Only 1–3% of testosterone circulates free; the rest is bound tightly to SHBG or loosely to albumin. Free and albumin-bound (bioavailable) testosterone reflect androgen activity better than total testosterone when SHBG is abnormal – for example in obesity, ageing, liver or thyroid disease.
How to use the Free Testosterone Calculator
- Total testosterone
- SHBG
- Albumin
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter total testosterone, SHBG and albumin from a morning blood test.
Inputs explained
| Field | What to enter |
|---|---|
| Total testosterone | Units: nmol/L, ng/dL |
| SHBG | In nmol/L |
| Albumin | Units: g/dL, g/L |
What each result means
- Free testosterone = (-[b] + sqrt([b]^2 + 4×[a]×[Total testosterone]×1e-9))÷(2×[a])×1e12
- Free testosterone (pg/mL) = [Free testosterone]×0.2884
- Free testosterone as % of total = [Free testosterone]÷1000÷[Total testosterone]×100
- Bioavailable testosterone = [Free testosterone]×[N]÷1000
Example values
How the main result changes when Total testosterone changes and everything else stays at the example values above:
| Total testosterone (nmol/L) | Free testosterone (pmol/L) |
|---|---|
| 7.5 | 127 |
| 11.25 | 198 |
| 15 | 274 |
| 18.75 | 354 |
| 22.5 | 439 |
| 30 | 624 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
FT = (−b + √(b² + 4a·TT)) ÷ 2a, with N = 1 + Ka × albumin, a = N × Kt, b = N + Kt × (SHBG − TT)
Ka (albumin) = 3.6 × 10⁴ L/mol, Kt (SHBG) = 10⁹ L/mol
Bioavailable T = FT × N
Worked example
Inputs — Total testosterone: 15 nmol/L; SHBG: 40 nmol/L; Albumin: 4.3 g/dL.
Results — Free testosterone = 274 pmol/L; Free testosterone (pg/mL) = 78.9; Free testosterone as % of total = 1.82 %; Bioavailable testosterone = 6.41 nmol/L.
Key terms
- SHBG
- Sex hormone-binding globulin.
Frequently asked questions
What is a normal free testosterone in men?
Roughly 200–600 pmol/L (60–170 pg/mL), depending on the lab and age.
Why calculate rather than measure?
Direct analogue assays are inaccurate; equilibrium dialysis is accurate but expensive.
When is free testosterone useful?
When total testosterone is borderline or SHBG is abnormal.