BMI Calculator for Kids and Teens
BMI Calculator for Kids and Teens
Body Mass Index (BMI) for children and adolescents is calculated differently than for adults. While adults use a fixed set of ranges, children's bodies are constantly changing as they grow. Therefore, a child's BMI must be interpreted relative to other children of the same age and sex. This is known as BMI-for-age.
This calculator uses the World Health Organization (WHO) growth standards to help parents and healthcare providers track a child's physical development from age 2 through 20.
What is BMI-for-Age?
BMI is a screening tool used to identify whether a child is underweight, healthy weight, overweight, or obese. Because children's body fat levels change with age and differ between boys and girls, BMI levels are plotted on growth charts to produce a percentile.
For example, if a 10-year-old boy is in the 75th percentile, it means his BMI is higher than 75% of other 10-year-old boys.
The Formula
The basic BMI formula is the same for everyone:
However, for children, the result is then compared to the reference population:
- Calculate BMI using metric units.
- Compare to Growth Charts: Find the corresponding percentile for the specific age and gender.
- Classify: Assign a category based on the percentile.
WHO BMI Classification for Children
| Category | Percentile Range | | :----------------- | :---------------------- | | Underweight | < 3rd Percentile | | Healthy Weight | 3rd to 85th Percentile | | Overweight | 85th to 97th Percentile | | Obese | > 97th Percentile |
How to Use This Calculator
- Select Gender: Growth patterns differ significantly between boys and girls.
- Enter Age: Use the child's current age in years.
- Input Height and Weight: You can use centimeters and kilograms or switch to feet/inches and pounds.
- Review Results: Look at the BMI value, the percentile ranking, and the health classification.
Worked Examples
Example 1: 8-Year-Old Girl
- Height: 128 cm (1.28 m)
- Weight: 25 kg
- Calculation:
- Result: Placed on the WHO chart, this is roughly the 50th percentile, indicating a Healthy Weight.
Example 2: 14-Year-Old Boy
- Height: 165 cm (1.65 m)
- Weight: 75 kg
- Calculation:
- Result: For a 14-year-old, this BMI is above the 97th percentile, categorizing the result as Obese.
Limitations and Disclaimer
BMI is a screening tool, not a diagnostic tool. It does not measure body fat directly. A child with high muscle mass (such as an athlete) may have a high BMI but not have excess body fat. Conversely, a child with low muscle mass may have a "normal" BMI but high body fat.
Medical Disclaimer: This tool is for informational purposes only and does not constitute medical advice. Always consult with a pediatrician or qualified healthcare provider regarding your child's growth and health.
FAQ
Why is BMI different for kids than adults?
Adults have stable growth, so fixed thresholds (like 25 for overweight) apply. Children grow at different rates, and their body composition changes as they age, requiring age-specific and sex-specific comparisons.
What if my child is in the 90th percentile?
Being in the 90th percentile means your child's BMI is higher than 90% of children their age. This is classified as "Overweight" by WHO standards. It is a prompt to look at lifestyle habits rather than a definitive health diagnosis.
Does this calculator work for toddlers under 2?
No. For children under age 2, healthcare providers use weight-for-length charts rather than BMI-for-age.
Can my child be "Obese" if they are just tall?
BMI accounts for height, but since it doesn't distinguish between muscle and fat, very tall or very athletic children may sometimes receive misleading classifications. A doctor will usually look at the growth trend over time rather than a single data point.
How often should I check my child's BMI?
Usually, BMI is checked once a year during a well-child visit. Frequent checking at home is generally not recommended unless advised by a doctor.