What Is BMI?
Body Mass Index (BMI) is a simple ratio of weight to height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet and is used today by the World Health Organization (WHO) and health services worldwide as a quick screening measure for weight categories in adults. Because it needs only two measurements and no equipment, it is useful for spotting trends across large populations and as a first check for individuals.
The formula
For example, a person who is 1.70 m tall and weighs 70 kg has a BMI of 70 ÷ (1.70 × 1.70) = 24.2, which falls in the healthy range.
WHO adult BMI categories
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal (healthy) weight |
| 25.0 – 29.9 | Overweight (pre-obese) |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
Some health authorities use lower cut-off points for people of Asian descent — for example, a BMI of 23 or above may indicate increased risk — because health risks can rise at lower BMI values in these populations.
What BMI doesn't tell you
- It doesn't distinguish muscle from fat. Athletes with a lot of muscle can have a high BMI while carrying little body fat.
- It ignores where fat is stored. Fat around the abdomen is linked with higher health risk; waist circumference or waist-to-height ratio adds useful information.
- It changes meaning with age. Older adults naturally lose muscle, so the same BMI can reflect a different body composition at 70 than at 30.
- It is not for children. Under-20s are assessed with age- and sex-specific BMI percentile charts.
- It is not suitable during pregnancy.
Using your result
Treat BMI as one piece of a bigger picture. If your result is outside the healthy range, it is a reason to look at other measures and talk to a doctor — not a verdict on your health. Pair it with our calorie calculator to understand your daily energy needs, and with the heart rate zone calculator to plan exercise at the right intensity for your age.
Frequently Asked Questions
Is BMI accurate?
BMI correlates reasonably well with body fat across populations, but it can misclassify individuals — especially very muscular people, older adults and some ethnic groups. It is a screening tool, not a diagnostic test.
Is BMI different for men and women?
The adult formula and categories are the same for men and women. Women tend to carry more body fat than men at the same BMI, which is one reason other measures are useful too.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the normal range. A value between 0.74 and 1.0 corresponds to the normal range; above 1.0 is above the normal range.
What is a healthy weight for my height?
The calculator shows the weight range that gives a BMI of 18.5 to 24.9 for your height. A healthy weight for you personally also depends on body composition, age and medical factors.
Disclaimer: This calculator provides general information only and is not a substitute for professional medical advice, diagnosis or treatment. Formulas are population averages and may not fit your individual situation. Always consult a qualified healthcare provider about your health.
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