Most blood tests report total testosterone, and most men treat that number as the answer. It isn't, because the majority of your testosterone is not available to your body at any given moment.
Roughly 40 to 60 per cent is bound tightly to sex hormone binding globulin, or SHBG. That portion is locked away and biologically inactive. Most of the rest is loosely bound to albumin, which releases it readily, and a small fraction, usually 1 to 3 per cent, circulates completely free.
Free testosterone plus albumin-bound testosterone is what actually reaches your tissues. That figure is called bioavailable testosterone, and it explains why two men with identical total readings can feel completely different.
This calculator uses the Vermeulen equation, published in 1999 and still the reference method for estimating free testosterone from a standard blood panel. It models the binding between testosterone, SHBG and albumin using established affinity constants.
Direct measurement of free testosterone by equilibrium dialysis is more accurate but expensive and rarely offered. Calculated free testosterone using this equation correlates well with dialysis results, which is why guidelines accept it.
You need three numbers, all of which appear on a standard hormone panel: total testosterone, SHBG, and albumin. If albumin was not measured, the calculator uses 4.3 g/dL, which is a reasonable population average for healthy adults.
Free testosterone below roughly 0.225 nmol/L (65 pg/mL) is generally considered low in adult men, and this threshold appears in Endocrine Society guidance. Ranges vary between laboratories and by age, so treat it as a guide rather than a diagnosis.
The situation this calculator exists for is a normal total testosterone alongside genuine symptoms. It happens most often when SHBG is high, because a large share of an apparently adequate total is locked up. High SHBG is associated with ageing, low body weight, restrictive dieting, hyperthyroidism, liver disease and some medications including anticonvulsants and opioids.
The reverse also occurs. Low SHBG, common with obesity, insulin resistance and hypothyroidism, can leave free testosterone in a reasonable range despite a total reading that looks low.
Timing changes the answer. Testosterone follows a daily rhythm, peaking in the early morning and falling through the day, so the sample should be taken before 11am. An afternoon reading can be 20 to 25 per cent lower with nothing wrong at all.
Because levels also vary between days, guidelines recommend confirming a low result with a second morning sample on a different day before drawing conclusions.
Acute illness suppresses testosterone substantially, so avoid testing while unwell or shortly after recovering.
It estimates a value from your blood results. It does not diagnose hypogonadism, which requires low levels confirmed on two occasions alongside consistent symptoms, plus LH and FSH to establish whether the problem lies in the testes or in the signal from the brain.
If your calculated free testosterone is low, the useful next step is a full panel with LH, FSH, prolactin and thyroid function, discussed with a doctor.
Related reading: Normal blood test but still low testosterone · What to check on a testosterone blood test · What causes low testosterone · Prolactin: the test that changes the diagnosis
Educational only, not medical advice. Blood results should be interpreted by a clinician who knows your history.