Testosterone calculator
Two calculations in one. The free & bioavailable estimator runs the Vermeulen equation on your total testosterone, SHBG and albumin to find the active fraction your total number hides. The TRT dose converter turns a prescribed milligram dose into millilitres and insulin-syringe units. Both are educational — not medical advice.
Uses the Vermeulen equation. If you don’t have an albumin value, the 4.3 g/dL default is standard. SHBG must be a lab value in nmol/L.
Free testosterone
10.8 ng/dL · 108 pg/mL · 0.38 nmol/L
- Bioavailable T
- 53%
- 264 ng/dL
- Total (entered)
- 500 ng/dL
- free + bound
Typical adult male free-testosterone ranges run roughly 9–30 ng/dL (≈ 50–210 pg/mL, ≈ 1.5–3% of total), but ranges are assay- and lab-specific. A number is not a diagnosis.
Why the free fraction matters
Most testosterone in your blood isn’t doing anything: roughly two-thirds is bound tightly to SHBG (sex-hormone-binding globulin) and most of the rest is bound loosely to albumin. Only the small free fraction — and the bioavailableportion (free plus the easily-released albumin-bound part) — can actually enter cells and act. That’s why two men with the same total testosterone can feel completely different: if one has high SHBG, far less of his total is free. The Vermeulen method estimates that free fraction from total T, SHBG and albumin by solving the binding equilibrium — the most widely-used calculated approach, validated against equilibrium dialysis (Vermeulen 1999, J Clin Endocrinol Metab). It’s an estimate, not a measurement, and it shifts at extreme SHBG or albumin, so read the result as a clinical figure to discuss — never a diagnosis on its own.
For what the numbers mean — reference ranges, how levels change with age, and why a single value isn’t a diagnosis — see testosterone levels by age. For whether and how low testosterone is treated, see our TRT evidence review, low-testosterone treatment options and the fertility-sparing enclomiphene route.
Common questions
- How do you calculate free testosterone?
- Free testosterone is estimated from three lab values — total testosterone, SHBG (sex-hormone-binding globulin) and albumin — using the Vermeulen equation, which solves the binding equilibrium for the unbound fraction. Most testosterone in blood is bound (tightly to SHBG, loosely to albumin); only the small free fraction (~1.5–3%) and the bioavailable fraction (free + albumin-bound) are biologically active. This calculator runs the Vermeulen method for you; enter SHBG in nmol/L and total T in ng/dL or nmol/L.
- What's the difference between free, bioavailable and total testosterone?
- Total testosterone is everything in the blood — most of it bound and inactive. Free testosterone is the unbound fraction that diffuses into cells. Bioavailable testosterone is free plus the loosely albumin-bound portion that readily dissociates, so it's also usable. When SHBG is high or low, total T can look 'normal' while free/bioavailable T tells a different story — which is why a free-T estimate is often the more informative number.
- Is the calculated free testosterone the same as a measured one?
- It's a well-validated estimate, not a direct measurement. The Vermeulen calculation depends on your SHBG assay and uses standard binding constants and a default albumin of 4.3 g/dL. It tracks measured (equilibrium-dialysis) free T closely in most people but can differ at extreme SHBG or albumin levels. Treat it as a clinical estimate to discuss with your provider, not a diagnosis.
- How do I convert my TRT dose in mg to insulin units?
- Switch to the TRT dose converter tab. Volume = dose ÷ concentration, and a U-100 insulin syringe reads 100 units per mL. For example, 100 mg of testosterone at 200 mg/mL is 0.5 mL — which is 50 units. The same dose is a different volume at 100 vs 200 vs 250 mg/mL, so always check your vial's concentration.
- What is a normal free testosterone level?
- Adult male free testosterone commonly runs roughly 9–30 ng/dL (about 50–210 pg/mL, ~1.5–3% of total), but the reference range is assay- and lab-specific, so compare against your own lab's range. A number alone isn't a diagnosis — low testosterone is diagnosed from symptoms plus consistently low values on repeat morning tests. See our explainer on testosterone levels by age.