Free Testosterone Calculator

Only a small fraction of the testosterone circulating in blood, roughly 1 to 3%, is actually free and biologically active. The rest is bound to SHBG (sex hormone-binding globulin) and, more loosely, to albumin. Most standard lab panels report only total testosterone, which can be misleading on its own, since two people with the same total testosterone can have very different free testosterone if their SHBG levels differ¹.

What This Calculator Measures

This calculates free and bioavailable testosterone from three lab values, rather than requiring the more expensive direct measurement method. Bioavailable testosterone includes the free fraction plus the weakly albumin-bound portion, since that bound fraction is still physiologically available to tissues.

The Formula

The Vermeulen equation solves for free testosterone using the law of mass action and the experimentally-derived binding association constants between testosterone and albumin, and testosterone and SHBG¹. This requires a quadratic solve, not simple algebra.

The calculator converts each input to moles per liter and then solves:

Free T = (−b + √(b × b + 4 × a × TT)) ÷ (2 × a)

Here TT is total testosterone, a = Kt × (1 + Ka × albumin) and b = 1 + Kt × SHBG + Ka × albumin − Kt × TT. The tool uses an SHBG binding constant (Kt) of 1,000,000,000 L/mol and an albumin binding constant (Ka) of 36,000 L/mol. Bioavailable testosterone is then free T × (1 + Ka × albumin).

Bioavailable testosterone = free testosterone + albumin-bound testosterone

How This Calculator Computes Your Result

Enter your total testosterone, SHBG, and albumin if you have it. If albumin isn't entered, a default of 4.3 g/dL, a commonly used population average, is applied and clearly labeled as a default rather than your actual value.

Total testosterone goes in as ng/dL, SHBG as nmol/L and albumin as g/dL. If your report gives total testosterone in nmol/L, multiply by 28.84 to get ng/dL. If albumin is reported in g/L, divide by 10. Albumin is the same value the Corrected Calcium Calculator uses.

Free testosterone is shown in pg/mL and bioavailable testosterone in ng/dL, each with its percentage of the total, rounded to one decimal place. The sex you select changes only the rough reference range shown, not the calculation.

Why Calculated Free Testosterone Is Often Preferred Over Direct Assays

This isn't just a cost-driven workaround. An Endocrine Society position statement reviewing testosterone measurement methods found that direct analog immunoassays, a cheaper, faster lab method some panels use, showed genuinely poor reliability: the same sample measured by the same method at different laboratories varied by a coefficient of variation of 13 to 32%, and results for the identical sample using the identical method varied 2- to 6-fold between runs².

Equilibrium dialysis remains the true gold standard, but it's expensive and not widely available, which is why a well-validated calculation like Vermeulen's is generally considered more trustworthy than a cheap direct assay, not just a more convenient substitute for one².

A Worked Example

Take a total testosterone of 500 ng/dL and an SHBG of 30 nmol/L, with albumin left blank so the 4.3 g/dL default applies. The calculator returns a free testosterone of 108.5 pg/mL, which is 2.2% of the total, and a bioavailable testosterone of 263.5 ng/dL, or 52.7% of the total.

Keep the same total testosterone but double SHBG to 60 nmol/L. Free testosterone falls to 68.7 pg/mL (1.4%) and bioavailable testosterone falls to 166.9 ng/dL (33.4%). Total testosterone did not change.

Why SHBG and Albumin Matter

SHBG binds testosterone tightly, so more SHBG leaves less free hormone. Albumin binds it loosely. In the first example, entering an albumin of 3.0 g/dL instead of the default raises free testosterone to 132.2 pg/mL but lowers bioavailable testosterone to 228 ng/dL, because the loosely bound pool shrinks.

Vermeulen's team found that calculated free testosterone was almost identical to equilibrium dialysis results across a wide range of SHBG levels, except in pregnancy. In pregnancy, estradiol occupies part of SHBG's binding sites, so the calculation reads lower than the dialysis result¹. The same study found that the free androgen index (100 × total testosterone ÷ SHBG) and direct analog free testosterone assays were not reliable measures of available testosterone¹.

Getting a Reliable Result

The Endocrine Society's guideline on testosterone in men recommends measuring total testosterone in a fasting, morning blood sample with an accurate assay, and repeating that morning measurement to confirm a low result³. It recommends checking free testosterone, by equilibrium dialysis or an accurate formula, when total testosterone is near the lower limit of normal or when a condition alters SHBG³.

The guideline also says a diagnosis of low testosterone should only be made when symptoms and signs are present and levels are consistently low³.