The “half your height” rule
The clearest everyday guide is waist-to-height ratio: keep your waist under half your height (a ratio below 0.5). It works across ages, sexes and ethnic groups and flags harmful central fat better than BMI alone.
Waist-size thresholds
As a rough guide, increased health risk starts around a waist of 94 cm (37 in) for men and 80 cm (31.5 in) for women, with high risk from about 102 cm (40 in) and 88 cm (34.5 in) respectively. Some ethnic groups reach higher risk at lower measurements.
How to measure your waist
Find the midpoint between the bottom of your ribs and the top of your hip bones (roughly level with your belly button). Breathe out gently, keep the tape snug but not tight, and don’t hold your stomach in.
Why central fat matters
Fat stored around the organs (visceral fat) is more strongly linked to type 2 diabetes, high blood pressure and heart disease than fat elsewhere — which is why waist size is such a useful check.