What is body mass index?
Body mass index (BMI) is a simple measure that compares your weight with your height. Doctors and public health organizations such as the World Health Organization use it as a quick screening tool to see whether weight may be a health risk. Enter your height and weight in US or metric units to see your BMI and your weight category. For adults the calculator also shows the healthy weight range for your height, your BMI Prime and your Ponderal index; for children and teens it shows the BMI-for-age percentile.
BMI formula
BMI = weight (kg) ÷ height (m)2. In US units: BMI = 703 × weight (lb) ÷ height (in)2.
Example: a person who is 5 ft 10 in tall and weighs 165 lb has a BMI of about 23.7, which is in the normal range. For that height, a normal BMI corresponds to about 128.9 to 174.2 lb. In metric units, 70 kg at 1.75 m gives a BMI of about 22.9.
BMI categories for adults
- Underweight: below 18.5
- Normal weight: 18.5 – 24.9
- Overweight: 25 – 29.9
- Obesity: 30 or more (class I 30 – 34.9, class II 35 – 39.9, class III 40 or more)
The adult categories are the same for men and women and apply from age 20.
BMI for children and teens
Children's body fat changes as they grow, and differently for boys and girls, so a child's BMI is compared with children of the same age and sex. The calculator uses the CDC growth charts for ages 2 to 19 and gives a percentile: a BMI at the 60th percentile is higher than that of 60% of children of the same age and sex.
- Underweight: below the 5th percentile
- Healthy weight: 5th to below the 85th percentile
- Overweight: 85th to below the 95th percentile
- Obesity: 95th percentile or above
Example: a 10-year-old girl with a BMI of 17 is at about the 52nd percentile, a healthy weight. For her age, a healthy BMI is from 14.0 to 20.0.
BMI Prime and Ponderal index
BMI Prime is your BMI divided by 25, the upper limit of the normal range. A value below 1 means you are under that limit; 1.2 means 20% above it. The Ponderal index divides weight by height cubed (kg/m³). It changes less with height than BMI, so it can be more useful for very tall or very short people.
Limits of BMI
BMI does not distinguish muscle from fat and does not show where fat is stored. Muscular athletes can have a high BMI while being healthy, and older adults can have a normal BMI with little muscle. Waist size, body fat percentage, blood pressure, blood tests and lifestyle give a fuller picture. BMI is not a diagnosis — talk to a healthcare provider about your health or your child's.
Frequently asked questions
What is a healthy BMI?
For adults, a BMI from 18.5 to 24.9 is considered a healthy or normal weight. The calculator also shows the weight range that corresponds to it for your height.
Is BMI the same for men and women?
For adults, yes: the formula and the categories are the same, although women typically have more body fat at the same BMI. For children and teens, sex matters, because the percentiles are different for boys and girls.
How is BMI worked out for children?
The BMI itself uses the same formula, but it is compared with the CDC growth charts for the child's age and sex and reported as a percentile. That is why the calculator asks for months as well for ages 2 to 19.
Is BMI accurate for athletes?
Not always. Muscle weighs more than fat, so very muscular people can have a high BMI without excess body fat.