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Most of your testosterone is bound up and unavailable — tightly to SHBG, loosely to albumin. Free and bioavailable testosterone are the fractions your body can actually use, and when SHBG runs high or low, a normal-looking total can hide the real story. This estimates both from your labs using the Vermeulen equation — the method that tracks closest to the gold standard.
You'll need three numbers from a blood test: total testosterone, SHBG, and — optionally — albumin. Everything runs in your browser; nothing you enter is saved or sent.
Method & important note
Calculated by the Vermeulen method: Vermeulen A, et al. J Clin Endocrinol Metab. 1999. It models testosterone's binding to SHBG and albumin; individual labs and calculators use slightly different constants, so treat the output as an estimate. Default albumin is 4.3 g/dL when left blank.
Not a diagnosis and not medical advice. Whether a value is "low" depends on your total testosterone, your symptoms, and your lab's own reference range — read it with a clinician. Read your labs walks through what each marker means.
Common questions
Most testosterone in your blood is bound — tightly to SHBG and loosely to albumin — and unavailable to tissues. Free testosterone is the small fraction (usually around 1.5–2.5% of total) circulating unbound. Bioavailable testosterone is that free fraction plus the loosely albumin-bound portion your body can still use.
Because SHBG changes the gap between them. When SHBG is high, total testosterone can look normal while the usable free fraction is low; when SHBG is low, the reverse. Calculated free testosterone estimates the usable fraction from total T, SHBG, and albumin — often closer to how you actually feel than the total alone.
For most people it tracks well with the gold-standard equilibrium dialysis, and it is more reliable than the older direct 'analog' free-testosterone immunoassays. It is still an estimate — labs and calculators use slightly different binding constants — so use it as context for a conversation with your clinician, not as a diagnosis.
If you don't have an albumin result, the standard default of 4.3 g/dL (43 g/L) is fine — albumin varies little between healthy people and changes the result only modestly. If you have your own value, use it.
Reference ranges vary by lab, assay, and age, which is why there is no single cutoff here. Bring the number to a clinician who can read it against your total testosterone, your symptoms, and the lab's own range.
No. The calculator runs entirely in your browser, and nothing you enter is saved or transmitted.