Why Free Testosterone Matters More Than Total T
Your total testosterone number is only half the story. Two men can both have a total T of 600 ng/dL, but if one has an SHBG of 20 and the other has an SHBG of 60, their free testosterone levels - the hormone actually available to receptors - will be dramatically different. This calculator reveals what your cells actually have access to.
Reference range: 264-916 ng/dL (quest)
Reference range: 16.5-55.9 nmol/L
Reference range: 3.5-5.5 g/dL (default: 4.3)
Total Testosterone
500
ng/dL
Free Testosterone
1.3
ng/dL (0.3%)
Bioavailable T
31.4
ng/dL (6.3%)
Total T / SHBG Ratio
5.0
High
Methodology: This calculator uses the Vermeulen method (linearized approximation), the same equation used by the International Society for the Study of the Aging Male (ISSAM) and referenced in clinical endocrinology. Results are estimates and may differ slightly from equilibrium dialysis (the gold standard laboratory method). Always share results with your physician for clinical interpretation.
Disclaimer: This calculator provides estimated values for educational purposes only. It is not a diagnostic tool. Lab results can vary between testing methodologies. Always consult your healthcare provider for interpretation of bloodwork and treatment decisions.
Understanding Testosterone Fractions
Testosterone circulates in your blood in three distinct forms, and understanding these fractions is essential for interpreting your bloodwork correctly. Each fraction behaves differently in the body and tells a different part of the clinical story.
SHBG-bound testosterone makes up approximately 60-70% of total testosterone in most men. Sex Hormone-Binding Globulin is a glycoprotein produced by the liver that binds testosterone with high affinity. Once testosterone is locked to SHBG, it is essentially inactive - it cannot bind to androgen receptors, cannot be converted to DHT by 5-alpha reductase, and cannot be aromatized to estradiol. SHBG-bound testosterone is biologically inert for practical purposes. Think of SHBG as a vault: testosterone stored there is safe from metabolism but also unavailable for use.
Albumin-bound testosterone accounts for approximately 25-35% of total testosterone. Albumin is the most abundant protein in blood plasma, and its binding to testosterone is much weaker than SHBG binding. Because of this weak affinity, albumin-bound testosterone readily dissociates and becomes available to tissues as it passes through capillary beds. This is why clinicians consider albumin-bound testosterone to be "bioavailable" - it may be technically bound, but it functions almost like free testosterone in practice.
Free testosterone represents only 1-3% of total testosterone. This tiny fraction is completely unbound and immediately available to enter cells, bind androgen receptors, and exert androgenic effects. Free testosterone drives libido, muscle protein synthesis, bone density maintenance, erythropoiesis, and mood regulation. When men describe "feeling" their testosterone - energy, drive, confidence - they are describing the effects of free testosterone.
The SHBG Problem: Why Total T Alone Is Misleading
Most primary care physicians order only a total testosterone test when evaluating a man for hypogonadism. While total T is a useful starting point, it can be profoundly misleading without knowing SHBG. Here is why.
SHBG levels vary enormously between individuals. Normal range spans 16.5 to 55.9 nmol/L - a 3.4x difference from bottom to top. A man with a total T of 500 ng/dL and an SHBG of 20 nmol/L may have a free T of 15+ ng/dL (solidly optimal). That same total T of 500 ng/dL with an SHBG of 55 nmol/L could yield a free T below 7 ng/dL (symptomatic hypogonadism). Same total number, completely different clinical picture.
Multiple factors influence SHBG production. Age is the biggest driver: SHBG increases approximately 1-2% per year after age 30, which is one reason men feel the effects of declining testosterone more sharply than the total T number alone would suggest. Their free T is dropping from both declining production and increasing SHBG binding.
Other factors that increase SHBG include hyperthyroidism, liver disease (including fatty liver in some stages), anticonvulsant medications, estrogen exposure, and low caloric intake. Factors that decrease SHBG include obesity, insulin resistance, type 2 diabetes, hypothyroidism, anabolic steroid use, and high-protein diets. Understanding your SHBG context is essential for making sense of your testosterone numbers.
When to Worry About SHBG
If your SHBG is above 50 nmol/L, even a "normal" total T may not be providing enough free testosterone for optimal function. Conversely, if your SHBG is below 20 nmol/L, your free T may be adequate even at a relatively low total T. Always request both total T and SHBG in any testosterone evaluation panel.
The Vermeulen Equation Explained
This calculator uses the Vermeulen method, published in 1999 by Alex Vermeulen, Lieve Verdonck, and Jean-Marc Kaufman in the Journal of Clinical Endocrinology and Metabolism. Their paper, "A critical evaluation of simple methods for the estimation of free testosterone in serum," validated multiple calculation methods against the gold standard of equilibrium dialysis and found that the Vermeulen equation produced the strongest correlation (r = 0.918).
