Free clinical tool
Free Testosterone Calculator
Free and bioavailable testosterone from total T, SHBG and albumin, by the Vermeulen equation — with the full conversion chart below.
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- Alpha Health Finder Editorial Team
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How free testosterone is calculated
Most testosterone in blood is not available to tissue. It is bound — tightly to sex hormone-binding globulin (SHBG), loosely to albumin. Only the small unbound fraction, plus the portion albumin releases readily, is usable.
The Vermeulen equation models both bindings and solves for the free concentration. This is why a man with a normal total testosterone and a high SHBG can still have a low free level, and why ordering SHBG alongside total changes the picture.
The formula
N = 1 + K(alb) · [albumin]
(N · K(SHBG)) · FT² + (N + K(SHBG) · ([SHBG] − [T])) · FT − [T] = 0
Solved for the positive root, where K(SHBG) = 1.0 × 10⁹ L/mol and K(alb) = 3.6 × 10⁴ L/mol. Bioavailable testosterone is N × FT.
Worked example
Total testosterone 500 ng/dL (17.34 nmol/L), SHBG 30 nmol/L, albumin 4.3 g/dL:
- N = 1 + 3.6×10⁴ × 6.47×10⁻⁴ = 24.30
- Free T = 10.8 ng/dL (108 pg/mL, 2.17% of total)
- Bioavailable = 264 ng/dL (53% of total)
Free testosterone chart — by total T and SHBG
Calculated free testosterone in ng/dL at albumin 4.3 g/dL. Read down for your total testosterone, across for your SHBG.
| Total T | SHBG 15 | SHBG 25 | SHBG 35 | SHBG 50 | SHBG 70 |
|---|---|---|---|---|---|
| 200 ng/dL6.9 nmol/L | 5.42.7% | 4.32.2% | 3.61.8% | 2.91.4% | 2.21.1% |
| 300 ng/dL10.4 nmol/L | 8.32.8% | 6.72.2% | 5.61.9% | 4.41.5% | 3.51.2% |
| 400 ng/dL13.9 nmol/L | 11.42.9% | 9.32.3% | 7.71.9% | 6.11.5% | 4.71.2% |
| 500 ng/dL17.3 nmol/L | 14.62.9% | 11.92.4% | 9.92.0% | 7.91.6% | 6.11.2% |
| 600 ng/dL20.8 nmol/L | 17.93.0% | 14.72.4% | 12.32.0% | 9.71.6% | 7.51.3% |
| 700 ng/dL24.3 nmol/L | 21.33.0% | 17.62.5% | 14.72.1% | 11.71.7% | 9.01.3% |
| 800 ng/dL27.7 nmol/L | 24.73.1% | 20.62.6% | 17.32.2% | 13.81.7% | 10.61.3% |
| 900 ng/dL31.2 nmol/L | 28.23.1% | 23.72.6% | 20.02.2% | 15.91.8% | 12.31.4% |
| 1000 ng/dL34.7 nmol/L | 31.83.2% | 26.82.7% | 22.82.3% | 18.21.8% | 14.01.4% |
Every figure computed by the same function the calculator uses, so the chart and the tool cannot disagree.
Harmonized total testosterone reference range
For healthy non-obese men aged 19–39, harmonized across four cohort studies in the United States and Europe.
| Percentile | ng/dL | nmol/L |
|---|---|---|
| 2.5th | 264 | 9.2 |
| 5th | 303 | 10.5 |
| 50th (median) | 531 | 18.4 |
| 95th | 852 | 29.5 |
| 97.5th | 916 | 31.8 |
Travison TG, et al. J Clin Endocrinol Metab. 2017;102(4):1161–1173 (PMID 28324103)
Frequently asked questions
- How do you calculate free testosterone?
- Free testosterone is calculated from total testosterone, SHBG and albumin using the Vermeulen equation. Testosterone binds tightly to SHBG and loosely to albumin; the equation models both bindings and solves a quadratic for the unbound fraction. At a total testosterone of 500 ng/dL with SHBG of 30 nmol/L and albumin of 4.3 g/dL, free testosterone is about 10.8 ng/dL — roughly 2.2% of total.
- What is a normal free testosterone level?
- Free testosterone is typically 1–3% of total testosterone in men, which puts most normal results between roughly 5 and 25 ng/dL (50–250 pg/mL). There is no single harmonized reference range for calculated free testosterone the way there is for total, so the figure must be read against your own laboratory’s range and your symptoms.
- Why is my total testosterone normal but my free testosterone low?
- High SHBG. SHBG binds testosterone tightly and makes it unavailable, so a man with a total of 500 ng/dL and SHBG of 100 nmol/L has a lower free testosterone than a man with a total of 300 ng/dL and SHBG of 20. This is the single most common reason a total-only result misleads, and it is why SHBG is worth ordering.
- What is the difference between free and bioavailable testosterone?
- Free testosterone is the unbound fraction. Bioavailable testosterone is the free fraction plus the portion loosely bound to albumin, which dissociates readily enough to be usable. Bioavailable is typically 30–60% of total, against 1–3% for free.
- Do I need my albumin value?
- Not usually. The calculation is relatively insensitive to albumin within the normal range, so 4.3 g/dL is used as a default. If your albumin is genuinely abnormal — liver disease, nephrotic syndrome, significant malnutrition — enter your measured value, because low albumin raises the calculated free fraction.
- How do I convert testosterone from nmol/L to ng/dL?
- Multiply nmol/L by 28.84 to get ng/dL, or multiply ng/dL by 0.0347 to get nmol/L. The factor comes from testosterone’s molecular weight of 288.42 g/mol. A total of 500 ng/dL is 17.34 nmol/L. The calculator above accepts either unit.
- Is calculated free testosterone as accurate as a direct measurement?
- The reference method is equilibrium dialysis, and calculated values are not interchangeable with it. Vermeulen and colleagues found calculated free testosterone tracked dialysis results closely across a wide SHBG range, which is why the calculation is widely used, but direct analog immunoassays for free testosterone are considered unreliable. Discuss which method your laboratory used with your clinician.
Sources
- [1]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(10):3666–3672
- [2]Travison TG, et al. Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies. J Clin Endocrinol Metab. 2017;102(4):1161–1173
- [3]Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744
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<script src="https://alphahealthfinder.com/embed/resize.js" async></script>Next steps
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- Bioavailable testosterone calculator
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Educational reference, not medical advice. A calculated estimate does not diagnose anything, and no result here replaces interpretation by your clinician.