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Your total testosterone only tells half the story. What matters is how much is actually free and available to your body. Enter your blood results to estimate your free and bioavailable testosterone using the clinically validated Vermeulen equation.
Most of the testosterone in your blood is not actually available to your body. The majority is bound tightly to a protein called sex hormone-binding globulin (SHBG), which locks it away. A smaller portion is loosely bound to albumin, and only a tiny fraction, usually around 1 to 3 percent, circulates completely unbound. That unbound fraction is your free testosterone, and it is the part that can enter your cells and do its job.
This is why two men with an identical total testosterone can feel completely different. If one has high SHBG, much of his testosterone is bound and unavailable, so his free testosterone, and how he feels, can be far lower than the total number suggests.
Three different measures, three different stories. The calculator works out the two that actually reflect how much testosterone your body can use.
Everything in your blood, bound and unbound. The standard test, but on its own it can be misleading.
The small unbound fraction that is biologically active and available to your tissues. Often the most telling number.
Free testosterone plus the loosely albumin-bound portion, the total amount your body can readily access.
Measuring free testosterone directly in a lab is expensive and not always reliable. Instead, clinicians calculate it from three values you already get on a standard blood panel, total testosterone, SHBG and albumin, using the Vermeulen equation, a method validated against direct measurement and widely used in research and clinical practice.
This calculator runs exactly that equation and returns your estimated free testosterone, in pmol/L and as a percentage of total, plus your bioavailable testosterone. It is an estimate, not a diagnosis, but it is the same calculation a clinician would use to read your results properly.
You need total testosterone and SHBG, both included in our at-home blood tests from 45 pounds.
Enter your values above to see your free and bioavailable testosterone instantly.
A UK GMC-registered doctor reads your numbers alongside your symptoms and tells you what they mean.
Our at-home blood tests include total testosterone and SHBG, reviewed by a UK doctor, everything you need to know where you really stand.
Free testosterone is the small fraction of testosterone in your blood, usually around 1 to 3 percent, that is not bound to proteins and is therefore available to enter your cells and act. The rest is bound to SHBG tightly or albumin loosely and is largely unavailable.
Total is everything in your blood, bound and unbound. Free is the unbound, active fraction. Bioavailable is the free portion plus the loosely albumin-bound portion, the total amount your body can readily use. Free and bioavailable often reflect how you feel better than total alone.
It is a clinically validated formula that calculates free and bioavailable testosterone from total testosterone, SHBG and albumin. It is widely used because directly measuring free testosterone in a lab is costly and less reliable. This calculator uses the Vermeulen method.
Your total testosterone and SHBG, both in nmol/L, which appear on any standard testosterone blood panel. Albumin in g/L is optional; if you leave it blank, the calculator uses the standard value of 43 g/L, which is accurate for most men.
Reference ranges for free testosterone in adult men sit very roughly between 225 and 700 pmol/L, but optimal levels depend on your age, symptoms and the lab used. A number near the bottom of the range, alongside symptoms, is worth discussing with a doctor. We treat the patient, not just the number.
No. It is a calculation based on values from a blood test you have already had, so it cannot measure anything itself, and it does not diagnose low testosterone. Only a clinician reading your full results alongside your symptoms can do that. If you do not have your numbers yet, an at-home test is the place to start.
This page and calculator are for general information and do not constitute medical advice or a diagnosis. If you are concerned about your health, speak to a qualified clinician.