Free testosterone calculator
Free testosterone is the small share of your testosterone that is not bound to SHBG or albumin, usually about 1 to 3 percent. Enter total testosterone, SHBG and albumin from your lab report to calculate free and bioavailable testosterone with the Vermeulen equation.
This is an educational estimate, not medical advice. It doesn’t recommend a dose. Real levels vary a lot between people; only a blood test shows yours. Talk to your prescriber before changing anything. Labs calculate and measure free testosterone differently; compare only with your own lab’s range.
– pg/mL
– ng/dL
- SHBG-bound
- Albumin-bound
- Free
Enter total testosterone and SHBG to see the result.
Log your labs once and TRTracker shows total, free and SHBG trends over time.
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How it works
Testosterone travels in the blood in three forms. Most is held tightly by sex hormone-binding globulin (SHBG). A large part is held loosely by albumin. A small share is free. The Vermeulen equation works out the free share from the law of mass action, using how strongly each protein binds testosterone:
N = 1 + Kalb × [albumin]
free T = (−b + √(b² + 4 · N · Kshbg · TT)) ÷ (2 · N · Kshbg)
b = N + Kshbg × ([SHBG] − TT)
All concentrations are in mol/L. Kshbg is 1 × 10⁹ L/mol and Kalb is 3.6 × 10⁴ L/mol, the constants Vermeulen used; albumin is converted with a molar mass of 69,000 g/mol. Bioavailable testosterone is free × N. These are the same constants the TRTracker app uses.
Units: 1 nmol/L of total testosterone is 28.8 ng/dL; 1 pg/mL of free testosterone is 3.47 pmol/L or 0.1 ng/dL.
The calculation is an estimate. It is usually close to measurement by equilibrium dialysis, but it can differ from your lab’s own free testosterone, especially with very high or low SHBG.
Sources
- 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
- Bhasin S et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab, 2018
- Mulhall JP et al. Evaluation and management of testosterone deficiency: AUA guideline. J Urol, 2018
Questions
What is free testosterone?
Most testosterone in the blood is bound to proteins: tightly to SHBG and loosely to albumin. Only a small share, usually about 1 to 3 percent, floats free. Free plus albumin-bound testosterone is called bioavailable testosterone.
How is free testosterone calculated?
With the Vermeulen equation (1999). It uses your total testosterone, SHBG and albumin and the binding strength of each protein to work out how much testosterone is left unbound. The formula and its constants are shown on this page.
Why is my lab’s free testosterone different?
Labs measure or calculate free testosterone in different ways (equilibrium dialysis, other formulas, or direct immunoassays), and each has its own reference range. Compare a result with the range printed on the same lab report, not with another lab’s.
What albumin should I use?
Use the albumin from the same blood test if you have it. If not, the calculator uses 4.3 g/dL, the usual default; albumin changes the result only a little unless it is unusually low or high.
Is there an optimal free testosterone level?
This calculator does not set targets. It shows your result next to the reference range your lab printed, if you enter it. What range is right for you is a conversation with your prescriber.
Is anything I type saved?
No. The calculation runs in your browser and nothing is stored or sent.
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Free tools for education only. Nothing you type here is stored; the maths runs in your browser, and your result is only sent if you ask us to email it. TRTracker does not provide medical advice, diagnosis or treatment. Privacy policy