Free Testosterone Calculator
Estimate your free testosterone from Total T, SHBG, and Albumin using the Vermeulen equation. See where you fall relative to reference ranges.
Typical male range: 264-916 ng/dL
Typical range: 10-57 nmol/L (male), 18-144 nmol/L (female)
Normal range: 3.5-5.0 g/dL. Most people can leave this at 4.3.
Estimated Free Testosterone
Above reference range
Free T (SI units)
Percentage free
Typical: 1-3% of total
What is Free Testosterone?
Only about 1-3% of your total testosterone circulates freely in the blood. The rest is bound to proteins: roughly 44-65% binds tightly to Sex Hormone-Binding Globulin (SHBG) and 33-54% binds loosely to albumin. Free testosterone is the fraction that can directly enter cells and activate androgen receptors, which is why many clinicians consider it the more clinically meaningful number.
Two people with identical total testosterone levels can have very different free T levels depending on their SHBG. Someone with high SHBG has more testosterone locked up and less available for use. This is why a total T of 500 ng/dL might feel optimal for one person and symptomatic for another.
This calculator uses the Vermeulen equation (law of mass action), the same method used in clinical practice and by the International Society for the Study of the Aging Male (ISSAM). It iteratively solves for the equilibrium between free testosterone, SHBG-bound testosterone, and albumin-bound testosterone.
Free T vs Total T
Total Testosterone
Measures everything in your blood — free + SHBG-bound + albumin-bound. It's the standard first test most clinicians order. Useful as a starting point, but can miss the full picture when SHBG is abnormally high or low. Normal male range is roughly 264-916 ng/dL (9.2-31.8 nmol/L), though "optimal" is debated.
Free Testosterone
Measures only the unbound fraction that's biologically active. Particularly important when total T is borderline or when SHBG is known to be high (common with oral estrogen use, aging, liver conditions, or hyperthyroidism). Direct measurement via equilibrium dialysis is the gold standard, but calculated free T using the Vermeulen method is widely accepted and more accessible.
Why SHBG Matters
SHBG is a protein produced by the liver that binds testosterone with high affinity. Higher SHBG means less free testosterone — even if your total T is normal. Several factors influence SHBG levels:
| Increases SHBG | Decreases SHBG |
|---|---|
| Aging | Obesity / high BMI |
| Oral estrogen (HRT) | Insulin resistance / Type 2 diabetes |
| Hyperthyroidism | Hypothyroidism |
| Liver disease (cirrhosis) | Anabolic steroid use |
| Anticonvulsant medications | High-dose testosterone |
Reference Ranges
| Measure | Male | Female | Notes |
|---|---|---|---|
| Free T (ng/dL) | 5-21 | 0.3-1.9 | Vermeulen calculation |
| Free T (pmol/L) | 170-730 | 10-66 | SI equivalent |
| Free T (%) | 1.0-3.0% | 0.5-1.5% | Percentage of total |
| Optimal TRT (ng/dL) | 9-25 | N/A | Clinical optimization target |
| SHBG (nmol/L) | 10-57 | 18-144 | Age-dependent |
| Total T (ng/dL) | 264-916 | 15-70 | Standard lab reference |
Frequently Asked Questions
How accurate is the Vermeulen calculation?+
The Vermeulen equation correlates well with equilibrium dialysis (the gold standard) in most clinical scenarios. It's the method recommended by the International Society for the Study of the Aging Male (ISSAM) and is used by most online medical calculators. It's less accurate at very high or very low SHBG values.
Why does my free T percentage seem low?+
Only 1-3% of total testosterone is free — that's normal physiology. If your free T percentage is below 1%, you likely have elevated SHBG. This is common with aging, oral estrogen use, and certain liver conditions. Discuss with your clinician whether addressing the underlying SHBG elevation is appropriate.
Should I use this instead of a lab test?+
No — this calculator provides an estimate based on the Vermeulen method. Direct measurement via equilibrium dialysis is more accurate. However, many labs use their own calculations rather than direct measurement, and the Vermeulen method is well-validated for clinical use. Always confirm important clinical decisions with actual lab work.
What albumin value should I use?+
If you have a recent albumin result from bloodwork, use that. If not, 4.3 g/dL is a reasonable default — it's the midpoint of the normal range (3.5-5.0 g/dL). Albumin has a smaller effect on the calculation than SHBG, so a small variance here won't dramatically change your result.
Why is free T important for TRT monitoring?+
During TRT, total testosterone often rises into the high-normal or supraphysiological range, but free T tells you how much is actually bioavailable. Some people on TRT have high total T but modest free T due to elevated SHBG. Free T is a better indicator of whether your dose is actually doing its job at the cellular level.
Does this work for people on HRT?+
Yes — the Vermeulen equation works regardless of the source of testosterone (endogenous or exogenous). For transmasculine individuals on testosterone HRT, this calculator can help monitor free T levels alongside total T. Note that oral estrogen can significantly raise SHBG, which may lower free T even with adequate total T levels.
Data 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;84(10):3666-3672.
ISSAM (International Society for the Study of the Aging Male). Recommendations for the calculation of free and bioavailable testosterone.
Ly LP, Handelsman DJ. Empirical estimation of free testosterone from testosterone and sex hormone-binding globulin immunoassays. Eur J Endocrinol. 2005;152(3):471-478.
Related Tools
Doseline provides informational tools only. It is not a medical device and does not provide medical advice. Always consult a qualified healthcare provider.
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