How does the BMI Calculator work?
BMI = weight (kg) ÷ height (m)². WHO bands: under 18.5 underweight, 18.5–25 normal, 25–30 overweight, 30+ obese.
Background & details
How to read your result
Your BMI places you in one of four WHO categories. What matters is not the second decimal but the broad position: are you clearly in the normal range (18.5–24.9), right on a boundary, or well above or below it? A BMI of 24.8 and one of 25.2 are practically identical – the cut-offs are statistical conventions, not medical deadlines.
What values are typical?
- Under 18.5: underweight – worth checking with a doctor if paired with persistent fatigue.
- 18.5–24.9: normal weight, statistically the lowest risk.
- 25–29.9: overweight (pre-obesity) – often still fine, depending on fat distribution.
- 30 and above: obesity class I–III, with rising risk of cardiovascular disease and diabetes.
Common mistakes
The biggest misconception is treating BMI as a body-fat measurement. It only knows your height and weight – not whether that weight is muscle, water or fat. A well-trained strength athlete can easily score a BMI above 27 and count as "overweight" on paper despite a low body-fat percentage. Conversely, someone with a normal BMI may carry plenty of belly fat and little muscle ("skinny fat"). Weighing yourself at the wrong time (after a meal, in clothes) also skews the number by a kilo or two.
Practical tips
Weigh yourself first thing in the morning, without clothes, on the same scale every time. The trend over several weeks tells you far more than a single reading. Pair the BMI with your waist circumference: above 88 cm (women) or 102 cm (men) is a risk factor regardless of BMI, because belly fat is more metabolically active than fat on the hips and legs.
If you are changing your weight, use the calculator as a tracking tool: note your weight and BMI every few weeks and watch the direction of travel. A change of half a BMI point per month is a healthy, sustainable pace. If your BMI jumps sharply within a few days, that is almost always water rather than fat – salt, carbohydrates and your cycle can move the scale by a kilo or two in the short term.
When BMI does not fit
For children and teenagers you need age- and sex-specific percentile charts instead of fixed cut-offs. BMI is also of limited use for pregnant women, very tall or very short people, bodybuilders and older adults losing muscle. In those cases a body-fat measurement, waist circumference or a clinical assessment says more than the number alone.