What it means
To calculate it, use the same units for both measurements. A 34-inch waist and a 68-inch height give a ratio of 0.5. The popular rule of thumb is to keep your waist to less than half your height.
Studies in adults suggest the ratio predicts heart and metabolic risk at least as well as BMI, and sometimes better, because it reflects belly fat and adjusts for height. In one large UK analysis, many people classed as low risk by BMI and waist size had a ratio of 0.5 or higher and showed more risk factors.
The 0.5 cutoff is widely used but not perfect. Research in children and in some ethnic groups suggests the best threshold may differ.
US federal guidelines still lean on BMI and waist circumference, so treat this ratio as an extra, easy check rather than a diagnosis.
Why it matters
It gives a quick read on belly fat that works across heights, useful when BMI does not tell the whole story.