What is BMI?
Body mass index (BMI) is a simple ratio of weight to height that doctors and public health bodies use to screen for weight categories that may affect health. It was developed in the 19th century by the statistician Adolphe Quetelet and is still used worldwide because it is quick, free and needs only a scale and a tape measure.
BMI is a screening tool, not a diagnosis. It does not distinguish muscle from fat, so muscular athletes can score as “overweight”, and it says nothing about where fat is stored. Some populations, including many people of South Asian, Chinese and other Asian heritage, face higher health risks at lower BMIs, and some guidelines use lower thresholds for them. It is also not designed for children, teenagers or pregnant women.
How to use the BMI calculator
- Choose metric (kilograms and centimeters) or imperial (pounds, feet and inches).
- Enter your weight and height.
- Read your BMI, your WHO category and the marker on the scale, plus the weight range that would put you in the healthy band for your height.
The BMI formula and categories
BMI = weight (kg) ÷ height (m)²
BMI = 703 × weight (lb) ÷ height (in)²
| BMI | WHO category for adults |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
Example
A person who weighs 70 kg and is 175 cm (1.75 m) tall has a BMI of 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9, in the healthy range. For that height, a BMI between 18.5 and 24.9 corresponds to a weight of about 56.7 to 76.3 kg.
In imperial units, 154 lb at 5 ft 9 in (69 inches) gives 703 × 154 ÷ 69² ≈ 22.7.
More worked examples
Imperial units: 165 lb at 5 ft 6 in gives a BMI of about 26.6, in the overweight range.
Metric units: 58 kg at 1.62 m gives 58 ÷ 1.62² ≈ 22.1, in the healthy range.
Common mistakes to avoid
- Using BMI for children. Under-18s need age-and-sex percentile charts instead.
- Reading BMI as body fat. Muscular people can score high with little fat; waist size adds context.
- Typing height in the wrong unit. Use the metric/imperial switch rather than mixing units. To estimate daily energy needs next, use the calorie calculator.
Sources and methodology
The calculations on this page follow these published standards and references. The code is covered by automated tests against known values.