BMI sees total mass and height, and cannot tell where the mass is or what it is made of. That produces two distinct failures, in opposite directions — and only one of them ever gets discussed.
Waist circumference divided by height. Both in the same units, so the ratio is dimensionless and the boundaries are the same everywhere — no separate chart for men and women, no adjustment for age, and nothing to look up.
Keep your waist to less than half your height, i.e. WHtR < 0.5. For
someone 175 cm tall that is a waist under 88 cm; at 190 cm it is under 95 cm. UK NICE
guidance uses this ratio alongside BMI for exactly this reason: it needs one measurement and
no interpretation.
Fat stored around the abdomen — visceral fat, packed around the organs — carries far more metabolic risk than the same mass stored on hips and thighs. Waist circumference is a crude but direct proxy for it. Total body mass is not: it counts muscle, bone and peripheral fat identically.
Someone 180 cm and 78 kg has a BMI of 24.1 — comfortably "healthy". With a 95 cm waist their WHtR is 0.53, which is in the increased-risk band. Meanwhile a 180 cm, 95 kg athlete has a BMI of 29.3 — "overweight" — and with an 82 cm waist a WHtR of 0.46, which is fine. The two measures disagree about both people, and the tape measure is right both times.
Midway between the bottom of your lowest rib and the top of your hip bone — roughly level with the navel for most people, though the landmarks matter more than the navel does. Directly on skin or over one thin layer, breathing out normally, tape snug but not compressing. Measuring over a waistband, or holding your breath in, is the commonest way to get a number that means nothing.
For predicting cardiometabolic risk in an individual, generally yes — and it takes one measurement rather than two. But it is still a proxy: it cannot distinguish visceral fat from subcutaneous, and it says nothing about fitness, blood pressure, lipids or blood glucose, all of which matter more than any body measurement.
Much less than BMI or raw waist circumference do, which is the main practical argument for it — a single 0.5 boundary works reasonably across sexes, ages above about 6, and ethnic groups where absolute waist thresholds need separate cut-offs. "Less adjustment" is not "none": some guidance uses lower boundaries for people of South Asian descent.
This page does not compute it, deliberately. It captures fat distribution rather than amount, which makes it informative but harder to use: it needs a second measurement, separate thresholds for men and women, and it can sit perfectly still while both numbers grow together. Waist-to-height needs one measurement and one boundary, which is most of why it is the more practical instrument.
An adult measuring accurately at the correct landmark. The 0.5 boundary is a general adult threshold, not a diagnosis, and a single number cannot describe anybody's health. Not modelled: age and sex-specific refinements, ethnicity-specific cut-offs, pregnancy, growth in children and adolescents, ascites or other conditions that change abdominal girth for reasons unrelated to fat, athletes with unusual trunk musculature, and every clinical measure — blood pressure, lipids, HbA1c, fitness — that predicts outcomes better than any tape measure.
No changes to this tool’s own behaviour since the earliest archived release (v1.22). The full history for the site is in the changelog.