Hardly any health figure is as widespread and as often misunderstood as the BMI. The BMI Calculator delivers not just the value but also its classification – this tutorial explains how to read both.
The formula
BMI = weight in kg ÷ (height in m)². At 1.80 m and 75 kg: 75 ÷ 3.24 = 23.1. The WHO classification: below 18.5 underweight, 18.5–24.9 normal weight, 25–29.9 overweight, from 30 obesity in three classes.
Step by step
- Enter height and weight: weighed in the morning, without clothes – that keeps values comparable
- State your age (optional): from 65 a slightly higher range counts as favourable
- Read the result: BMI, WHO category, your personal normal-weight range and the 'New BMI'
Why the 'New BMI' is shown alongside
The classic formula scales with the square of height, but bodies grow in three dimensions. Consequence: tall people systematically get values that are too high, short people too low. Oxford mathematician Nick Trefethen therefore proposed 1.3 × weight ÷ height^2.5 – for people around 1.90 m this quickly makes a point of difference, and thus sometimes a whole category.
Where BMI fails
The index knows neither muscle mass nor fat distribution. A strength athlete at 95 kg and 1.85 m (BMI 27.8) is not overweight in the health sense; a slim person with a lot of abdominal fat can carry elevated metabolic risk despite a BMI of 23. The waist-to-height ratio is more informative then: a waist circumference below half your height counts as favourable. Children and teenagers have their own percentile curves, and for pregnant women the BMI is unsuitable. In short: the BMI is a good first compass – diagnoses are for the doctor.
