Free testosterone is calculated using a formula that combines three blood test values: total testosterone, sex hormone-binding globulin (SHBG), and albumin. Only 1% to 4% of your circulating testosterone floats unbound in the bloodstream, and this “free” fraction is what many clinicians consider the most biologically active. Because directly measuring free testosterone in a lab is expensive and time-consuming, most people and clinicians rely on a calculated estimate instead.
Why Free Testosterone Is Calculated, Not Measured
The gold standard for measuring free testosterone is a lab technique called equilibrium dialysis. A blood sample sits inside a special membrane for about 23 hours, allowing only unbound testosterone molecules to pass through while bound molecules stay trapped. It’s accurate, but it’s also slow, labor-intensive, and expensive, which keeps it confined mostly to research settings.
The most common alternative you’ll see on routine lab work is a direct analog immunoassay. This test is fast and cheap, but it has a well-documented accuracy problem: the values it produces are a fraction of what equilibrium dialysis and calculated methods return, and some studies suggest it doesn’t reliably detect truly unbound testosterone at all. For this reason, the Endocrine Society recommends either equilibrium dialysis or a validated calculation formula when free testosterone levels matter for a clinical decision.
The Three Blood Values You Need
To calculate free testosterone, you need results from a standard blood draw:
- Total testosterone, which measures all testosterone in your blood, both bound and unbound.
- SHBG (sex hormone-binding globulin), the protein that locks onto testosterone most tightly. Testosterone bound to SHBG is essentially inactive because it can’t detach easily.
- Albumin, a general-purpose blood protein that binds testosterone loosely. Many calculators assume a default albumin level of 4.3 g/dL (43 g/L) if you don’t have a specific lab result, since albumin stays fairly stable in healthy people.
Your lab report will list total testosterone in units like ng/dL or nmol/L, and SHBG in nmol/L. Make sure you note the units before plugging numbers into a calculator, since using the wrong unit will produce a wildly incorrect result.
The Vermeulen Equation
The most widely used calculation is the Vermeulen equation, published by Alex Vermeulen and colleagues. It uses the law of mass action, which describes how molecules compete for binding spots on proteins. The formula accounts for how strongly SHBG grabs testosterone (using an association constant of 1 × 10⁹ L/mol at body temperature) and how weakly albumin holds it (association constant of 3.6 × 10⁴ L/mol). The huge difference between those two numbers reflects reality: SHBG binds testosterone roughly 25,000 times more tightly than albumin does.
You don’t need to solve the equation by hand. The International Society for the Study of the Aging Male (ISSAM) hosts a widely used online calculator at issam.ch/freetesto.htm. You enter your total testosterone, SHBG, and albumin values, select your units, and it returns both free and bioavailable testosterone. Several other validated calculators exist online, and they all use variations of the same underlying math.
A 2018 study comparing calculation methods found that the Vermeulen formula overestimates free testosterone by about 19% compared to equilibrium dialysis, but this overestimation stays consistent regardless of your SHBG, albumin, or testosterone levels. That consistency is actually its strength: the formula performs reliably across a wide range of patients, making it the most robust option for clinical use.
Other Calculation Formulas
Two alternatives are worth knowing about, though neither has displaced the Vermeulen equation in routine practice.
The Ly and Handelsman formula uses empirically derived equations, with separate versions for testosterone levels above and below 5 nmol/L (roughly distinguishing male-range from female-range values). Its median accuracy is impressive, hitting a ratio of 1.0 against equilibrium dialysis. The catch is that its accuracy depends heavily on SHBG and testosterone levels. At low SHBG or low testosterone, it increasingly underestimates free testosterone, with individual results ranging from 50% to 150% of the true value.
The Zakharov formula uses a more complex model that accounts for how SHBG changes shape when testosterone binds to it. In theory, this should be more accurate. In practice, it overestimates free testosterone by roughly a factor of two on average, and its accuracy swings widely depending on SHBG concentration, producing values anywhere from equal to the true result to triple it at high SHBG levels.
Free vs. Bioavailable Testosterone
These two terms describe different slices of your total testosterone, and they’re often confused. Free testosterone is only the completely unbound fraction, typically 1% to 4% of the total. Bioavailable testosterone is a larger pool: it includes free testosterone plus the portion loosely bound to albumin. Together, these make up roughly 35% to 58% of your total testosterone.
The reasoning behind the bioavailable concept is straightforward. Albumin holds testosterone so weakly that when blood passes through capillaries, albumin-bound testosterone can detach and enter tissues almost as easily as free testosterone does. SHBG-bound testosterone cannot do this. So “bioavailable” captures all the testosterone your tissues can actually access. The same online calculators that produce free testosterone typically output bioavailable testosterone as well, since the math is nearly identical.
When Calculating Free Testosterone Matters Most
Total testosterone alone is enough for most clinical evaluations. Free testosterone becomes important in two specific situations identified by the Endocrine Society. First, when total testosterone falls near the lower boundary of normal and symptoms are present, free testosterone can clarify whether there’s a genuine deficiency or whether high SHBG is simply making the total number look low. Second, when a medical condition alters SHBG levels, total testosterone becomes unreliable as a standalone marker.
Several common conditions raise or lower SHBG. Aging, liver disease, and hyperthyroidism tend to push SHBG up, which can make total testosterone look adequate even when free testosterone is low. Obesity, type 2 diabetes, and hypothyroidism tend to push SHBG down, which can make total testosterone appear low even when free testosterone is normal. In these situations, calculating free testosterone gives a more accurate picture of what’s actually available to your body.
Normal Ranges for Free Testosterone
Reference ranges for free testosterone vary by age and sex. For adult men, typical values in pg/mL are:
- Ages 20 to 29: 9.3 to 26.5 pg/mL
- Ages 30 to 39: 8.7 to 25.1 pg/mL
- Ages 40 to 49: 6.8 to 21.5 pg/mL
- Ages 50 to 59: 7.2 to 24.0 pg/mL
- Over 59: 6.6 to 18.1 pg/mL
For adult women over 19, the reference range is 0.0 to 4.2 pg/mL. Pediatric reference ranges have not been firmly established. Keep in mind that different labs and different calculation methods can produce slightly different numbers, so it’s best to compare your results against the specific range provided by the lab or calculator you used.