Quick answer: Body mass index is weight in kilograms divided by height in meters squared. For adults the World Health Organization calls a BMI of 18.5 to 24.9 a healthy weight, 25 to 29.9 overweight and 30 or more obesity. A person who is 175 cm tall and weighs 70 kg has a BMI of 22.9.
Body mass index (BMI) is a quick way to compare your weight with your height. Doctors and public health agencies use it as a first screening step because it needs nothing more than a tape measure and a scale.
It is only a screening number. BMI cannot tell muscle from fat, and it does not show where fat is stored. A muscular athlete can land in the overweight range with very little body fat, and some people with a normal BMI carry more fat around the waist than is healthy. Use it as one piece of information alongside waist size, fitness and advice from your doctor.
How BMI is calculated
BMI = 703 × weight (lb) ÷ height (in)²
The World Health Organization groups adult BMI into these ranges:
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 and above: obesity (class 1: 30 to 34.9, class 2: 35 to 39.9, class 3: 40 and above)
These ranges are for adults aged 18 and over. Children and teenagers are assessed with growth charts that account for age and sex. Some Asian populations face higher health risk at lower BMI values, and local guidance may use lower cut-offs.
A worked example
Example. A person who is 170 cm tall and weighs 70 kg has a BMI of 70 ÷ (1.70 × 1.70) = 24.2, which falls in the healthy weight range. The healthy range for 170 cm is 53.5 to 72.0 kg.
Real-world example
Two people are 175 cm tall. One weighs 70 kg, a BMI of 22.9, in the healthy range. The other weighs 100 kg, a BMI of 32.7, in obesity class 1. If the second person is a bodybuilder, the number overstates their body fat, which is why a doctor looks at more than BMI.
BMI categories for adults
| BMI (kg/m²) | WHO category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity, class 1 |
| 35.0 to 39.9 | Obesity, class 2 |
| 40.0 or more | Obesity, class 3 |
These are the standard adult categories used by the World Health Organization. For people of Asian descent, health authorities suggest that risk starts at lower BMI values, around 23, so discuss your own target with a doctor.
Common mistakes to avoid
- Treating BMI as a diagnosis. It compares weight with height and cannot tell muscle from fat, so athletes and older adults can be misclassified.
- Using adult categories for children and teenagers, whose healthy ranges depend on age and sex and are read from growth charts.
- Ignoring where the weight is carried. Waist size and other measures add useful information that BMI does not capture.
Go deeper: read our guide BMI Explained: Categories, Limits and Alternatives.
Frequently asked questions
Is BMI accurate?
It is a reasonable screening tool for groups of people, but it is a rough guide for individuals. It ignores muscle mass, bone density, age and fat distribution. A doctor may combine it with waist measurement and other tests.
What is a healthy BMI?
For most adults, the WHO defines 18.5 to 24.9 as the healthy weight range. The calculator shows the weight range that matches this for your height.
Does BMI differ for men and women?
The adult formula and categories are the same for men and women, although women tend to carry more body fat at the same BMI.
Can I use this for my child?
No. Use a growth chart designed for children and teenagers, and ask your paediatrician. This calculator is only for adults.
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Sources and further reading
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