Free Testosterone Calculator with Bioavailable Levels

What This Calculator Does

This tool estimates your free testosterone and bioavailable testosterone levels using standard lab values. It applies clinically recognized formulas to provide a clearer picture of your androgen status beyond what total testosterone alone can show.

Free testosterone represents the small fraction of testosterone not bound to proteins in your blood. Bioavailable testosterone includes both free testosterone and the portion loosely bound to albumin. These values are often more clinically relevant than total testosterone for assessing symptoms related to low or high testosterone.

How the Calculation Works

The calculator uses the Vermeulen formula, a widely accepted method for estimating free testosterone from total testosterone, sex hormone-binding globulin (SHBG), and albumin levels.

The calculation accounts for the binding affinities between testosterone and its carrier proteins:

• Total testosterone — the overall amount measured in your blood
• SHBG — a protein that binds tightly to testosterone, reducing its availability
• Albumin — a protein that binds testosterone more loosely, allowing some release into tissues

Bioavailable testosterone is derived by combining free testosterone with the albumin-bound fraction, which is considered readily available for tissue uptake.

Required Inputs

You will need recent lab results for the following values:

• Total Testosterone — typically reported in ng/dL or nmol/L
• SHBG — reported in nmol/L
• Albumin — reported in g/dL or g/L

All values must come from the same blood draw for the calculation to be meaningful.

Understanding Your Results

The output provides two key values:

• Free Testosterone — the unbound fraction, typically 1–3% of total testosterone in healthy adults
• Bioavailable Testosterone — the sum of free and albumin-bound testosterone, often 15–30% of total

These values can help identify cases where total testosterone appears normal but free or bioavailable levels are low, which may explain symptoms such as fatigue, reduced libido, or mood changes.

Reference ranges vary by age, sex, and laboratory. Compare your results against the reference range provided by your testing lab rather than general population averages.

Common Mistakes When Using This Calculator

• Using mismatched units — ensure all inputs use compatible units. Mixing ng/dL and nmol/L without conversion will produce incorrect results.
• Entering values from different dates — SHBG and albumin levels fluctuate. Using values from separate blood draws reduces accuracy.
• Misinterpreting the result as a diagnosis — this calculator provides an estimate, not a clinical diagnosis. Always discuss results with a healthcare provider.

Limitations

The Vermeulen formula assumes standard binding constants and may be less accurate in certain conditions:

• Pregnancy or estrogen therapy, which alter SHBG levels significantly
• Severe obesity, which can affect albumin binding
• Medications that compete for SHBG binding sites
• Extremely high or low SHBG values outside typical ranges

Direct measurement of free testosterone via equilibrium dialysis remains the gold standard, but it is not routinely available. This calculator offers a practical alternative for estimation.

When to Use This Tool

• Reviewing lab results with your doctor and wanting additional context
• Tracking changes in testosterone status over time
• Assessing whether symptoms might be related to low free testosterone despite normal total levels
• Understanding the impact of SHBG changes on testosterone availability