What is BMI?
Body mass index (BMI) is a simple screening measure that compares your weight to your height. It is calculated as weight in kilograms divided by height in meters squared: BMI = kg ÷ m². In US units, the equivalent formula is 703 × lb ÷ in². This calculator converts imperial measurements to metric internally, so both give identical results.
BMI categories for adults
- Below 18.5: Underweight
- 18.5 – 24.9: Normal (healthy) weight
- 25 – 29.9: Overweight
- 30 and above: Obese
These ranges come from the World Health Organization and the U.S. Centers for Disease Control and Prevention and apply to adults aged 20 and older. Children and teens use age- and sex-specific BMI percentiles instead.
Limitations of BMI
BMI is popular because it is quick and free, but it doesn't measure body fat directly. It can't distinguish muscle from fat, so very muscular people may register as overweight. It doesn't account for where fat is stored, which matters for health risk; waist circumference adds useful information. It may also be less accurate for older adults, who often have less muscle, and the relationship between BMI and health risk varies among ethnic groups.
Because of this, BMI is best used as a starting point for a conversation with a healthcare professional rather than a diagnosis.
Healthy weight tips
If your BMI is outside the healthy range, small sustainable changes usually work better than drastic diets: regular physical activity, more vegetables, fiber and protein, fewer sugary drinks, and consistent sleep. A doctor or registered dietitian can help set realistic goals.
This calculator is for informational purposes only and is not medical advice.
Frequently asked questions
What is a healthy BMI?
For adults, a BMI between 18.5 and 24.9 is considered within the healthy weight range.
Is BMI accurate for athletes?
Not always. Muscle weighs more than fat, so athletes may have a high BMI with low body fat.
How is BMI calculated in pounds?
BMI = 703 × weight in pounds ÷ (height in inches)².