The equation models the equilibrium between three testosterone states (free, albumin-bound, and SHBG-bound) using known association constants for each binding protein. The association constant for SHBG binding is approximately 1.0 x 10^10 L/mol, while the albumin association constant is much lower at approximately 3.6 x 10^4 L/mol. This 278,000-fold difference in binding affinity is why SHBG dominates the equation and why changes in SHBG have such a disproportionate impact on free T.
The International Society for the Study of the Aging Male (ISSAM) and the Endocrine Society both reference the Vermeulen calculation as an acceptable method for estimating free testosterone when equilibrium dialysis is not available. Most online calculators, including those used by major laboratories, implement some variant of this equation.
How to Interpret Your Results
The calculator provides four key outputs. Here is how to read each one and what to discuss with your provider.
Free Testosterone (ng/dL) is the primary output. Optimal range for adult men is generally 10-20 ng/dL, though some optimization-focused clinicians target 15-25 ng/dL. Below 5 ng/dL is typically considered clinically low and often correlates with symptoms such as reduced libido, fatigue, depressed mood, and decreased muscle mass.
Free T Percentage indicates what fraction of your total T is unbound. A healthy free T percentage is typically 2-3%. If your percentage is below 1.5%, it suggests SHBG is binding an unusually large amount of testosterone, and interventions targeting SHBG reduction (weight loss, boron supplementation, addressing thyroid function) may be beneficial even before considering TRT.
Bioavailable Testosterone combines free testosterone with albumin-bound testosterone. This number represents the total testosterone fraction available to tissues and is sometimes used as a more inclusive measure of androgenic potential. Optimal bioavailable T is generally considered above 150 ng/dL.
Total T / SHBG Ratio is a quick screening metric. A ratio below 0.25 often indicates functionally low testosterone regardless of the total T number, while a ratio above 0.7 suggests adequate free hormone availability.
Do Not Self-Diagnose from Calculator Results
This calculator provides estimates for educational purposes. Hormone levels should always be interpreted by a qualified healthcare provider in the context of clinical symptoms, medical history, and confirmatory laboratory testing. Never start, stop, or adjust hormone therapy based solely on a calculator output.
Factors That Affect Your Numbers
Blood test timing matters significantly. Testosterone levels peak in the early morning (6-8 AM) and can be 20-30% lower by the afternoon. The Endocrine Society recommends drawing testosterone levels between 7-10 AM, fasting, for the most accurate baseline assessment. If you are on TRT, the timing relative to your last injection also matters: trough levels (drawn right before your next injection) give the most clinically relevant picture.
Acute stress, poor sleep, illness, and heavy exercise in the 24-48 hours prior to testing can all temporarily suppress testosterone levels. If your results seem unexpectedly low, consider whether any transient factors may have affected the draw and discuss retesting with your provider before making treatment decisions.
Albumin is typically stable and most people do not need to test it separately unless they have liver or kidney disease. The default value of 4.3 g/dL is a reasonable assumption for most healthy men. If your lab panel includes albumin (common in a comprehensive metabolic panel), enter your actual value for more precise results.
Frequently Asked Questions
What is free testosterone?
Free testosterone is the fraction of total testosterone in your blood that is not bound to any protein. Only about 1-3% of your total testosterone is free. Despite being a small fraction, free T is biologically the most important form because it is the only form that can directly enter cells and activate androgen receptors. It drives libido, muscle growth, bone density, mood, and energy levels.
What is a good free testosterone level?
For adult men, optimal free testosterone is generally 10-20 ng/dL. Optimization-focused clinicians often target 15-25 ng/dL. Values below 5 ng/dL are typically associated with symptoms of hypogonadism. However, individual sensitivity to testosterone varies, and some men feel optimal at levels that would be "low" for others. Symptoms should always be considered alongside lab values.
How can I increase my free testosterone?
There are two strategies: increase total testosterone production or decrease SHBG binding. Natural approaches to increase total T include resistance training, adequate sleep (7-9 hours), stress management, maintaining a healthy body composition, and ensuring adequate zinc and vitamin D intake. To lower SHBG, address insulin resistance through diet and exercise, optimize thyroid function, and consider boron supplementation (10mg daily has shown modest SHBG reduction in some studies). If natural approaches are insufficient, TRT directly increases both total and free testosterone.
Should I get a direct free T test or use this calculator?
The gold standard for measuring free testosterone is equilibrium dialysis, but this test is expensive and not widely available. The Vermeulen calculation is the next best option and is endorsed by the Endocrine Society. Avoid the "analog free T" test offered by some labs - it is known to be inaccurate, especially at low testosterone levels. If your physician orders free T, ask specifically for equilibrium dialysis or use this calculator with your total T and SHBG values.