Body mass index (BMI) assesses how your weight relates to your height. It indirectly reflects how much body fat you have. Doctors and other professionals use your BMI as one factor to predict your risk for health problems and to measure or calculate various health factors and recommendations.
Calculating Your BMI
You calculate your BMI by dividing your weight in pounds by your height in inches squared (lbs/in²). Using the metric system, it is weight in kilograms, divided by height in meters squared (kg/m²).
Our BMI calculator above makes it easier to do this calculation:
- Click on the button that corresponds to the system of measurement you prefer.
- Enter your weight and height in the appropriate boxes.
- Click "Calculate" and the calculator will display your BMI.
- Click "Clear Results" to perform a new calculation.
The same formula works to calculate the BMI of women, men, and children.
Understanding How to Use It
Your body mass index is an indirect assessment of your level of body fat and is a simple and inexpensive health tool, according to the Centers for Disease Control and Prevention (CDC). The BMI correlates well with more direct measures of body fat, such as skinfold thickness. The CDC notes it also correlates with various health risk factors related to being overweight or obese, such as diabetes and heart disease.
Not Correlated With Muscle Mass
You can have the same BMI as another person but look thinner if you have less body fat and more muscle. The BMI accounts for weight but not for a person's lean muscle mass, which weighs more than fat. This is important when assessing athletes and others who exercise rigorously and have more muscle.
Adult women and men older than 20 years of age have the same weight categories for each BMI rang, although women tend to have more body fat than men. Although calculated by the same formula as adults, the BMI of children and teens aged 20 and younger vary with age and gender, because of the age and gender-specific changes in body fat in that group.
Adult BMI Ranges
The following chart summarizes the World Health Organization's (WHO) international adult women and men BMI ranges and weight categories. The United States uses the same classification.
The BMI ranges are in kilograms per meter squared (kg/m²)
| BMI Ranges || Weight Categories |
| Less than 18.50 || Underweight |
| 18.50 to 24.99 || Normal Weight |
| 25.00 to 29.99 || Overweight |
| 30 and over || Obese |
Note that the BMI does not take into account that body fat differs between ethnic groups, according to the CDC and the WHO.
Classification for Children and Teens
A BMI calculation for a child or teen up to age 20 is charted on a gender-specific BMI-for-age percentile growth chart, which you can access on the CDC's website. This gives you the percentile range of his BMI compared to other children of the same age and reflects his weight category and growth pattern.
The following chart shows the weight categories for the standard BMI percentiles for children and teens.
| BMI Percentiles || Weight Categories |
| Less than 5th percentile || Underweight |
| 5th percentile to less than 85th percentile || Normal Weight |
| 85th to less than 95th percentile || Overweight |
| Equal to or greater than the 95th percentile || Obese |
A Simple Health Tracker
Healthcare providers use the body mass index to help assess risk for diseases that increase with being overweight or obese. The BMI is also useful to guide treatments and health advice, such as the recommendations for weight gain in pregnancy. Calculate your own BMI with our BMI calculator to keep track of your weight, diet, and health status.