Free Testosterone Calculator
Total testosterone can read perfectly normal while the amount your body can actually use is low. Most of it is bound to SHBG and unavailable — this works out how much is free and bioavailable, using the Vermeulen equation.
Why free testosterone matters
Roughly 98% of the testosterone in your blood is bound — tightly to SHBG, and loosely to albumin. Only the free fraction, plus what comes off albumin easily, is available to your tissues.
That's why two men with the same total testosterone can feel completely different. High SHBG binds more of it up, so symptoms of low testosterone can appear while a total testosterone result still sits inside the reference range. It's also why a good assessment measures SHBG rather than total alone.
What this testosterone calculator does
This uses the Vermeulen equation (Vermeulen, Verdonck & Kaufman, Journal of Clinical Endocrinology & Metabolism, 1999), which solves the binding equilibrium between testosterone, SHBG and albumin. It's the method most clinical calculators and papers use.
It gives you two numbers your lab report usually doesn't: free testosterone, the fraction circulating unbound, and bioavailable testosterone, which adds what comes off albumin easily. Both are worked out from figures already on a standard blood test, so you don't need anything extra measured.