Your numbers · 3 min read
BMI: what it can and cannot tell you
BMI is your weight divided by your height squared, a formula devised in the 1830s by a statistician describing populations rather than patients. It cannot distinguish muscle from fat or tell you where fat sits. It is a reasonable first screen at scale and a poor verdict on one person. Waist measurement adds more than a second decimal place ever will.
What it measures
BMI is a ratio, not a measurement of anything in your body. Weight in kilograms divided by height in metres squared. It contains no information about what your weight is made of.
That is the design rather than a flaw in it. The ratio came out of nineteenth-century population statistics, where individual variation averages out and the correlation with body fat is strong enough to be useful for public health. WHO still uses it that way, counting 25 and above as overweight and 30 and above as obesity.
Applied to one person, that averaging is exactly what you lose. A muscular person and a sedentary person of the same height and weight get the same number, and it means something different for each.
Where it fails
It cannot tell muscle from fat. This is the failure people know about, and it matters most for people who lift, for athletes, and for anyone who has recently changed their body composition without changing much weight.
It cannot tell you where fat is. Fat around the organs carries more metabolic risk than the same weight sitting under the skin, which is the reasoning behind NICE asking for a waist-to-height ratio alongside BMI. Two people with identical BMIs can differ a lot on that second number.
And it does not adjust for age or for the differences in typical body composition between populations, which is why NICE lowers its overweight and obesity thresholds by 2.5 kg/m2 for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background. The reason it gives is that these groups tend toward central adiposity, and so carry weight-related risk at a lower BMI.
| Measure | Tells you | Misses |
|---|---|---|
| BMI | Rough weight-for-height, useful at scale | Muscle vs fat, fat location |
| Waist circumference | Central fat, which carries the most risk | Overall size |
| Waist-to-height ratio | Central fat adjusted for build | Muscle mass |
| Body fat estimate | Composition | Where the fat sits |
What it is still good for
Tracking yourself over time. If your height is fixed, BMI moves only when your weight moves, so as a trend line for one person it is exactly as useful as tracking weight, which is to say reasonably useful.
As a first screen. If a number is well outside the usual range, that is worth a conversation with a doctor, and BMI will catch that.
What it is not good for is a verdict. Sitting a point either side of a threshold does not change your health on the day you cross it.
What to look at instead, or as well
Waist circumference is the single most useful addition and it costs nothing but a tape measure. It captures central fat, which is where the metabolic risk concentrates, and it changes visibly when your composition changes even if the scale does not move.
Waist-to-height ratio is the measure NICE now asks clinicians to record alongside BMI for anyone under a BMI of 35, and the target is a waist under half your height. It adjusts naturally for build.
And if you are fasting, expect BMI to be a lagging and noisy indicator in the first weeks. Water and glycogen shifts of two to three kilograms are normal and mean nothing about fat.
Related tool: BMI calculator
Sources
- World Health Organization. Obesity and overweight: fact sheet. Defines BMI as weight (kg) divided by height squared (m2), with overweight at 25 and above and obesity at 30 and above.
- National Institute for Health and Care Excellence. Overweight and obesity management (NG246): identifying and assessing overweight, obesity and central adiposity. Recommends a waist-to-height ratio below 0.5 for adults with a BMI under 35, and thresholds lowered by 2.5 kg/m2 for several ethnic groups.
Every source here has been checked to exist and to say what this article says it says. Findings from animal or laboratory studies are labelled, because they often do not transfer to people.