How BMI is calculated
BMI is your weight in kilograms divided by the square of your height in metres (kg/m²). Someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9. A second example: a 60 kg adult at 1.60 m gets 60 ÷ (1.60 × 1.60) ≈ 23.4 — also in the healthy range, despite the different height and weight.
BMI categories (adults)
- Under 18.5 — Underweight
- 18.5–24.9 — Healthy weight
- 25–29.9 — Overweight
- 30+ — Obese
Where the formula comes from
BMI was devised in the 1830s by Belgian mathematician Adolphe Quetelet as a population statistic, not an individual diagnostic tool. It became the standard clinical screening measure decades later because it is cheap to calculate and correlates reasonably well with body fat across large groups — even though, as Quetelet himself intended it, it was never designed to assess one person's health on its own.
What BMI can and can't tell you
BMI is a fast screening tool, not a diagnosis. Because it only uses height and weight, it can't tell muscle from fat — very muscular people often read "overweight" despite low body fat, and older adults can read "healthy" while carrying more fat and less muscle than the number suggests. Pair it with a body-fat estimate for a fuller picture.
Using your result
If your BMI sits outside the healthy range, small consistent habits beat crash diets: steady hydration, 7–9 hours of sleep, regular movement, and balanced meals. Track the trend over weeks, not single days.