Free Testosterone Calculator with Bioavailable Levels
Calculate free testosterone and bioavailable testosterone from total testosterone, SHBG, and albumin using the validated Vermeulen formula.
Enter total testosterone, sex hormone-binding globulin (SHBG), and albumin values to calculate free and bioavailable testosterone levels for accurate hormone assessment.
Free Testosterone Calculator with Bioavailable Levels
Calculate free testosterone and bioavailable testosterone from total testosterone, SHBG, and albumin using the validated Vermeulen formula.
About the Free Testosterone Calculator
Total testosterone measurements alone can be misleading when assessing androgen status, because most circulating testosterone is tightly bound to proteins and cannot exert hormonal effects. This calculator uses the validated Vermeulen method to determine how much testosterone is actually available to cells.
Testosterone in the bloodstream exists in three fractions: about 44–65% is bound to sex hormone-binding globulin (SHBG), 30–54% is loosely bound to albumin, and only 1–3% circulates as free (unbound) testosterone. Free testosterone directly enters cells and binds androgen receptors. Bioavailable testosterone includes both free testosterone and the albumin-bound fraction, since albumin-bound testosterone dissociates readily and is biologically accessible.
The Vermeulen 1999 formula calculates free testosterone using two binding constants: Ka (3.6 × 10⁴ L/mol) for albumin and KSHBG (5.97 × 10⁸ L/mol) for SHBG. The free fraction equals 1 divided by (1 + Ka × [Albumin] + KSHBG × [SHBG]). Bioavailable testosterone uses the same denominator but includes the albumin-bound portion: total T × (1 + Ka × [Albumin]) / denominator. These constants were derived from equilibrium dialysis studies and are widely endorsed in clinical endocrinology.
SHBG levels vary considerably with age, body weight, thyroid status, liver function, and medications. SHBG rises with aging and hyperthyroidism, which can lower free testosterone even when total testosterone is normal — a condition sometimes called SHBG excess. SHBG falls with obesity, hypothyroidism, anabolic steroid use, and insulin resistance, which may keep free testosterone higher than total T would suggest.
Reference ranges for free testosterone differ by laboratory and method. Common adult male reference ranges for free testosterone are approximately 50–210 pg/mL (Vermeulen method), while bioavailable testosterone typically falls between 130–430 ng/dL. Women have substantially lower values, typically 0.3–19 pg/mL free testosterone. These ranges are age-dependent and should be interpreted by a clinician in the context of symptoms and clinical history.
This calculator is an educational tool and does not replace professional medical evaluation. Testosterone testing and interpretation require clinical context including symptom assessment, physical examination, and sometimes repeat confirmatory testing.
Free testosterone calculation examples
Click any example button below the calculator to load these hormone profiles.
| Input Values | Free Testosterone | Interpretation |
|---|---|---|
| TT 650 ng/dL, SHBG 35 nmol/L, Albumin 4.3 g/dL | Free T ≈ 144 pg/mL, BioavailT ≈ 354 ng/dL | Normal adult male — free T well within reference range |
| TT 280 ng/dL, SHBG 45 nmol/L, Albumin 4.3 g/dL | Free T ≈ 55 pg/mL, BioavailT ≈ 134 ng/dL | Low testosterone — both total and free T below typical male reference range |
| TT 500 ng/dL, SHBG 80 nmol/L, Albumin 4.0 g/dL | Free T ≈ 68 pg/mL, BioavailT ≈ 166 ng/dL | High SHBG — normal total T but borderline-low free T due to elevated binding |
How to use the free testosterone calculator
- Enter your total testosterone value and select the units (ng/dL or nmol/L) from your lab report.
- Enter your SHBG (sex hormone-binding globulin) level in nmol/L — this is usually reported on the same panel.
- Enter your albumin level in g/dL (default is 4.3 g/dL if you do not have a recent value).
- Click Calculate Levels to see free testosterone in pg/mL and nmol/L, plus bioavailable testosterone in ng/dL.
- Compare results to reference ranges and discuss with your healthcare provider, as clinical context matters.
Free testosterone calculator FAQ
What is free testosterone and why does it matter?
Free testosterone is the small fraction of total testosterone not bound to SHBG or albumin. It can diffuse directly into cells and activate androgen receptors, making it the biologically active form. Even when total testosterone is normal, a high SHBG can leave free testosterone low, causing symptoms of androgen deficiency.
What is bioavailable testosterone?
Bioavailable testosterone includes free testosterone plus the fraction loosely bound to albumin. Unlike SHBG-bound testosterone, albumin-bound testosterone readily dissociates and enters tissues. Bioavailable testosterone is often considered a more clinically useful measure than free testosterone alone because it captures both accessible fractions.
What is the Vermeulen formula?
The Vermeulen formula (published in 1999) uses equilibrium binding constants for SHBG and albumin to calculate the free and bioavailable fractions of total testosterone. It is widely endorsed in clinical endocrinology as the preferred method when direct free testosterone measurement by equilibrium dialysis is not available.
What SHBG value should I enter if I do not have one?
SHBG is ideally obtained from the same blood draw as total testosterone. Typical adult male values range from 10–57 nmol/L and female values from 18–144 nmol/L. Without a measured SHBG, the calculation will be inaccurate. Ask your doctor to add SHBG to your next testosterone panel.
Why might my free testosterone be low even with normal total testosterone?
High SHBG is the most common reason. SHBG increases with aging, hyperthyroidism, liver disease, certain medications, and low body weight. When SHBG is elevated, more testosterone is bound and unavailable, reducing free and bioavailable fractions even when the total level appears normal.
Is this calculator a substitute for a blood test?
No. The calculated free testosterone is an estimate based on mathematical modeling, not a direct measurement. Direct equilibrium dialysis is the gold standard for measuring free testosterone. Calculated values can differ by up to 20–30% from dialysis values. Always interpret results with your healthcare provider.