How to Use This Tool
BMI takes two numbers you already know and produces one that public health bodies have used since the 1970s. It is genuinely useful for what it was designed to do — compare populations — and genuinely limited for what people mostly use it for, which is judging themselves. This calculator gives you the number, and then tells you honestly when it does not apply to you.
Entering your measurements
Switch between metric and imperial at the top. In imperial mode you can enter height either as total inches or as separate feet and inches, whichever you actually know. Weight should be your typical weight, not a morning-after-a-run reading; normal daily fluctuation of one to two kilograms is water, and it moves BMI by about half a point.
What the number means
The coloured bar places you against the standard WHO bands. Alongside it you get the healthy weight range for your height — usually more actionable than the index itself, because it is expressed in the unit you actually step onto a scale in. If you are outside the range, the second box shows the distance in kilograms or pounds rather than in abstract index points.
The Asian cutoffs box applies the lower thresholds recommended by the WHO for people of South and East Asian descent, where metabolic risk rises at a lower BMI: overweight begins at 23 rather than 25. If that describes you, that is the row to read.
The four situations where BMI misreads you
- Muscular builds. Muscle is denser than fat, so athletes and regular lifters routinely score "overweight" at low body fat. If you train seriously with weights, BMI is measuring the wrong thing.
- Older adults. Muscle and bone density decline with age, so an unchanged BMI can hide substantial loss of lean mass. A BMI slightly above the "healthy" band is associated with better outcomes after about 65.
- Very tall or very short people. The formula divides by height squared while bodies scale closer to a cube. This inflates BMI for tall people and deflates it for short people, a flaw Quetelet himself pointed out in 1832.
- Pregnancy, and children. BMI is not valid during pregnancy, and children need age-and-sex percentile charts rather than the adult bands.
The cross-check worth doing
Enter your waist measurement in the box near the bottom. Waist-to-height ratio — keeping your waist under half your height — predicts metabolic and cardiovascular risk better than BMI in most published comparisons, because it captures where fat sits rather than only how much mass there is. It takes one tape measure and ten seconds, and if the two measures disagree, the waist ratio is generally the one to trust.