BMI vs waist-to-height: which actually matters?

BMI is a fast, useful screen across a population, but for an individual it cannot tell muscle from fat or show where you carry weight. Waist-to-height ratio, which the NHS now recommends alongside BMI, captures central fat around the organs, the kind most linked to health risk. A simple rule is to keep your waist under half your height: a ratio of 0.5 or above signals increased risk regardless of BMI. Use BMI as a first look and waist-to-height as the sanity check. Neither is a diagnosis.

What BMI does well, and badly

BMI is your weight in kilograms divided by your height in metres squared, sorted into the NHS bands: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and above obese. It is cheap, quick and good at tracking a whole population, which is why national surveys use it. The weakness is at the individual level: a muscular rugby player and someone carrying excess fat can share a BMI of 28, because the number knows nothing about body composition.

Why waist-to-height ratio improves on it

Waist-to-height ratio is your waist measurement divided by your height, in the same units. It targets visceral fat, the fat stored around the abdominal organs that is most strongly tied to type 2 diabetes and heart disease. The NHS guidance is to keep your waist to less than half your height, so a ratio of 0.5 or above is a flag, and 0.6 or above is high. Because it focuses on where fat sits rather than total mass, it often catches risk that BMI misses, especially in people who look a healthy weight but carry fat centrally.

How to use both together

• Start with BMI for a quick category.
• Measure your waist at the midpoint between your lowest rib and the top of your hip bone, then check the waist-to-height ratio.
• If BMI says healthy but your waist is half your height or more, treat that as the more important signal.
• If you are very muscular, expect BMI to overstate risk; waist-to-height will be the fairer read.

The BMI and waist-to-height calculator works out both at once and shows the maths behind each. For how UK adults compare on body weight overall, see the UK fitness statistics.

Informational only, not a medical diagnosis. BMI and waist-to-height ratio are screening tools; a clinician can assess your full risk.