Free & Bioavailable Testosterone Calculator

Most testosterone in your blood isn't available to your tissues. Roughly two-thirds is bound tightly to sex hormone-binding globulin (SHBG), and most of the rest is bound loosely to albumin; only a small slice — usually around 1.5 to 2 percent — circulates completely unbound. That unbound slice is 'free' testosterone, and together with the loosely albumin-bound portion it makes up 'bioavailable' testosterone — the fraction that can actually act on your body. It matters because your total testosterone can sit in the normal range while your free (usable) testosterone is low, or vice versa, depending on how much SHBG you have. When symptoms and total-T don't line up, a free or bioavailable estimate often explains why.

This tool uses the Vermeulen equation — a law-of-mass-action formula that estimates free and bioavailable testosterone from your total testosterone, SHBG, and albumin. It's the calculation most guidelines accept as a good, inexpensive stand-in for the gold-standard measured methods (equilibrium dialysis and ultracentrifugation), which are accurate but slow and not widely available. Many direct 'free testosterone' immunoassays, by contrast, are considered unreliable. A calculated value is still an estimate: it depends on the accuracy of your total-T and SHBG assays and on the assumptions built into the equation, so treat the number as a well-grounded approximation, not a precise measurement.

That usually comes down to SHBG. SHBG binds testosterone tightly and holds it out of circulation, so when SHBG is high — which happens with aging, liver disease, hyperthyroidism, some medications, and other conditions — a larger share of your total testosterone is locked up, leaving less free and bioavailable testosterone even though the total looks fine. The reverse happens when SHBG is low (common with obesity, insulin resistance, or hypothyroidism): free-T can look adequate despite a lower total. This mismatch is exactly why clinicians add SHBG and a free or bioavailable estimate when a man has symptoms of low testosterone but a borderline or normal total level.

There's no single universal number, because reference ranges depend on the lab, the assay, and the population, and they shift with age. As rough orientation, many labs report an adult-male calculated free-testosterone range on the order of about 5 to 21 ng/dL (roughly 1.5 to 2 percent of total), but yours may differ, and that's the whole point of comparing against the range printed on your own report. This calculator shows where your estimate falls against that general context only to help you read your result — it is not a diagnostic threshold. A value slightly below a range isn't automatically abnormal, and a diagnosis is never made from one calculation.

No. A low estimate is a reason to talk with a clinician, not a prescription. Testosterone deficiency is diagnosed by a clinician using symptoms plus low levels measured on blood drawn in the morning after an overnight fast, and repeated on a second morning because levels fluctuate — not from a single calculation or a home number. If low testosterone is confirmed, whether treatment is appropriate is a shared decision that weighs benefits against risks (including effects on fertility) and requires monitoring. Sometimes a treatable underlying cause — sleep, weight, medications, thyroid or pituitary problems — is behind the numbers. This tool never recommends testosterone therapy; it helps you understand a lab result before that conversation.

Yes. This calculator runs entirely in your browser. The values you enter are used to compute an estimate on your own device and are never sent to a server, saved, or shared — close the tab and they're gone. It's general educational information based on the published Vermeulen equation; it isn't medical advice, a diagnosis, or a test result, and it can't replace the clinician who ordered your labs.