BMI Calculator
How BMI is calculated
Body Mass Index relates your weight to your height with a single formula: BMI = weight (kg) ÷ height (m)². It was devised in the 1830s by the Belgian statistician Adolphe Quetelet to describe populations, and was renamed "body mass index" by physiologist Ancel Keys in 1972. Today the World Health Organization uses it as a first screening tool for underweight and excess weight in adults.
Worked example
Someone who weighs 70 kg and is 1.75 m tall:
- Square the height: 1.75 × 1.75 = 3.0625
- Divide the weight by it: 70 ÷ 3.0625 = 22.86 kg/m²
- 22.86 falls between 18.5 and 24.9, the "normal weight" range.
If you work in pounds and inches, the equivalent formula is BMI = 703 × weight (lb) ÷ height (in)². You can also convert your values first with our weight converter.
WHO categories for adults
| BMI (kg/m²) | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight (pre-obesity) |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
What BMI does not tell you
BMI is a ratio, not a body-composition measurement, so it should be read with care:
- Muscle vs fat: a muscular rugby player of 1.80 m and 95 kg has a BMI of 29.3 — "overweight" on paper while having little body fat. Our body fat calculator (US Navy method) gives a better picture in that case.
- Fat distribution: fat stored around the abdomen carries more health risk than fat on the hips. A waist above 94 cm for men or 80 cm for women is associated with increased risk, whatever the BMI.
- Age: older adults naturally lose muscle, so the same BMI can hide more fat at 70 than at 30.
- Ethnicity: a WHO expert consultation found that for many Asian populations health risks start rising at lower values, and suggested 23 and 27.5 as additional action points.
- Children, teenagers and pregnancy: adult categories do not apply. Children are assessed with age- and sex-specific BMI percentile charts.
Use your BMI as a starting point for a conversation with a doctor, not as a diagnosis. The ideal weight and daily calorie calculators can help you turn the number into a concrete plan.
Frequently Asked Questions
What is a healthy BMI?
For adults, the WHO considers a BMI between 18.5 and 24.9 as the normal range. Within that range, other factors such as waist size, fitness and diet matter more than the exact number.
Why is my BMI high when I am muscular?
BMI only uses weight and height, so it cannot tell muscle from fat. Muscle is dense, which pushes BMI up. A body fat estimate or a waist measurement is more informative for athletic people.
Can I use this BMI calculator for my child?
No. For children and teenagers, BMI must be compared to growth charts for their age and sex (percentiles). Ask your paediatrician or use the official WHO or CDC child charts.
My height is in centimetres — what should I type?
Enter it in metres: 175 cm becomes 1.75. The calculator squares this value, so entering 175 would give a meaningless result.