Free testosterone is calculated using a formula that combines three lab values: your total testosterone, sex hormone-binding globulin (SHBG), and albumin. Only about 2% of the testosterone in your blood is actually “free,” meaning unbound to any protein and available for your body to use. The calculation estimates that small but important fraction without needing a specialized lab test.
Why Free Testosterone Is Calculated, Not Measured
Most testosterone in your bloodstream is bound to proteins. Roughly 44% attaches to SHBG, about 50% binds to albumin, around 4% binds to a third protein called cortisol-binding globulin, and only about 2% floats freely. That free portion is the most biologically active, so knowing its level matters when total testosterone alone doesn’t explain your symptoms.
The gold standard for measuring free testosterone directly is a lab technique called equilibrium dialysis, but it’s expensive, slow, and available at very few labs. The most common alternative, an analog immunoassay, has been discouraged by the Endocrine Society and international andrology groups because it produces values far lower than those from equilibrium dialysis and may not reliably reflect true free testosterone levels. Calculated free testosterone, by contrast, correlates extremely well with equilibrium dialysis (r = 0.986), making it the preferred method in most clinical settings.
What Lab Values You Need
To run the calculation, you need three inputs from a blood draw:
- Total testosterone. Often reported in ng/dL in the United States or nmol/L in countries using SI units. To convert ng/dL to nmol/L, multiply by 0.0347.
- SHBG. Reported in nmol/L.
- Albumin. Reported in g/dL. If your lab panel didn’t include albumin, the standard assumption is 4.3 g/dL, which is a normal value for most adults.
All three values should come from the same blood draw, ideally taken in the morning when testosterone levels peak.
How the Vermeulen Equation Works
The most widely used calculation is the Vermeulen method, sometimes called the “one-ligand/two-protein” model. It treats testosterone as a single molecule competing for binding spots on two proteins: SHBG and albumin. Each protein grabs testosterone with a known strength, expressed as an association constant.
The two constants at the heart of the formula are:
- SHBG binding constant: 1 × 10⁹ L/mol (very strong binding, which is why SHBG-bound testosterone is essentially locked up)
- Albumin binding constant: 3.6 × 10⁴ L/mol (much weaker binding, which is why albumin-bound testosterone can still become available to tissues)
Using these constants along with your total testosterone, SHBG, and albumin concentrations, the formula sets up a quadratic equation. Solving it gives the concentration of testosterone that isn’t bound to either protein. The math is straightforward enough to run on a spreadsheet, but in practice most people use an online calculator that does it instantly.
Using an Online Calculator
You don’t need to solve the equation by hand. Several free online tools plug in the Vermeulen formula for you. The most commonly referenced one is hosted by the International Society for the Study of the Aging Male (ISSAM), though other versions exist on lab and endocrinology websites. The steps are simple: enter your total testosterone (in ng/dL or nmol/L), your SHBG (in nmol/L), and your albumin (in g/dL or left at the default 4.3). The calculator returns your estimated free testosterone, usually in ng/dL or pg/mL, and often your “bioavailable” testosterone as well, which includes both the free fraction and the loosely albumin-bound fraction.
One common stumbling point is unit mismatch. If your lab reports total testosterone in ng/dL but the calculator asks for nmol/L, multiply by 0.0347. Getting this wrong will produce wildly inaccurate results.
What the Results Mean
Calculated free testosterone is typically reported in pg/mL (picograms per milliliter). In men, levels vary substantially by age. The highest average values appear in the twenties, around 114 pg/mL, and decline roughly 12.7% per decade. By age 80 and older, the average drops to about 46 pg/mL. The steepest single-decade drop happens between the twenties and thirties, where levels fall by nearly 17%.
There are no universally agreed-upon cutoffs, and individual variation within any age group is large. That’s why free testosterone is interpreted alongside symptoms, not as a standalone number. A calculated value that falls well below the typical range for your age group, combined with symptoms like fatigue, low libido, or loss of muscle mass, is what prompts further evaluation.
When the Calculation Can Be Misleading
The Vermeulen equation assumes your SHBG and albumin levels are relatively normal. If something is pushing your SHBG unusually high or low, the calculation still works mathematically, but the clinical picture becomes more complicated because your SHBG level itself is telling an important story.
Conditions that raise SHBG include hyperthyroidism (where levels can reach two to three times normal), liver disease, aging, and estrogen-containing medications. Conditions that lower SHBG include obesity, insulin resistance, metabolic syndrome, hypothyroidism, and elevated prolactin levels. Even acute psychosocial stress can temporarily push SHBG higher. Certain medications and alcohol consumption also shift SHBG in either direction.
When SHBG is abnormal, the calculated free testosterone still tends to correlate well with equilibrium dialysis results. However, the calculation does consistently overestimate free testosterone slightly compared to dialysis measurements. This small systematic bias is worth keeping in mind if your values fall right at a borderline level.
Calculated vs. Bioavailable Testosterone
Many calculators report two numbers: free testosterone and bioavailable testosterone. Free testosterone counts only the completely unbound 2%. Bioavailable testosterone adds in the albumin-bound portion, roughly 50% of total testosterone, because albumin’s weak grip means that fraction can still detach and enter cells fairly easily. Some clinicians prefer bioavailable testosterone as a more complete picture of what your tissues can actually access, while others rely on the free fraction alone. Both are derived from the same calculation.