Clinical Hormone Calculator
Check Your Active Testosterone
Calculate your free and bioavailable testosterone using the gold-standard Vermeulen equation. Used by endocrinologists and men’s health clinics worldwide to assess active hormone levels from standard blood work.
Normal range: 300–1,000 ng/dL for adult men
Normal range: 10–80 nmol/L for adult men
Normal range: 3.5–5.0 g/dL
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Enter your lab values and press Calculate
Uses the Vermeulen formula for clinical accuracy
Reference Ranges – Free & Bioavailable Testosterone
Clinical reference ranges for adult men. Values vary by age, time of day, and laboratory standards.
For Men (Age 18–50)
| Marker | Low | Normal | High |
|---|---|---|---|
| Free T (ng/dL) | < 5.0 | 5.0–25.0 | > 25.0 |
| Free T (pg/mL) | < 50 | 50–250 | > 250 |
| Bioavailable T (ng/dL) | < 100 | 100–400 | > 400 |
| Total T (ng/dL) | < 300 | 300–1,000 | > 1,000 |
For Men (Age 50+)
| Marker | Low | Normal | High |
|---|---|---|---|
| Free T (ng/dL) | < 4.0 | 4.0–20.0 | > 20.0 |
| Free T (pg/mL) | < 40 | 40–200 | > 200 |
| Bioavailable T (ng/dL) | < 80 | 80–350 | > 350 |
| Total T (ng/dL) | < 250 | 250–900 | > 900 |
Clinical Note: Free testosterone is the active fraction not bound to SHBG. Bioavailable testosterone includes free T plus T loosely bound to albumin. These are the clinically relevant markers for diagnosing low testosterone (hypogonadism).
Vermeulen Formula – Calculate It Yourself
The Vermeulen equation is the most widely accepted method for calculating free and bioavailable testosterone from total testosterone, SHBG, and albumin. Used in endocrinology and men’s health clinics worldwide.
The Vermeulen Equation
Calculate the association constant (K):
K = 1.0 × 10⁹ L/mol
Calculate free testosterone using the quadratic equation:
Free T = (Total T) / (1 + (K × SHBG))
Calculate bioavailable testosterone:
Bioavailable T = Free T + (Albumin-bound T)
Note: The full Vermeulen equation accounts for both SHBG and albumin binding. This calculator implements the clinically validated formula for maximum accuracy.
Example Calculation
Patient: 35-year-old male
Input:
- Total Testosterone: 550 ng/dL
- SHBG: 35 nmol/L
- Albumin: 4.3 g/dL
Step 1: Calculate free testosterone
Free T = 550 / (1 + (1.0 × 10⁹ × 35 × 10⁻⁹))
Free T = 550 / 36 = 15.3 ng/dL
Step 2: Calculate bioavailable T
Albumin-bound T = Total T × (Albumin binding fraction)
Albumin-bound T ≈ 550 × 0.17 = 93.5 ng/dL
Bioavailable T = 15.3 + 93.5 = 108.8 ng/dL
Result: Free T = 15.3 ng/dL (Normal) | Bioavailable T = 108.8 ng/dL (Normal)
Unit Conversions:
- Total T: ng/dL × 0.0347 = nmol/L
- Free T: ng/dL × 10 = pg/mL
- SHBG: nmol/L (standard unit)
- Albumin: g/dL (standard unit)
Why Free & Bioavailable Testosterone Matter
Total testosterone alone doesn’t tell the full story. Understanding your active hormone levels is critical for accurate diagnosis.
Free Testosterone
Free testosterone is the unbound fraction that is biologically active and available to bind to androgen receptors. Only 1–3% of total testosterone is free, but this fraction is responsible for most of the effects on muscle, bone, brain, and sexual function.
Bioavailable Testosterone
Bioavailable testosterone includes free T plus T loosely bound to albumin. This fraction can dissociate from albumin and become active in tissues. It represents 40–60% of total testosterone and is the best marker for symptomatic hypogonadism.
SHBG
SHBG binds tightly to testosterone, making it inactive. High SHBG (common in older men, hyperthyroidism, or liver disease) lowers free testosterone. Low SHBG (common in obesity, diabetes, or hypothyroidism) increases free T but may indicate other metabolic issues.
⚕️ Clinical Use: The Vermeulen formula is recommended by the Endocrine Society for calculating free and bioavailable testosterone when direct measurement is unavailable. It’s the standard tool used in men’s health clinics and endocrinology practices worldwide.
Medical Disclaimer: This free and bioavailable testosterone calculator is for educational and informational purposes only. It does not provide medical advice, diagnosis, or treatment. Always consult your physician or a qualified healthcare provider regarding your hormone levels and health concerns. The Vermeulen formula is a research tool and should not replace clinical judgment. Individual results may vary based on genetics, health conditions, and laboratory methodology.
