What the Vermeulen equation does
Testosterone doesn't float freely in your blood. Most of it is bound to two proteins, and the binding follows the law of mass action - which means if you know the concentrations and the binding strengths, you can solve for how much is left unbound.
SHBG binds testosterone tightly, with an association constant of about 1 × 10⁹ L/mol. Albumin binds it far more weakly, at about 3.6 × 10⁴ L/mol - roughly 28,000 times weaker - but there's an enormous amount of albumin in blood, so it still accounts for a large share of the total.
Published by Vermeulen and colleagues in 1999, the equation solves the resulting equilibrium as a quadratic. It has become the standard calculated method, and it tracks equilibrium dialysis - the reference measurement - closely enough to be clinically useful.
Reading the three fractions
| Fraction | Typical share | Available to tissue |
|---|---|---|
| Free | 1-3% | Yes |
| Albumin-bound | 33-54% | Yes, released at the capillary |
| SHBG-bound | 44-65% | Generally not |
Bioavailable testosterone is the first two added together. The exact split shifts with your SHBG and albumin, which is what the bar in the readout is showing.
Why the number can disagree with your lab
If your lab also reported a calculated free testosterone and it differs from this one, that's expected rather than alarming. There are a few reasons.
Different calculators use different association constants. Some use 1.0 × 10⁹ for SHBG binding, others 5.97 × 10⁸, and the choice moves the answer. Some use an albumin molecular weight of 66,500 rather than 69,000. Some labs report a directly measured free testosterone by dialysis, which is a different thing entirely and is the more accurate of the two.
The constants this page uses are listed in the working at the bottom, so you can check them against whatever your lab used.
Free testosterone questions
What units should I enter?
Total testosterone accepts either ng/dL, which is standard in the US, or nmol/L, which is standard in most of the rest of the world. SHBG is entered in nmol/L, which is how essentially every lab reports it. Albumin takes g/dL or g/L.
I don't have an albumin result. Does it matter?
Less than you might think. Albumin binds testosterone weakly and its concentration varies over a narrow range in healthy people, so the default of 4.3 g/dL is usually close. SHBG is the input that genuinely drives the result. If you have liver disease, kidney disease or significant malnutrition, get an actual albumin figure.
Why is bioavailable so much higher than free?
Because it includes the albumin-bound fraction, which is typically 30 to 50% of your total. Albumin holds testosterone loosely enough that it dissociates during a single pass through a capillary bed, so it's treated as available to tissue even though it's technically bound.
What's a normal free testosterone?
There isn't one universal answer, which is why this page doesn't state one. Reference ranges differ by laboratory, by assay, and considerably by age. Use the range printed on your own lab report. A calculated value compared against another lab's range can be misleading.
Is this as good as a measured free testosterone?
No. Equilibrium dialysis is the reference standard and a calculated value is an estimate from a model. The estimate is good enough to be widely used clinically, and it's far better than judging from total testosterone alone, but if there's a genuine clinical question the measured test is the better answer.
Does this work for women?
The equation itself is not sex-specific - it's a binding model, and the chemistry is identical. What differs is the reference range, which is roughly an order of magnitude lower for women, and the fact that assay precision at those low concentrations is considerably worse.