How BMI is calculated
Body mass index is weight in kilograms divided by height in metres squared: BMI = kg ÷ m². Someone who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. With imperial units the equivalent is (pounds ÷ inches²) × 703 — this calculator converts your inputs to metric and applies the same formula, so both unit systems give identical results.
The WHO adult categories
| BMI (kg/m²) | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
These are the World Health Organization’s adult reference ranges. Several countries apply lower cut-offs — a number of Asian guidelines treat a BMI of 23 and above as overweight — because the health risk attached to a given BMI differs between populations.
Your healthy weight range
The range shown by the calculator inverts the formula: multiplying 18.5 and 24.9 by your height in metres squared gives the weight span that corresponds to a “normal” BMI. At 1.75 m that is 56.7 kg to 76.3 kg. It is a population-level reference band, not a personal target — where in (or near) that band is right for you depends on build, age and health history.
What BMI can and cannot tell you
BMI is useful precisely because it is crude: two numbers, one result, comparable across whole populations. But it cannot distinguish muscle from fat — muscular athletes routinely read as “overweight” with low body fat, and older adults can read as “normal” while carrying little muscle. It says nothing about where fat is carried, which matters for health risk. It does not apply to children and teenagers, who are assessed against age- and sex-specific growth charts, nor during pregnancy. Treat the number as a screening signal to discuss with a doctor, not a verdict — and if you track it, the trend over months is far more informative than any single reading.
