BMI cannot distinguish muscle from fat, or fat under the skin from fat around your organs. Waist-to-height ratio addresses the second problem, which is the one that matters most for health.
Visceral fat, the fat packed around your liver, pancreas and intestines, behaves differently from the fat under your skin. It releases inflammatory signals and free fatty acids directly into the portal circulation, which is why it drives insulin resistance in a way subcutaneous fat does not.
Your waist measurement reflects that fat. Your weight does not.
Keep your waist to less than half your height. That is the whole rule, and it applies to men and women, and across ethnic groups, which is why it has been recommended in preference to BMI in several guidelines including NICE.
So someone 175 cm tall should aim for a waist under 87.5 cm.
Rough bands: below 0.4 may indicate underweight, 0.4 to 0.49 is healthy, 0.5 to 0.59 suggests increased risk, and 0.6 or above suggests high risk.
Most people measure in the wrong place, which makes the number meaningless.
Measure midway between the bottom of your ribs and the top of your hip bones, roughly level with your navel. Not at the narrowest point, and not where your trousers sit.
Stand up, breathe out normally, and do not pull the tape tight. Measure on bare skin, at the same time of day, ideally in the morning before eating.
Consistency matters more than precision if you are tracking change.
Visceral fat is metabolically active tissue, and its effects run in both directions.
In men, it contains aromatase, which converts testosterone into oestradiol. More visceral fat means more conversion and lower testosterone, which in turn makes fat easier to store. The loop is why waist circumference predicts low testosterone better than weight does.
In women, falling oestrogen through menopause shifts fat from hips and thighs to the abdomen. Weight can stay identical while waist grows several centimetres, and that redistribution is a genuine metabolic change rather than a cosmetic one.
This is the number to track through midlife, not the scales.
Visceral fat responds well to intervention, often faster than total weight.
Resistance training, adequate protein, reduced alcohol and improved sleep all help. So does simply reducing the total energy surplus, since visceral fat is usually the first to go.
Related reading: Why your belly fat will not budge · Why weight gathers around your middle · Insulin resistance calculator · Testosterone and type 2 diabetes
Educational only, not medical advice.