How to Calculate BMI: Formula, Categories, and Limits
Learn the BMI formula for metric and imperial units, what the standard adult categories mean, and when BMI can be misleading.
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What BMI measures
Body mass index relates weight to height. It is a quick screening number, not a diagnosis, and it does not measure body fat directly.
BMI formula
Example: a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 70 / 3.0625, which is about 22.9.
- Metric: BMI = weight in kg / (height in meters)²
- Imperial: BMI = 703 × weight in pounds / (height in inches)²
Standard adult BMI categories
The World Health Organization uses these ranges for adults:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal range |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
Limits of BMI
This information is general and not medical advice. Talk to a healthcare professional about your health.
- It does not distinguish muscle from fat, so athletes can score high.
- Ranges can be interpreted differently for children, older adults, and some ethnic groups.
- It says nothing about diet, fitness, or other health markers.
FAQ
What is a healthy BMI?
For most adults, a BMI from 18.5 to 24.9 is considered the normal range, but individual health depends on many factors.
Does BMI work for children?
Children use age- and sex-specific percentile charts instead of the adult ranges.
Is BMI the same for men and women?
The adult formula and ranges are the same, though body composition differs between people.
