Free Testosterone Calculator
Calculate your free and bioavailable testosterone levels using the Vermeulen method. Enter your total testosterone, SHBG, and albumin values to get instant results with age-based reference ranges. 100% private — nothing leaves your browser.
What Is Free Testosterone?
Testosterone circulates in the blood in three forms: tightly bound to sex hormone-binding globulin (SHBG), loosely bound to albumin, and free (unbound). Only about 1-3% of total testosterone is free, yet this fraction is the most biologically active. Free testosterone can enter cells directly and activate androgen receptors, driving muscle growth, bone density, mood regulation, and sexual function. Bioavailable testosterone includes both free testosterone and the albumin-bound fraction, since albumin binding is weak enough that testosterone can dissociate and become active at target tissues.
Measuring total testosterone alone can be misleading. Two men with identical total testosterone levels can have very different free testosterone levels depending on their SHBG concentration. High SHBG binds more testosterone, leaving less free, while low SHBG leaves more available. This is why calculating free and bioavailable testosterone gives a more complete picture of androgenic status.
How the Vermeulen Method Works
The Vermeulen method, published by Alex Vermeulen and colleagues, is the gold standard for calculating free testosterone from total testosterone, SHBG, and albumin. It uses the known binding affinities of testosterone to SHBG (association constant Ka = 1.0 x 10^9 L/mol) and to albumin (association constant Ka = 3.6 x 10^4 L/mol) to solve a system of mass-action equations iteratively. The calculation starts with an initial estimate of free testosterone, then repeatedly adjusts until the sum of free, SHBG-bound, and albumin-bound testosterone equals the measured total. This method correlates closely with equilibrium dialysis, the direct laboratory measurement, making it reliable for clinical and research use.
Normal Free Testosterone Levels by Age
Free testosterone naturally declines with age. For men aged 20-29, the normal range is approximately 5.05-19.8 ng/dL. In the 30-39 range, it drops to 4.65-18.1 ng/dL. Men aged 40-49 typically fall between 4.06-15.6 ng/dL, while those 50-59 see ranges of 3.87-13.9 ng/dL. For men 60-69, the range is 3.47-12.2 ng/dL, and for those 70 and older, 2.99-10.6 ng/dL. These ranges help determine whether your levels are within expected bounds for your age group. Levels below the lower reference limit may warrant clinical evaluation for hypogonadism, especially when accompanied by symptoms like fatigue, reduced libido, or loss of muscle mass.
Why SHBG Matters for Testosterone Assessment
Sex hormone-binding globulin is a protein produced primarily by the liver that binds testosterone with high affinity. SHBG levels are influenced by many factors: they increase with age, hyperthyroidism, liver disease, estrogen use, and low body weight. They decrease with obesity, insulin resistance, hypothyroidism, and androgen use. Because SHBG can vary so widely between individuals, measuring total testosterone alone can be unreliable. A man with high SHBG may have normal total testosterone but low free testosterone, experiencing symptoms of deficiency despite seemingly adequate levels. Conversely, a man with low SHBG may have low-normal total testosterone but adequate free testosterone. This calculator accounts for SHBG to give you the clinically meaningful free and bioavailable values that drive your actual hormonal health.
Frequently Asked Questions
What is free testosterone?
Free testosterone is the unbound fraction of total testosterone in your blood, typically 1-3% of the total. Unlike SHBG-bound testosterone, free testosterone can enter cells directly and activate androgen receptors, making it the most biologically active form responsible for muscle growth, libido, and mood regulation.
What is the Vermeulen method?
The Vermeulen method is a validated mathematical calculation that estimates free testosterone from total testosterone, SHBG, and albumin using known binding affinity constants. It uses iterative mass-action equations and correlates closely with equilibrium dialysis, the gold-standard laboratory method.
What is bioavailable testosterone?
Bioavailable testosterone is the sum of free testosterone plus albumin-bound testosterone. Since albumin binds testosterone weakly, this fraction can dissociate and become active at target tissues. Bioavailable testosterone represents approximately 30-50% of total testosterone and gives a more complete picture than free testosterone alone.
What are normal free testosterone levels by age?
Normal free testosterone ranges decline with age. For men: 20-29 years: 5.05-19.8 ng/dL, 30-39: 4.65-18.1, 40-49: 4.06-15.6, 50-59: 3.87-13.9, 60-69: 3.47-12.2, and 70+: 2.99-10.6 ng/dL. Levels below the lower limit for your age may warrant clinical evaluation.
Why is SHBG important for testosterone assessment?
SHBG (sex hormone-binding globulin) binds testosterone with high affinity, making it biologically inactive. SHBG varies widely between individuals due to age, weight, thyroid function, and liver health. Two people with identical total testosterone can have very different free testosterone levels depending on SHBG, which is why this calculation is clinically important.
Is this calculator private?
Yes. All calculations run entirely in your browser using the Vermeulen equation. No blood work data, hormone levels, or personal information is sent to any server. Your inputs are saved locally for convenience and can be cleared at any time.