Testosterone is reported in nmol/L in the UK, Europe and most of the world, and in ng/dL in the United States. Comparing your result to advice written for the other system without converting produces genuine confusion.
The conversion factor is 0.0347. So 300 ng/dL is 10.4 nmol/L, and 12 nmol/L is 346 ng/dL.
This matters because most online discussion of testosterone thresholds uses American units, and most UK lab reports do not.
Different bodies use slightly different cut-offs, which is worth knowing before treating any single number as decisive.
Below 8 nmol/L (231 ng/dL) is generally accepted as low, and treatment is usually considered if symptoms are present.
8 to 12 nmol/L (231 to 346 ng/dL) is a grey zone. Symptoms, free testosterone and clinical judgement guide what happens next.
Above 12 nmol/L (346 ng/dL) is generally considered adequate, though symptoms in this range do occur, particularly when SHBG is high.
American guidance often uses 300 ng/dL, which is 10.4 nmol/L, as the threshold.
Most of your testosterone is bound to SHBG and unavailable. A total of 15 nmol/L with high SHBG can leave less usable hormone than 11 nmol/L with normal SHBG.
This is the most common reason a man with real symptoms is told his level is fine. The free testosterone calculator on this site works out the available fraction from your total, SHBG and albumin.
Three things determine whether a result means anything.
Timing. Testosterone peaks in the early morning and falls through the day. Blood should be taken before 11am, and an afternoon reading can be 20 to 25 per cent lower with nothing wrong.
Repetition. Levels vary between days. Guidelines recommend confirming a low result with a second morning sample before drawing conclusions.
Health at the time. Acute illness suppresses testosterone substantially, so a sample taken while unwell will underestimate your baseline.
Related reading: Blood test: what to check · Normal blood test, still low testosterone · Testosterone levels by age · Free testosterone calculator
Educational only, not medical advice. Blood results should be interpreted by a clinician.