PonyatnoHow it's calculated
Free testosterone is calculated with the Vermeulen equation (1999), a mass-action model of binding to SHBG and albumin. All values are converted to mol/L; albumin in g/L is divided by 69,000 g/mol. Binding constants: SHBG Kt = 1.0 × 10⁹ L/mol, albumin Ka = 3.6 × 10⁴ L/mol.
First N = 1 + Ka × albumin, then a = Kt × N, b = N + Kt × (SHBG − total T) and free T = (−b + √(b² + 4 × a × total T)) ÷ (2 × a). The result is shown in nmol/L and pmol/L.
Bioavailable T = free T × N (free plus loosely albumin-bound). Free % = free ÷ total × 100. FAI = total T (nmol/L) ÷ SHBG (nmol/L) × 100.
Unit conversion: ng/dL ÷ 28.842 = nmol/L, ng/mL × 3.4672 = nmol/L, albumin g/dL × 10 = g/L. If no albumin is entered, 43 g/L is used.
What to know
Why calculate free testosterone
About half of circulating testosterone is tightly bound to SHBG, most of the rest loosely to albumin, and only a few percent is free. Total testosterone depends on SHBG, which rises with age, hyperthyroidism, liver disease and oestrogen use, and falls with obesity, insulin resistance and hypothyroidism. So when total testosterone is borderline, doctors often look at calculated free testosterone.
The Vermeulen equation agrees well with the reference method, equilibrium dialysis, and is usually more accurate than “direct” immunoassays for free testosterone.
When the estimate is less reliable
The equation uses average binding constants. Errors grow at very high or very low SHBG, and SHBG assays differ between labs, so the calculated value will differ too. In pregnancy binding proteins and hormones change so much that the calculation is not reliable.
Women have low testosterone levels, where standard immunoassays are imprecise; mass spectrometry (LC-MS/MS) is more reliable. Testosterone is best measured in the morning, ideally fasting, and repeated if the result is unexpected.
FAI and women
The free androgen index is used when assessing hyperandrogenism and polycystic ovary syndrome (PCOS). FAI norms depend on the lab and method, so this calculator does not rate it — compare it with your lab's reference range. In men FAI is of little use; free testosterone is the better guide.
This calculator is a reference tool, not a diagnosis. Laboratory ranges differ, and the result depends on your health, medicines and other tests. Discuss it with your doctor.
FAQ
What free testosterone level is considered low in men?
According to EAU guidelines, a value below 0.225 nmol/L supports a diagnosis of testosterone deficiency, together with symptoms and a repeat morning test. Lab ranges may differ.
Why calculate free testosterone if I already have total?
Total testosterone depends on SHBG. With high SHBG total can look normal while free is low, and the reverse with low SHBG. The calculation helps when total is borderline or SHBG is unusual.
What is a normal FAI for women?
There is no single normal value — it depends on the lab and method. FAI is used in assessing hyperandrogenism and PCOS, and a doctor interprets it alongside symptoms and other tests.
Do I need to enter albumin?
No. Without it 43 g/L is used, and with a normal albumin the result changes little. If your albumin is clearly low (liver or kidney disease, malnutrition), enter the real value.
Can I use this calculator in pregnancy?
No, the calculation is not reliable in pregnancy because SHBG and other hormones change a lot. Your pregnancy care team should interpret hormone results during this time.
Sources
Formulas and cut-offs follow clinical guidelines — see sources below.
- 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.
- EAU Guidelines on Sexual and Reproductive Health — Male Hypogonadism. European Association of Urology, 2024.