Bloodwork & Labs
Last updated: June 2026
Free Testosterone Index: A Worked Calculation Example
The free testosterone index (also called the free androgen index, FAI) estimates how much of your total testosterone is potentially active by comparing it to your SHBG. This page works through the arithmetic step by step using real lab numbers.
The free testosterone index calculation example, in one line
The free testosterone index (FTI), more formally the free androgen index (FAI), is a single ratio. You divide total testosterone by sex hormone-binding globulin (SHBG), then multiply by 100:
FTI = (Total testosterone ÷ SHBG) × 100
The catch that trips most people up is units. Both numbers must be in the same units — nanomoles per litre (nmol/L) — before you divide. The index itself has no unit; the ×100 is just there to turn a small fraction into a readable whole number. This page walks through a full free testosterone index calculation example, including the unit conversion most lab reports force you to do first.
Step 1 — get total testosterone and SHBG into the same units
SHBG is almost always reported in nmol/L. Total testosterone is reported in nmol/L in most of the world but in nanograms per decilitre (ng/dL) on many US lab reports. You cannot put ng/dL and nmol/L into the same ratio — convert first.
Testosterone has a molecular weight of 288.4 g/mol, so:
- ng/dL → nmol/L: divide by 28.84
- nmol/L → ng/dL: multiply by 28.84
Example
A US report shows total testosterone of 600 ng/dL. Converting: 600 ÷ 28.84 = 20.8 nmol/L. SHBG on the same report reads 35 nmol/L — already in the right units, so it stays as 35.
For more on this conversion see testosterone units explained: nmol/L vs ng/dL.
Step 2 — plug the numbers into the formula
With both values in nmol/L, the calculation is a single division and one multiplication.
Example
Total testosterone = 20.8 nmol/L, SHBG = 35 nmol/L.
FTI = (20.8 ÷ 35) × 100
FTI = 0.594 × 100
FTI = 59.4
That is the entire mechanism. The same lab values typed into the Free Testosterone Index Calculator return the same number — the tool simply removes the conversion and rounding steps.
Worked comparison: same 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 have very different indices. Here are three worked rows, each using the same total testosterone of 20.8 nmol/L.
| Total T (nmol/L) | SHBG (nmol/L) | Calculation | FTI |
|---|---|---|---|
| 20.8 | 20 | (20.8 ÷ 20) × 100 | 104.0 |
| 20.8 | 35 | (20.8 ÷ 35) × 100 | 59.4 |
| 20.8 | 70 | (20.8 ÷ 70) × 100 | 29.7 |
Identical total testosterone, but the index falls from 104 to 30 as SHBG rises. High SHBG mathematically lowers the index; low SHBG raises it. This is why a normal total testosterone with very high SHBG can still leave someone with a low calculated index. The role of the binding proteins is covered in SHBG and albumin explained.
How to read the result
The FTI is a dimensionless ratio, not a concentration, so it is interpreted against population reference ranges rather than a hormone level. Reported ranges vary by laboratory and assay, and the index behaves differently between the sexes.
- In women, an FAI above roughly 5 is commonly used as a threshold for biochemical hyperandrogenism, for example when investigating PCOS — though published cut-offs vary by assay and population.
- In men, typical values are far higher and far more variable, and the FAI is a weaker surrogate for free testosterone; one comparison study found it correlated only poorly with calculated free testosterone and over-estimated it at low SHBG.
Because of this, the FAI is a rough surrogate that should not be read as a free testosterone concentration on its own. Always read your own result against the reference 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 walkthrough of a hormone panel, see reading TRT bloodwork.
FTI vs the Vermeulen calculated free testosterone
The free testosterone index is the simplest of the available estimates. A more detailed method, the Vermeulen equation (1999), 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.
The FTI is quicker and needs one fewer input, but it does not output a concentration and is less informative where SHBG or albumin are unusual. In published comparisons the index tracks calculated free testosterone only loosely, so the concentration-based methods are generally regarded as the better calculated estimate when equilibrium dialysis is unavailable. If you have an albumin value to hand, the concentration-based approach in the free testosterone calculator guide may suit you better than the index.
FAQs
What is a worked free testosterone index calculation example?
Take total testosterone and SHBG, both in nmol/L, then compute (Total T ÷ SHBG) × 100. For example, total testosterone 20.8 nmol/L and SHBG 35 nmol/L gives (20.8 ÷ 35) × 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.
Why does the index drop when SHBG goes up?
SHBG is the denominator in the formula. 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 is a rough surrogate that correlates only loosely with measured free testosterone, especially in men. For an estimated concentration, the Vermeulen equation, which also uses albumin, is generally preferred.
What is a normal free androgen index?
Reference ranges depend on the lab, assay and sex. In women, a FAI above about 5 is often used as a marker of biochemical hyperandrogenism, though published cut-offs vary. In men, values are much higher and more variable. Read your result against your own lab's printed range and discuss it with your clinician.
Read next
Free Androgen Index ExplainedSources
- 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. academic.oup.com/jcem/article/84/10/3666/2660660.
- 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.
- Kelsey TW, Li LQ, Mitchell RT, Whelan A, Anderson RA, Wallace WHB. A validated age-related normative model for male total testosterone shows increasing variance but no decline after age 40 years. PLoS One. 2014;9(10):e109346. pmc.ncbi.nlm.nih.gov/articles/PMC4190174.
- Fritz KS, McKean AJS, Nelson JC, Wilcox RB. Analog-based free testosterone test results linked to total testosterone concentrations, not free testosterone concentrations. Clin Chem. 2008;54(3):512-516. pubmed.ncbi.nlm.nih.gov/18171714.
This guide is for general educational purposes only and does not constitute medical advice. Always follow your prescriber's specific instructions.