Waist circumference
"Why the waist and not the scale?" Because the scale counts all tissue equally and the body does not.
Why abdominal fat is different
Fat stored around the organs — visceral fat — behaves differently from fat under the skin: it is more metabolically active and more closely tied to the risks that obesity guidelines are actually about. Two people of the same weight and height can carry very different amounts of it, and BMI cannot tell them apart. Waist circumference can, roughly, which is why the WHO expert consultation on the subject treats it as a measure of risk in its own right and the newer diagnostic frameworks pair it with BMI.
How to measure
The number is only useful if it is taken the same way every time:
- Standing, feet together, arms relaxed at your sides, at the end of a normal breath out.
- A soft tape, horizontal, against the skin, midway between the lowest rib and the top of the hip bone — for most people about the level of the navel, but the landmark is the bones, not the navel.
- Snug without compressing; read to the nearest half centimetre.
- Same time of day, same conditions, once a month. The waist moves with water and gut contents from day to day, like the scale; the trend is the signal.
The thresholds and their limits
The WHO consultation gives population thresholds at which risk rises — about 94 centimetres for men and 80 for women as "increased", 102 and 88 as "substantially increased" — and is explicit that they were derived mainly from populations of European origin and that appropriate cut-offs differ by ethnicity. The waist-to-height ratio, with the working rule "keep your waist under half your height", is a simpler screen that travels better across heights. Neither is a diagnosis; both are a reason to look further.
In the app
The app does not track the waist yet. Measure it monthly alongside the weight trend; a falling waist with a flat scale is what a strength programme looks like in its first weeks, and the BMI page explains why the two can disagree.