How to Use the Free Testosterone Calculator
Enter total testosterone, SHBG, and albumin from the same blood test panel. Choose whether total testosterone is reported in ng/dL or nmol/L, and choose whether albumin is reported in g/dL or g/L.
If your report only lists total testosterone and you need a unit conversion, use the ng/dL to nmol/L Conversion Calculator first. This page estimates calculated free testosterone, which requires SHBG and albumin.
What Is Free Testosterone?
Most testosterone in blood is bound to proteins. SHBG binds testosterone tightly, albumin binds it more loosely, and a small fraction remains free. Free testosterone is the unbound fraction available in circulation.
Calculated free testosterone can be useful when total testosterone does not match symptoms or when SHBG is unusually high or low. It is commonly reviewed with total testosterone, SHBG, albumin, and clinical history.
Free Testosterone Formula
This calculator uses a mass-action approach commonly associated with the Vermeulen calculation. It converts total testosterone, SHBG, and albumin into mol/L, then solves for free testosterone based on binding constants.
Ka albumin = 3.6 x 10^4 L/mol; Kt SHBG = 1.0 x 10^9 L/mol Total T = free T + albumin-bound T + SHBG-bound T Results are estimates, not direct measurements. Equilibrium dialysis is often treated as a reference method for measured free testosterone, while calculated free testosterone depends on the formula and input quality.
Example Free Testosterone Calculation
With total testosterone of 600 ng/dL, SHBG of 40 nmol/L, and albumin of 4.3 g/dL, the calculator estimates free testosterone at about 11.32 ng/dL, or 113.2 pg/mL.
The same result is about 0.392 nmol/L, with percent free testosterone around 1.89%. These numbers should be compared with the reference range from the laboratory or clinician using the result.
Example Results Table
These examples show how SHBG can change calculated free testosterone even when total testosterone looks similar.
| Total testosterone | SHBG | Albumin | Calculated free T | Percent free |
|---|---|---|---|---|
| 300 ng/dL | 20 nmol/L | 4.3 g/dL | 6.81 ng/dL | 2.27% |
| 500 ng/dL | 30 nmol/L | 4.3 g/dL | 10.08 ng/dL | 2.02% |
| 600 ng/dL | 40 nmol/L | 4.3 g/dL | 11.32 ng/dL | 1.89% |
| 800 ng/dL | 60 nmol/L | 4.3 g/dL | 13.47 ng/dL | 1.68% |
Calculated Free Testosterone vs Direct Free Testosterone
Calculated free testosterone is estimated from total testosterone, SHBG, and albumin. Direct free testosterone is measured by a lab method, and different methods may not agree perfectly.
If your free testosterone result came from a direct assay, do not assume it will exactly match this calculator. Use the same method and reference range when tracking trends over time whenever possible.
Common Mistakes to Avoid
Do not enter free testosterone into the total testosterone field. This calculator starts with total testosterone, SHBG, and albumin. Also check whether your total testosterone is reported in ng/dL or nmol/L before entering the value.
Do not use this result as a treatment target by itself. Testosterone decisions require symptoms, timing of blood draw, repeat testing, assay quality, medications, fertility goals, and a qualified clinician.
Frequently Asked Questions
What values do I need?
You need total testosterone, SHBG, and albumin. If albumin was not measured, some calculators assume a default value, but a measured value is better when available.
What albumin value should I use if my lab does not include it?
A common default is 4.3 g/dL, but albumin can differ with illness, nutrition status, liver disease, kidney disease, and other factors. For medical use, ask your clinician whether a default is appropriate.
Can I use this calculator for women?
The same binding concept applies, but reference ranges and clinical interpretation differ substantially by sex, age, assay, and clinical situation. Use a laboratory-specific reference range and medical guidance.
Is percent free testosterone the same as free androgen index?
No. Percent free testosterone is calculated from estimated free and total testosterone. Free androgen index is usually total testosterone divided by SHBG and multiplied by 100. They are related concepts, but they are not the same calculation.
References
- Vermeulen et al.: Estimation of Free Testosterone in Serum
- Endocrine Society: Testosterone Therapy Clinical Practice Guideline
Conclusion
This Free Testosterone Calculator estimates calculated free testosterone from total testosterone, SHBG, and albumin. It is most useful when you want to understand how binding proteins may affect the active fraction of testosterone.
Use the result as an educational estimate, then interpret it with your lab report, symptoms, and healthcare provider.