One tape measure, one division, and a signal BMI cannot give you
BMI tells you how heavy you are for your height. It cannot tell you where that weight sits, and where it sits turns out to matter. Fat carried around the abdomen — around the organs rather than under the skin — is more strongly associated with cardiovascular and metabolic risk than fat carried on the hips and thighs. Two people with identical BMI can have quite different risk profiles for this reason alone.
Waist-to-height ratio is the cheapest way to capture that difference. It needs a tape measure and one division.
Ratio = waist circumference ÷ height, in the same units.
A waist of 84 cm and a height of 175 cm gives 84 ÷ 175 = 0.48.
NICE advises keeping your waist to less than half your height — a ratio below 0.5. That single boundary is the appeal of the measure: because it scales with height, one number applies whether you are 155 cm or 195 cm, which is not true of raw waist circumference.
NICE recommends it specifically for adults with a BMI below 35, used alongside BMI rather than instead of it. Above that, the guidance is that waist-to-height adds little, because central adiposity can be assumed.
| Ratio | Broad interpretation |
|---|---|
| 0.4 – 0.49 | Healthy central adiposity — no increased health risk on this measure |
| 0.5 – 0.59 | Increased central adiposity, and increased health risk |
| 0.6 or more | High central adiposity, and further increased health risk |
Most of the error in this measure comes from measuring in the wrong place. The waist is not where your trousers sit.
The two measures answer different questions. BMI asks how much total mass you carry for your height; waist-to-height asks where it sits. A high BMI with a low ratio often points toward muscle. A healthy BMI with a high ratio is the combination most likely to be missed by BMI alone, and it is the one worth catching.
It is not designed for children, who are assessed on age- and sex-specific charts, as covered on BMI for children and teens. It is not appropriate during pregnancy. And like every simple anthropometric measure, it is a screening signal rather than a diagnosis — it describes a population-level association, not a prediction about you.
What to do with the number. A ratio above 0.5 is a reason to raise the subject at your next appointment, not a reason to start an aggressive plan on your own. Blood pressure and blood markers measure the risk directly rather than by proxy, and your provider can arrange both.
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