How BMI is calculated
Body Mass Index (BMI) is a simple ratio of weight to height used worldwide to screen for weight categories in adults:
BMI categories for adults
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
These ranges apply to adults aged 20 and over. For children and teens, BMI is assessed against age- and sex-specific growth percentiles instead.
What BMI can and can't tell you
BMI is a fast, inexpensive screening tool, and at the population level it correlates well with body fat and health risk. But it has known limits:
- Muscle vs. fat: muscular athletes can register as "overweight" while carrying very little fat.
- Fat distribution: BMI says nothing about where fat is stored — abdominal fat carries more health risk than fat elsewhere.
- Age and ethnicity: health risk at a given BMI varies across populations; some guidelines use lower cutoffs for certain groups.
Treat your BMI as a starting point, not a verdict — pair it with waist circumference, activity level, and advice from a healthcare professional.
Frequently asked questions
How is BMI calculated?
Weight in kilograms divided by height in meters squared (kg/m²). In imperial units, multiply weight in pounds by 703 and divide by height in inches squared.
What is a healthy BMI?
For most adults, 18.5–24.9 is the healthy range. Below 18.5 is underweight, 25–29.9 is overweight, and 30+ is classified as obesity.
Is BMI accurate for everyone?
It's a screening tool, not a diagnosis. It can misclassify muscular people as overweight and doesn't account for fat distribution, age or ethnicity.
Does BMI work for children?
Not with adult cutoffs — children's BMI is compared to growth-chart percentiles for their age and sex. This calculator is designed for adults 20+.