Free Testosterone Calculator (Vermeulen Method)
The short answer
Enter total testosterone, SHBG and (optionally) albumin, and the calculator returns free testosterone in pg/mL and ng/dL, the free fraction as a percent, and bioavailable testosterone, using the Vermeulen mass-action equation. It's the same math behind most lab-reported 'calculated free testosterone' values, and it explains why a normal total can hide a low free level.
- Method
- Vermeulen 1999
- Inputs
- Total T · SHBG · albumin
- Outputs
- Free · bioavailable · %
- Cost
- Free
Free testosterone
93.2 pg/mL
9.32 ng/dL · 0.323 nmol/L
Free fraction
1.86%
Typical range ≈ 1.5–3% of total
Bioavailable testosterone
218 ng/dL
7.57 nmol/L (free + albumin-bound)
Vermeulen equation; Ka 3.6×10⁴ L/mol (albumin), Ks 1×10⁹ L/mol (SHBG). Educational — compare against the free testosterone reference range and discuss with a clinician.
How does SHBG change free testosterone?
Four worked examples at albumin 4.3 g/dL. Notice the third row: a normal total with high SHBG yields a lower free level than a low-normal total with low SHBG.
| Scenario | Total T (ng/dL) | SHBG (nmol/L) | Free T (pg/mL) | Free % |
|---|---|---|---|---|
| Mid-range total, average SHBG | 500 | 40 | 93.2 | 1.86% |
| Low-normal total, low SHBG | 320 | 20 | 82.0 | 2.56% |
| Normal total, high SHBG | 450 | 85 | 46.6 | 1.04% |
| High total, high SHBG | 800 | 70 | 107.5 | 1.34% |
Reference range for free testosterone (adults): Male, age 18+ 47.0 – 244 pg/mL; Female, age 18+ 0.100 – 6.40 pg/mL. See the free testosterone page for sources.
What is the Vermeulen equation?
Testosterone circulates in three pools: tightly bound to sex hormone-binding globulin (SHBG), loosely bound to albumin, and free. Vermeulen, Verdonck and Kaufman (1999) showed that free testosterone can be solved from total testosterone, SHBG and albumin using the law of mass action with two association constants — 3.6 × 10⁴ L/mol for albumin and 1 × 10⁹ L/mol for SHBG. The result agrees closely with equilibrium dialysis, the reference method, across the normal range.
In practice: free T = [−b + √(b² + 4·N·T)] / (2·N·Ks), where N = 1 + Ka·[albumin] and b = N + Ks·([SHBG] − [T]), with all concentrations in mol/L. Bioavailable testosterone is the free fraction plus the albumin-bound fraction, i.e. free T × N.
The Endocrine Society's 2018 hypogonadism guideline recommends measuring free testosterone by equilibrium dialysis or calculating it this way when SHBG may be altered — obesity, aging, thyroid disease, liver disease, or certain medications — rather than relying on direct analog immunoassays.
- Vermeulen A, Verdonck L, Kaufman JM. J Clin Endocrinol Metab 1999;84:3666–72. doi:10.1210/jcem.84.10.6079
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. 2018. doi:10.1210/jc.2018-00229
Frequently asked questions
Why calculate free testosterone instead of measuring it?
Direct free-T immunoassays are notoriously unreliable, so the Endocrine Society recommends either equilibrium dialysis (the reference method, expensive and slow) or a calculation from accurately measured total testosterone and SHBG. The Vermeulen calculation tracks equilibrium dialysis closely for most people and is what appears on most lab reports as 'calculated free testosterone'.
What SHBG and albumin values should I use?
Use the SHBG from the same blood draw as your total testosterone — it varies with age, weight, thyroid status and insulin resistance. If albumin wasn't measured, 4.3 g/dL is the standard assumed value and changes the result very little in healthy adults.
Why is my total testosterone normal but my free testosterone low?
Because most testosterone is bound to SHBG and unavailable to tissues. When SHBG is high (common with aging, thyroid excess, liver disease or some medications), more of the total is locked up, so the free fraction falls even while the total looks fine. That is exactly the situation this calculator makes visible — and why testing SHBG alongside total T matters.
Does this calculator diagnose low testosterone?
No. Diagnosis needs symptoms plus a confirmed low morning total testosterone on at least two separate days, interpreted by a clinician. The calculation helps explain a result; it doesn't replace that evaluation.
Which biomarkers does this tool cover?
Which tests give you these numbers?
- Free & Total Testosterone + SHBG Test$69.99
- Free & Total Testosterone Test$54.99
- Testosterone Test (Total)$33.99
Physician-authorized, drawn at Quest, Labcorp or BioReference. Results in 1–3 business days for most tests.