BMI Explained: What It Tells You and What It Doesn’t
Body mass index, or BMI, is one of the most widely used health screening numbers in the world. It is simple, free and quick, but it is also widely misunderstood. Here is what it measures and where it can mislead.
How BMI is calculated
BMI equals your weight in kilograms divided by your height in meters squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. If you prefer pounds and inches, multiply weight in pounds by 703 and divide by height in inches squared. Our BMI calculator does either for you.
The standard adult categories
- Below 18.5: underweight
- 18.5 to 24.9: normal weight
- 25 to 29.9: overweight
- 30 and above: obese
These thresholds come from population studies linking BMI to health risks such as heart disease and type 2 diabetes.
Where BMI falls short
- It ignores body composition. Muscle is denser than fat, so a muscular athlete can have a high BMI with little body fat.
- It ignores fat distribution. Fat stored around the waist carries more health risk than fat on the hips and thighs.
- It varies by group. Some health bodies suggest different thresholds for people of South Asian and other backgrounds, since risk can start at lower BMI values.
- Age matters. Older adults may be healthier at a slightly higher BMI, and the adult categories do not apply to children.
Better companions to BMI
Waist circumference, waist-to-height ratio and body fat percentage give a more complete picture. You can estimate body fat with only a tape measure using the body fat calculator. Blood pressure, blood sugar, cholesterol, fitness and sleep matter too.
What to do with your number
Treat BMI as a prompt, not a verdict. If it is outside the normal range, speak with a doctor, who can look at your whole health. Focus on habits rather than the scale alone: regular movement, plenty of vegetables, enough protein, adequate sleep and stress management. If you want to change your weight, our calorie calculator shows how many calories you need to maintain, lose or gain gradually.
This article is general information and not medical advice.