Body mass index, or BMI, is one of the most widely used health screening numbers in the world. It is quick to calculate and needs only your height and weight. But it is also frequently misunderstood. This article explains how BMI is calculated, what the categories mean and, just as important, what BMI cannot tell you.
How to calculate BMI
The formula is simple:
BMI = weight (kg) ÷ height (m)²
If you use pounds and inches, the formula becomes BMI = 703 × weight (lb) ÷ height (in)².
Example 1: A person who weighs 70 kg and is 1.70 m tall: 1.70 × 1.70 = 2.89, and 70 ÷ 2.89 = 24.2.
Example 2: A person who weighs 95 kg and is 1.80 m tall: 1.80 × 1.80 = 3.24, and 95 ÷ 3.24 = 29.3.
Adult BMI categories
The World Health Organization uses these standard ranges for adults:
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25.0 to 29.9: overweight
- 30.0 and above: obesity
These categories are statistical tools. Across large populations, risks for conditions such as type 2 diabetes, high blood pressure and heart disease tend to rise as BMI moves above or far below the healthy range. They do not diagnose any individual.
Why BMI is useful
- It is free, instant and does not require equipment.
- It allows comparison across large groups, which is why public-health researchers use it.
- It can flag when a conversation with a doctor might be worthwhile.
The limits of BMI
BMI measures body size, not body composition. That leads to several well-known blind spots:
- Muscle mass. Muscle is denser than fat. A strong, athletic person can have a BMI in the overweight range with low body fat.
- Age. Older adults often lose muscle and may have a "normal" BMI while carrying more fat than is healthy.
- Fat distribution. Fat stored around the abdomen carries more health risk than fat stored elsewhere, and BMI cannot see where fat is located.
- Ethnicity. Some groups have higher health risk at lower BMI values, and some health bodies suggest different cut-offs for them.
- Height extremes. Very tall or very short people can be misclassified.
- Children and teenagers. They should not use adult categories. Paediatric growth charts compare BMI with age and sex percentiles.
Measures that add context
- Waist circumference: a larger waist suggests more abdominal fat.
- Waist-to-height ratio: keeping your waist under half of your height is a commonly used guide.
- Blood pressure, cholesterol and blood sugar: these directly measure health markers that BMI only hints at.
- Fitness level: how far you can walk or how quickly you recover is a strong predictor of health.
What to do with your result
If your BMI is in the healthy range, keep up the habits that support it: regular movement, a varied diet and good sleep. If it is outside the range, treat it as a prompt, not a verdict. Talk to a doctor or registered dietitian who can look at the full picture. Gradual, sustainable changes, such as walking more, eating more vegetables and whole foods, and reducing sugary drinks, are more effective and safer than crash diets.
Underweight results also deserve attention. Being too light can signal nutritional deficiencies or underlying health issues.
Try the calculator
Use our BMI Calculator with metric or imperial units to get your number in seconds. Remember it is intended for adults and is for information only, not medical advice.