Every so often a form asks me for my height and weight, and I have noticed the units keep changing on me. American forms want feet, inches and pounds; anything international wants meters and kilograms, and I end up doing conversion arithmetic in a waiting room.
The one formula that sits behind all those forms is body mass index, and it is worth understanding what the number actually is — and is not.
The formula and why height gets squared
BMI equals weight in kilograms divided by height in meters squared, so its unit is kilograms per square meter. A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 divided by about 3.06, roughly 22.9.
The squared height is the interesting part. A taller person naturally carries more mass, so dividing by height alone would still penalize tall people. Dividing by height squared roughly accounts for how body mass scales with stature, which is what makes the index comparable across different heights.
Doing the math in pounds and inches
For those of us raised on imperial units, the same formula needs a conversion factor of 703: multiply weight in pounds by 703, then divide by height in inches squared. Someone who is 69 inches tall and weighs 154 pounds gets 154 times 703 divided by 4761, about 22.7. The 703 is just the metric formula in disguise, bundling the pound-to-kilogram and inch-to-meter conversions into one constant.
Two mistakes to watch for:
- Mixing systems, such as kilograms with inches, which produces nonsense numbers
- Using centimeters instead of meters, which makes BMI ten thousand times too small — if your result is 0.0023, the arithmetic went wrong, not your health
Reading the number sensibly
The standard adult categories used by health organizations put a BMI below 18.5 as underweight, 18.5 to 24.9 as the healthy range, 25 to 29.9 as overweight, and 30 or above as obese. These cutoffs are population-level screening lines, not personal verdicts, and some health bodies apply somewhat lower cutoffs for certain Asian populations, reflecting differences in typical body composition.
The blind spots are well known. BMI cannot distinguish muscle from fat, so muscular athletes often score as overweight while carrying little fat. It says nothing about where fat sits on the body, even though abdominal fat matters more for health. And for children, BMI must be read against age- and sex-specific growth charts, not the adult cutoffs.
A screening tool, not a diagnosis
Doctors use BMI as a quick first filter precisely because it needs only a scale and a tape measure. Anything it flags is a prompt for better measurements — waist circumference, blood work, an actual conversation — rather than a conclusion in itself. I find that framing takes a lot of the sting and the superstition out of the number.
Know your height in meters and weight in kilograms, and you can check your BMI anywhere with ten seconds of arithmetic, which is exactly what a good screening tool should allow.
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