# Free Testosterone Calculator Estimate free and bioavailable testosterone from total testosterone, SHBG, and albumin using the Vermeulen calculation method. ## What this calculates Only about 1 to 3% of your total testosterone circulates in its free, unbound form. The rest is bound to SHBG or albumin. This calculator uses the Vermeulen method to estimate your free and bioavailable testosterone from standard lab values. ## Inputs - **Total Testosterone** (ng/dL) — min 1, max 2000 — Total testosterone from a blood test. - **SHBG** (nmol/L) — min 1, max 200 — Sex Hormone-Binding Globulin level. - **Albumin** (g/dL) — min 1, max 7 — Serum albumin. Default 4.3 g/dL is the standard assumption if not measured. ## Outputs - **Free Testosterone** — Estimated free (unbound) testosterone - **Free Testosterone (%)** — Free testosterone as a percentage of total - **Bioavailable Testosterone** — Free + albumin-bound testosterone (not bound to SHBG) - **Bioavailable (%)** — Bioavailable testosterone as a percentage of total ## Details Testosterone in the blood exists in three forms: tightly bound to sex hormone-binding globulin (SHBG), loosely bound to albumin, and free (unbound). SHBG-bound testosterone is generally considered inactive because the binding is too tight for tissues to use. Albumin-bound testosterone can dissociate easily and is considered bioavailable along with the free fraction. The Vermeulen method calculates free testosterone using the law of mass action, factoring in the binding affinities of SHBG and albumin. It solves a quadratic equation based on total testosterone, SHBG concentration, and albumin level. This approach is more accurate than simple ratio methods (like total T divided by SHBG) and is widely used in clinical research. Free testosterone is clinically important because some patients have normal total testosterone but low free testosterone due to elevated SHBG. SHBG rises with age, liver disease, hyperthyroidism, and estrogen use. It decreases with obesity, hypothyroidism, and androgen use. Measuring or calculating free testosterone gives a clearer picture of androgen status than total testosterone alone. ## Frequently Asked Questions **Q: What is a normal free testosterone level?** A: Normal free testosterone ranges depend on the lab and method used, but general adult male reference ranges are roughly 5 to 21 ng/dL (or 1 to 3% of total). For adult females, the range is about 0.1 to 0.9 ng/dL. Levels decline with age in both sexes. Always compare your result to the reference range provided by your specific lab. **Q: Why is SHBG important?** A: SHBG binds testosterone tightly, making it unavailable to tissues. High SHBG means less free testosterone even if total testosterone is normal. This is why two people with the same total testosterone can have very different symptoms. SHBG increases with age, liver disease, and estrogen, and decreases with obesity and insulin resistance. **Q: How accurate is the Vermeulen calculation?** A: The Vermeulen method correlates well with equilibrium dialysis, which is considered the gold standard for measuring free testosterone. It is more accurate than simple ratios or analog immunoassay methods. However, it still relies on the accuracy of the total testosterone and SHBG measurements from your lab work. **Q: What is the difference between free and bioavailable testosterone?** A: Free testosterone is the small fraction (1 to 3%) that circulates completely unbound. Bioavailable testosterone includes both the free fraction and the portion loosely bound to albumin (roughly 25 to 50% of total). Since albumin releases testosterone easily, the bioavailable fraction better represents the testosterone your tissues can actually use. --- Source: https://vastcalc.com/calculators/health/free-testosterone Category: Health & Fitness Last updated: 2026-04-08