Free labs & hormones tool
Free Testosterone Calculator (Vermeulen)
Free testosterone is calculated from total testosterone, SHBG and albumin with the Vermeulen equation. For a total of 500 ng/dL (17.3 nmol/L), SHBG of 40 nmol/L and albumin of 4.3 g/dL, calculated free testosterone is about 9.3 ng/dL (93 pg/mL), roughly 1.9% of total. The calculator also gives bioavailable testosterone.
Prepared by Prospyr · Reviewed October 3, 2026 · Free, no sign-up, runs in your browser
Sex hormone-binding globulin, as reported by the lab.
If there is no albumin result, 4.3 g/dL (43 g/L) is the average the ISSAM calculator uses.
Calculated free testosterone (Vermeulen)
9.3ng/dL
93 pg/mL, or 323 pmol/L. That is 1.86% of total testosterone.
Bioavailable T
218 ng/dL
7.57 nmol/L
Bioavailable share
43.7%
Free + albumin-bound
Decision support only; it does not replace clinical judgment. The equation is not valid in pregnancy, and this page does not interpret the result.
Vermeulen free testosterone equation
With total testosterone T and SHBG S in mol/L, and N = 1 + Ka × [albumin], free testosterone F is the positive root of:
Kt × N × F² + (N + Kt × (S − T)) × F − T = 0
Kt = 1 × 109 L/mol (SHBG) and Ka = 3.6 × 104 L/mol (albumin), the constants used by the ISSAM calculator. Albumin of 4.3 g/dL gives N = 23.43. Bioavailable testosterone is F × N.
Worked example
Total testosterone 500 ng/dL, SHBG 40 nmol/L, albumin 4.3 g/dL.
- 500 ÷ 28.842 = 17.3 nmol/L total testosterone.
- N = 1 + 36,000 × (43 ÷ 69,000) = 23.43.
- Solving the quadratic gives F = 0.323 nmol/L, which is 9.3 ng/dL or 93 pg/mL, 1.9% of total.
- Bioavailable = 0.323 × 23.43 = 7.57 nmol/L (218 ng/dL, 43.7% of total).
Free, bioavailable and total testosterone
Total testosterone includes everything in serum. Free testosterone is the unbound part. Bioavailable adds the albumin-bound part. Vermeulen, Verdonck and Kaufman compared several methods and found calculated free testosterone from total T and SHBG correlated closely with equilibrium dialysis, which is why the calculation is used in routine work.
What this page does not do
It calculates the published equation. It does not interpret the result, set a reference range or advise on treatment. If your practice records hormone labs with the visit, Prospyr's charting keeps them with the patient record, and the med spa software page shows the wider workflow. For other lab math, see the eGFR calculator.
Frequently asked questions
How is free testosterone calculated?
The Vermeulen method solves the binding equilibrium of testosterone to SHBG and albumin. It uses the total testosterone, the SHBG concentration, the albumin concentration, an association constant of 1 × 10^9 L/mol for SHBG and 3.6 × 10^4 L/mol for albumin. The result is the unbound fraction.
What is bioavailable testosterone?
Bioavailable testosterone is free testosterone plus the albumin-bound portion, which is weakly bound. Vermeulen and colleagues describe it as the fraction of serum testosterone not bound to SHBG.
How do I convert total testosterone from ng/dL to nmol/L?
Divide ng/dL by 28.842 to get nmol/L. 500 ng/dL is 17.3 nmol/L. Free testosterone in pg/mL equals ng/dL times 10.
What albumin value should I use?
Use the albumin from the same blood draw if you have it. If not, 4.3 g/dL (43 g/L) is the average value used by the ISSAM free testosterone calculator.
What is a normal free testosterone level?
Reference intervals depend on the lab, the method, age and sex, so this page does not label a result. Use the interval printed on the lab report.
When is the Vermeulen calculation not valid?
Vermeulen and colleagues found calculated and measured free testosterone were almost identical except during pregnancy, when estradiol occupies a substantial share of SHBG binding. This calculator does not interpret results or give treatment guidance.
Sources and scope
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab 1999;84:3666-72
Peer-reviewed. The calculation of free testosterone from total T and SHBG; not valid in pregnancy.
- ISSAM: Free and bioavailable testosterone calculator parameters
Association constants (SHBG 10^9, albumin 3.6 × 10^4), default albumin 4.3 g/dL and the quadratic form.
Decision support only; it does not replace clinical judgment. The tool applies the published equation. It does not interpret results, give reference ranges or recommend treatment.