What this free testosterone calculator does
Total testosterone alone does not always tell the full story. A significant portion of testosterone is bound to proteins in blood, mainly SHBG and albumin. Only a small fraction circulates as free testosterone, which is often considered the biologically active portion.
This calculator estimates free testosterone from:
- Total testosterone
- SHBG (sex hormone-binding globulin)
- Albumin
It also reports percent free testosterone and estimated bioavailable testosterone (free + albumin-bound fraction).
How the calculation works
The tool uses a classic equilibrium approach (often referred to as a Vermeulen-type model). In simple terms:
- Testosterone can remain free, bind weakly to albumin, or bind strongly to SHBG.
- Association constants are used to model this binding in plasma.
- The equation is solved for free testosterone concentration.
Constants used in this implementation:
- Albumin binding constant (Ka): 3.6 × 104 L/mol
- SHBG binding constant (Ks): 1.0 × 109 L/mol
- Albumin molecular weight: 66,500 g/mol
Input guide
1) Total testosterone
Enter your lab value and choose the correct unit: ng/dL or nmol/L. The calculator converts automatically.
2) SHBG
SHBG should be entered in nmol/L. Higher SHBG usually means a lower free testosterone estimate for the same total testosterone.
3) Albumin
Albumin is usually around 4.0–5.0 g/dL (or 40–50 g/L). If you are not sure, use your exact lab result when available.
How to interpret your result
The output includes multiple units because labs report differently:
- pg/mL (common in many endocrine panels)
- pmol/L (SI unit)
- ng/dL (sometimes used in reports)
A rough range hint is shown based on the profile you choose (male or female), but this is only a convenience feature. Lab methods vary, and ranges differ by assay type, age, sex, time of day, medications, and health status.
Why free testosterone can differ from total testosterone
- High SHBG: may lower free testosterone despite normal total testosterone.
- Low SHBG: may increase free testosterone for a given total testosterone.
- Albumin changes: can slightly affect calculated free and bioavailable values.
- Timing: testosterone has diurnal variation, especially in younger males.
Limitations and important notes
No online calculator replaces a clinical assessment. Different free testosterone methods (equilibrium dialysis, ultrafiltration, analog immunoassays, calculated methods) may not match perfectly. If symptoms and labs conflict, discuss repeat testing and assay method with your clinician.
Always consider the full context: LH, FSH, estradiol, prolactin, thyroid panel, metabolic markers, liver function, medications, sleep quality, body composition, and overall health goals.
Quick FAQ
Is this calculator free?
Yes. It is a free educational calculator you can use directly in your browser.
Can this diagnose low testosterone?
No. Diagnosis requires symptoms, repeated morning labs, and medical interpretation.
Should I optimize free testosterone only?
Not in isolation. Clinical decisions should include symptoms, fertility goals, cardiovascular risk, sleep, and broader endocrine context.