OptiPin
Free testosterone calculator
Enter total testosterone, SHBG, and albumin to calculate free and bioavailable testosterone using the Vermeulen equation - the formula used in clinical practice.
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About free & bioavailable testosterone
Why free testosterone matters. Most circulating testosterone is bound to SHBG and is not biologically active. Free testosterone (~1–3% of total) and the loosely albumin-bound fraction make up bioavailable testosterone - the portion available to tissues. When SHBG is high or low, total testosterone can look normal while free testosterone is not. → free vs total testosterone explains the mechanism, what shifts SHBG, and the thresholds that matter.
The Vermeulen equation. This tool uses the Vermeulen et al. (1999) law-of-mass-action method with association constants KSHBG = 1.0×10⁹ L/mol and Kalbumin = 3.6×10⁴ L/mol. Calculated free testosterone tracks closely with equilibrium dialysis, the reference method.
Typical adult-male reference range. Free testosterone is roughly 5–21 ng/dL (≈174–729 pmol/L) in adult men, though ranges vary widely by lab and age. Ranges differ for women and are not shown here.
Free testosterone chart by total T and SHBG
The same Vermeulen equation the calculator runs, precomputed across the combinations people actually have. It exists to make one point visible: at a fixed total testosterone, SHBG alone can more than halve your free testosterone. A man at 600 ng/dL total with SHBG 15 has roughly the free testosterone of a man at 1,000 ng/dL with SHBG 45.
| Total T | SHBG 15 | SHBG 25 | SHBG 35 | SHBG 45 | SHBG 60 | SHBG 80 |
|---|---|---|---|---|---|---|
| 300 ng/dL | 8.3 | 6.7 | 5.6 | 4.8 | 3.9 | 3.1 |
| 400 ng/dL | 11.4 | 9.3 | 7.7 | 6.6 | 5.3 | 4.3 |
| 500 ng/dL | 14.6 | 11.9 | 9.9 | 8.5 | 6.9 | 5.5 |
| 600 ng/dL | 17.9 | 14.7 | 12.3 | 10.5 | 8.5 | 6.7 |
| 700 ng/dL | 21.3 | 17.6 | 14.8 | 12.6 | 10.2 | 8.1 |
| 800 ng/dL | 24.7 | 20.6 | 17.3 | 14.8 | 12.0 | 9.5 |
| 900 ng/dL | 28.2 | 23.7 | 20.0 | 17.1 | 13.9 | 10.9 |
| 1,000 ng/dL | 31.8 | 26.8 | 22.8 | 19.6 | 15.9 | 12.5 |
Free testosterone in ng/dL, albumin held at the 4.3 g/dL population default. Amber = below the typical adult-male range of 5–21 ng/dL, blue = above it. SHBG is in nmol/L. Read down a column to see how much free testosterone moves when only SHBG changes.
Why SHBG drives the number
SHBG binds testosterone tightly, and what it binds is not available to tissue. That makes it the dominant term in the equation and the reason two men with identical total testosterone can have very different free levels. SHBG is not fixed, which is why a free testosterone estimate is a snapshot rather than a constant.
- Ageing, steadily, across adult life
- Hyperthyroidism
- Liver disease and cirrhosis
- Oestrogen, including oral oestrogen therapy
- Anticonvulsants such as phenytoin and carbamazepine
- Sustained caloric restriction and low body fat
- Obesity and insulin resistance
- Type 2 diabetes and metabolic syndrome
- Hypothyroidism
- Exogenous androgens, including TRT itself
- Glucocorticoids
- Growth hormone excess
This is why the calculation is worth running alongside a total testosterone result rather than instead of it. A normal total with high SHBG and a normal total with low SHBG are different clinical pictures, and the total on its own does not distinguish them. See insulin resistance and testosterone and thyroid, testosterone and SHBG for the mechanisms.
How free testosterone is calculated, and why methods disagree
There is no single agreed method, and the one your lab or calculator uses changes the number. The spread between methods is a real source of confusion when comparing a calculated result against a lab-reported one.
Physically separates the free fraction and measures it directly. It is the standard everything else is judged against, but it is slow, expensive, and not what most panels order.
A law-of-mass-action model using KSHBG = 1.0×109 L/mol and Kalbumin = 3.6×104 L/mol. It tracks equilibrium dialysis closely across most of the range and is the method most published calculators implement, which makes it the sensible default for comparing your own results over time.
A cheap direct measurement that some panels report as “free testosterone”. It is widely criticised for tracking total testosterone more than the actual free fraction, and it can read substantially low. If your panel reports free testosterone without also reporting SHBG, this is often the method behind it.
Practical consequence: compare like with like. A calculated Vermeulen value from this page and a direct-immunoassay value from your lab are not interchangeable, and a difference between them is usually the method rather than a change in your physiology.
Frequently asked questions
What is the Vermeulen equation?
A law-of-mass-action model published by Vermeulen, Verdonck and Kaufman in 1999 that estimates free testosterone from total testosterone, SHBG and albumin. It solves a quadratic derived from the binding equilibria, using association constants of 1.0×109 L/mol for SHBG and 3.6×104 L/mol for albumin. It is the method this calculator implements.
What is bioavailable testosterone?
Free testosterone plus the fraction loosely bound to albumin. Because the albumin bond is weak enough to dissociate at the capillary, that fraction is generally considered available to tissue, whereas the SHBG-bound fraction is not. Bioavailable testosterone is typically 30 to 50 percent of total, against roughly 1 to 3 percent for free alone.
How do I calculate free testosterone by hand?
You would convert total testosterone and SHBG to mol/L, convert albumin to mol/L using its molecular weight of 66,437 g/mol, then solve the quadratic a·FT² + b·FT + c = 0 where a = (Kalb·Calb + 1)·KSHBG. It is arithmetic rather than anything conceptually hard, but it is fiddly enough that entering the three numbers above is faster and less error-prone.
What is a normal free testosterone level?
Roughly 5 to 21 ng/dL (about 174 to 729 pmol/L) in adult men, though this varies meaningfully by lab, assay and age, and differs for women. Treat the range as orientation rather than a threshold: a result near either edge means considerably less than the trend across several draws taken under the same conditions.
Why is my calculated free testosterone different from my lab’s?
Almost always the method. Labs variously report equilibrium dialysis, a calculated value, or a direct analog immunoassay, and those do not agree with each other. A calculated Vermeulen result is best compared against other calculated Vermeulen results, which is the case for keeping your own history in one place.
Do I need my albumin measured?
Not usually. Albumin varies far less between people than SHBG does, so the 4.3 g/dL population default is a reasonable stand-in and moving it within the normal range shifts the result only slightly. If you have a measured albumin, especially with liver or kidney disease where it can be genuinely abnormal, enter it.
Is this a substitute for a lab free testosterone test?
No. It is an educational estimate calculated from numbers you already have, not a diagnosis and not a measurement. Interpreting a hormone result needs clinical context this page does not have: symptoms, history, draw timing, assay and repeat testing. Do not start, change or stop treatment based on it.