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Calculate BMI for You and Your Child

BMI for Children and AdultsCalculate Body Mass Index

Body Mass Index (BMI) is commonly used as a preliminary diagnoses in identifying health risks associated with excessive or insufficient weight. It is a simple but useful numeric measurement of fatness or thinness that applies to the general population having average body composition.

The index is based on a person's height and weight using the following formula:

BMI = kg / m2

where kg is a person's weight in kilograms and m2 is their height in metres squared.

BMI is a good general purpose formula for the populace but it does not take body composition into consideration. It does not distinguish between weight due to muscle mass and that of fat mass. Since muscle tissue is denser than fat, it weighs more than fat on a comparative volume basis. Consequently, muscle builders and athletes may show a higher BMI than normal but may not be at all at a greater health risk. In contrast, children, while still growing and developing muscles and the elderly who may be losing muscle may show a lower BMI. Additionally, the body composition of pregnant or lactating women changes dramatically during these periods and so BMI cannot be considered applicable for them.

BMI also does not take body fat distribution into consideration. Studies have shown that excess abdominal fat can also contribute to higher health risks. Use the BMI calculator in conjunction with other online calculators found on this site such as the Body Shape Index and Waist-to-Hip Ratio calculators for further assessment of your health risk status.

The standard weight categories for BMI is shown in the table below and applies to the average population of adults 20 years or older.

BMI Category
Below 18.5 Underweight
18.5 - 24.9 Normal Weight
25 - 29.9 Overweight
30 or greater Obese

This calculator will calculate your BMI based on your height and weight using the standard BMI formula. It will also calculate the weight for your height at each of the BMI category boundaries using an inversion of the BMI formula:

kg = BMI * m2

where kg is weight in kilograms, BMI is the index at a category boundary, and m2 is your height in meters squared. From this calculation, you will be able to see what the BMI normal healthy weight range is for your height.

Body Mass Index for Children and Teenagers

Since 1980, the percentage of children who are obese has more than tripled in the U.S., from 5.5% to 17% and a similar trend is occurring worldwide. Overweight children are getting so prevalent that they are now perceived by some as normal. The extra weight can pose significant health risks during the development years and into adulthood. There is an 80% chance that they may stay overweight for their entire lives.

Excessive weight is evaluated differently for children and teens. Unlike BMI for adults, age and sex is taken into consideration because boys and girls grow at different rates and differ in their amount of body fat as they grow. Consequently, so does their relation to body fatness. For children from 2 and up to 19 years of age, BMI is calculated using the same formula as adults but their BMI is compared with growth charts. From the growth chart, a BMI percentile is produced from the child's BMI, age and sex. Their BMI is often referred to as BMI-for-age.

BMI-for-age percentile is a useful indicator of how a child’s weight compares with other children of the same age and sex. If for example, the child is in the 80th percentile, it means that the child’s weight is greater than that of 80% of the other children of the same age and sex.

This calculator first calculates the child's BMI then looks up the corresponding BMI-for-age percentile for the child's sex and age. The percentile is derived from the Centers for Disease Control and Prevention growth charts data.

The table shown below are the BMI-for-age percentile ranges and corresponding weight categories. A BMI-for-age percentile between the 5th and 85th is considered a healthy weight.

Percentiles Category
less than 5th Underweight
5th to 85th Healthy Weight
85th to 95th Overweight
95th or greater Obese

Note that neither BMI nor growth charts measure body fat. Having a BMI between the 85th and 95th percentile could mean that the excessive weight is due to muscle and may not be all attributed to fat. Being athletic or having lean builds will show a high BMI without necessarily having excessive fat. Studies have found that as much as 15% of children fall into this category. However, in almost all children with percentiles above the 95th, the higher weight was found to be due to mostly fat.

Related Calculators

References:
  1. CDC Growth Charts
    National Center for Health Statistics
    Centers for Disease Control and Prevention
  2. BMI Classification
    World Health Organization

Last Update: Sept. 29, 2020 13:41