What is the waist-to-height ratio and why use it?
It is your waist circumference divided by your height, in the same units. Research over the past two decades has found it a better predictor of cardiometabolic risk than BMI, because it measures where fat is carried rather than how much you weigh in total. Abdominal fat around the organs is metabolically active and far more strongly associated with cardiovascular disease and type 2 diabetes than fat elsewhere on the body.
What is a healthy ratio?
The widely used rule of thumb is to keep your waist under half your height — a ratio below 0.5. The UK's NICE guidance uses this threshold for adults. Between 0.5 and 0.6 indicates increased risk, and 0.6 or above indicates high risk. The elegance of the 0.5 rule is that it needs no chart or calculator and works across most adult heights, ages and ethnic groups, which is not true of BMI cut-offs.
Why is this considered better than BMI?
BMI cannot distinguish muscle from fat, nor tell you where fat sits. A muscular athlete and a sedentary person can share a BMI while having very different risk profiles. Two people with identical BMI can also carry fat quite differently — one around the abdomen, one on the hips and thighs — and the abdominal pattern carries substantially more risk. Waist-to-height captures that distinction with a single tape measure.
How do I measure my waist correctly?
Measure at the midpoint between the bottom of your ribs and the top of your hip bones, roughly level with the navel for most people — not at the narrowest point and not where your trousers sit. Stand relaxed, breathe out normally, and keep the tape snug without compressing the skin. Measure at the same time of day for comparability, ideally in the morning, since abdominal measurement varies noticeably after meals.
Does the threshold change for children or different ethnic groups?
For children over about five, the same 0.5 boundary is generally applied and appears to work well, though paediatric assessment should involve a clinician. For adults, one of the ratio's strengths is that it needs less ethnic adjustment than BMI or waist circumference alone, both of which require different cut-offs for South Asian and East Asian populations. That said, no single number captures individual risk.
What are the limitations?
It is a screening measure, not a diagnosis. It cannot account for pregnancy, certain medical conditions affecting abdominal size, or unusual body proportions. It says nothing about blood pressure, cholesterol, blood glucose, fitness or family history — all of which matter as much or more. Use it as one input alongside those, and discuss anything concerning with a healthcare professional rather than acting on the ratio alone.