Free Testosterone Calculator
Enter total testosterone, SHBG, and albumin from the same blood draw. If albumin was not measured, 4.3 g/dL (43 g/L) is the standard assumed value and is already filled in.
Educational tool, not a diagnosis. Calculated free testosterone by the Vermeulen equation is widely preferred to the direct analog immunoassay, which is known to be unreliable — but it is still an estimate, and reference ranges differ between laboratories and assays. Testosterone is also strongly diurnal: a morning draw is the standard. Interpret any result with the lab’s own reference interval and with your physician.
Why total testosterone can mislead you
Most of the testosterone circulating in your blood is not available to your tissues. Roughly two thirds is bound tightly to SHBG and effectively locked away; most of the rest is bound loosely to albumin and can still be released. Only about 1 to 3 percent circulates completely unbound.
That matters because SHBG varies enormously between people and rises with age, thyroid excess, liver disease, and several medications. Two men with an identical total testosterone of 500 ng/dL can have very different free levels if one has an SHBG of 20 and the other 60. This is the single most common reason a result looks reassuring on paper while symptoms persist — and it is exactly what this calculation is for. The distinction is covered in more depth in free testosterone versus total testosterone.
Why calculated, not the direct test
Many labs offer a direct free testosterone immunoassay. It is inexpensive and widely regarded as unreliable. The reference method, equilibrium dialysis, is accurate but costly and not always available. Calculated free testosterone using the Vermeulen equation agrees closely with equilibrium dialysis in most patients, which is why it has become the practical standard — and why it is worth running the numbers yourself if your report only shows a direct value.
Getting the units right
Two unit errors account for most wrong answers from calculators like this one. The first is albumin: US reports use g/dL (typically around 4.3) while many other countries use g/L (around 43), and entering one for the other silently produces a plausible but wrong result. The second is the output: free testosterone in pg/mL is ten times the ng/dL figure, so a value of 110 pg/mL and 11 ng/dL are the same number. The unit switches above handle both.
If albumin was not measured on your panel, 4.3 g/dL is the standard assumed value and is prefilled. Using the assumption introduces only a small error in most people, but it matters more in liver disease, nephrotic syndrome, or significant malnutrition, where albumin genuinely deviates.
Where to go next
Testosterone is diurnal and varies between draws, so a morning sample and a repeat before any conclusion are both standard practice. For context on how levels shift across a lifetime, see testosterone levels by age; for what a specific mid-range number means, see whether a testosterone of 400 is normal; and for the younger cohort, low testosterone in men under 30.