dL (2.6 nmol/L), SHBG 22 nmol/L.

FAI = (2.6 ÷ 22) × 100 = 11.8 — the same total T with low SHBG crosses common PCOS-investigation cut-offs.

Worked example 7 — borderline-low male total T

Total testosterone 310 ng/dL → 310 ÷ 28.84 = 10.75 nmol/L. SHBG 40 nmol/L.

FAI = (10.75 ÷ 40) × 100 = 26.9

Worked example 8 — high total T, mid SHBG

Total testosterone 923 ng/dL (32.0 nmol/L), SHBG 38 nmol/L.

FAI = (32.0 ÷ 38) × 100 = 84.2

How to read your result

The FAI is a dimensionless ratio, not a concentration, so it is read against population reference ranges rather than a hormone level — and those ranges vary by laboratory, assay and sex. In women, an FAI above roughly 5 is commonly used as a marker of biochemical hyperandrogenism, for example when investigating PCOS, though published cut-offs differ by assay and population. In men, values are far higher and far more variable, and the index is a weaker surrogate: one comparison study found the FAI correlated only poorly with calculated free testosterone and over-estimated it at low SHBG.

Treat the FAI as a rough surrogate, not a free testosterone concentration. Read your number against the range printed on your lab report and discuss it with the clinician who ordered the test — this page is a maths walkthrough, not medical advice. For a wider read of a hormone panel see reading Testosterone (TRT) bloodwork, and for the biology, free testosterone explained.

FAI vs the Vermeulen calculated free testosterone

The free androgen index is the simplest available estimate. The Vermeulen equation (1999) is the more detailed method: it uses total testosterone, SHBG and albumin together with mass-action binding constants to return an actual free testosterone concentration (in pmol/L or ng/dL) rather than a unitless index. That extra albumin input is the practical difference — if your report includes albumin, the concentration-based method generally tells you more.

Worked example 9 — where the FAI and Vermeulen diverge

Total testosterone 600 ng/dL (20.8 nmol/L), SHBG 18 nmol/L gives a high FAI of (20.8 ÷ 18) × 100 = 115.6. At that low SHBG the FAI over-estimates free testosterone, which is exactly the regime where Ho et al. found it least reliable — so an albumin-aware Vermeulen estimate is the better call here.

The index tracks calculated free testosterone only loosely, so concentration-based methods are the better estimate when equilibrium dialysis is unavailable. If you have an albumin value, the approach in the free testosterone calculator guide may suit you better than the index.

So, how do you calculate the free testosterone index?

The free testosterone index equals total testosterone divided by SHBG, both in nmol/L, multiplied by 100. If your total testosterone is in ng/dL, convert it first by dividing by 28.84 before applying the formula. The result is a unitless ratio — for example, 600 ng/dL (20.8 nmol/L) divided by SHBG 35 nmol/L, times 100, gives an FAI of 59.4. To skip the arithmetic, use the free testosterone index calculator.

FAQs

How do you calculate the free testosterone index?
Put total testosterone and SHBG into the same units (nmol/L), then compute (total testosterone ÷ SHBG) × 100. For example, 20.8 nmol/L ÷ 35 nmol/L × 100 = 59.4. The result is a unitless index.
My total testosterone is in ng/dL — how do I use it?
Convert it to nmol/L first by dividing by 28.84 (testosterone's molecular weight is 288.4 g/mol). For example, 600 ng/dL ÷ 28.84 = 20.8 nmol/L. Only then divide by SHBG, which is already in nmol/L.
Where does albumin come into the free androgen index?
It does not. The FAI uses only total testosterone and SHBG. Albumin is needed for concentration-based estimates such as the Vermeulen 1999 calculated free testosterone, which returns an actual free-T figure rather than a unitless ratio.
Why does the index drop when SHBG goes up?
SHBG is the denominator. With total testosterone held constant, a larger SHBG value makes the ratio smaller, so the index falls. Physiologically, more SHBG binds more testosterone, leaving less potentially available.
Is the free testosterone index the same as free testosterone?
No. The index is a unitless ratio, not a hormone concentration. It correlates only loosely with measured free testosterone, especially in men, and over-estimates it at low SHBG. For an estimated concentration, the Vermeulen equation, which also uses albumin, is preferred.

Related guides

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;84(10):3666-3672. pubmed.ncbi.nlm.nih.gov/10523012.
  • Ho CKM, Stoddart M, Walton M, Anderson RA, Beckett GJ. Calculated free testosterone in men: comparison of four equations and with free androgen index. Ann Clin Biochem. 2006;43(5):389-397. pubmed.ncbi.nlm.nih.gov/17036414.
  • Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744. pubmed.ncbi.nlm.nih.gov/29562364.
  • Sizar O, Leslie SW, Schwartz J. Male hypogonadism. StatPearls. NCBI Bookshelf, updated 2024. ncbi.nlm.nih.gov/books/NBK532933.
  • Kelsey TW, Li LQ, Mitchell RT, Whelan A, Anderson RA, Wallace WHB. A validated age-related normative model for male total testosterone. PLoS One. 2014;9(10):e109346. pmc.ncbi.nlm.nih.gov/articles/PMC4190174.

This guide is for general educational purposes only and does not constitute medical advice. The free androgen index is a surrogate, not a measured free testosterone concentration. Always follow your prescriber's specific instructions and read your result against your own lab's reference range.

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