Child BMI Percentile Calculator
Based on CDC Growth Charts for Ages 2–20 — Age & Sex-Specific Percentiles
Understanding Child BMI
Why BMI-for-Age Differs from Adult BMI
In adults, BMI is interpreted using fixed categories (e.g., a BMI of 25 or above is considered overweight regardless of age or sex). For children and adolescents aged 2 to 20, however, the same BMI value can mean very different things depending on the child's age and sex. This is because body composition changes dramatically during growth and development. A 7-year-old boy and a 15-year-old girl with the same BMI may fall into entirely different weight categories.
Growth Patterns During Childhood
During childhood, body fatness changes substantially. BMI typically decreases during the preschool years, reaching its lowest point around ages 5 to 6 (a phenomenon called the adiposity rebound), and then increases through adolescence and into adulthood. Girls and boys follow different growth trajectories, with girls generally accumulating more body fat during puberty while boys tend to gain more lean muscle mass. These natural variations mean that the same BMI number has different implications at different ages and for each sex.
Why Percentiles Matter
Because of these age and sex differences, child BMI is expressed as a percentile rather than a raw number. A percentile indicates where a child's BMI falls relative to other children of the same age and sex in the CDC reference population. For example, a BMI at the 75th percentile means the child's BMI is higher than 75% of children of the same age and sex. Percentiles allow healthcare providers to track a child's growth trajectory over time and identify potential concerns early, long before they become serious health issues.
Growth Charts Explained
How CDC Charts Work
The CDC growth charts were developed in 2000 using data from national health examination surveys conducted between 1963 and 1994. They represent the distribution of BMI values across a large, nationally representative sample of U.S. children. The charts include smoothed percentile curves (3rd, 5th, 10th, 25th, 50th, 75th, 85th, 90th, 95th, and 97th) that allow clinicians to plot a child's BMI and visually assess where it falls within the population distribution.
The LMS Method Simplified
The CDC charts use a statistical technique called the LMS method to create smooth percentile curves. LMS stands for three parameters that describe the BMI distribution at each age:
- L (Lambda): The Box-Cox transformation power that accounts for the skewness (asymmetry) of the BMI distribution. At most ages, the BMI distribution is right-skewed, meaning there are more children with BMIs above the median than below it.
- M (Mu): The median BMI value for children of a given age and sex. This represents the "typical" BMI for that age group.
- S (Sigma): The coefficient of variation, which captures the spread (variability) of BMI values around the median.
Using these three values, any BMI measurement can be converted to a z-score (standard deviation score) and then to a percentile. This approach ensures that the percentile calculations account for the fact that the BMI distribution is not perfectly symmetrical at any given age.
Why Age and Sex Matter
Boys and girls have different body composition patterns, especially during puberty. Girls typically begin puberty earlier and accumulate a higher percentage of body fat, while boys tend to develop more lean muscle mass. The CDC provides separate LMS reference tables for boys and girls, and the parameters change at every age point. This is why a BMI of 20 in a 10-year-old boy might be at the 85th percentile, while the same BMI of 20 in a 16-year-old boy might be at the 50th percentile. Always interpret BMI in context of the child's specific age and sex.
Healthy Habits for Kids
Nutrition Guidelines
A balanced diet is the foundation of healthy growth. The Dietary Guidelines for Americans recommend that children eat a variety of fruits, vegetables, whole grains, lean proteins, and low-fat dairy products. Key nutrition tips include:
- Offer fruits and vegetables at every meal and as snacks
- Choose whole grains over refined grains (whole wheat bread, brown rice, oatmeal)
- Limit added sugars to less than 10% of daily calories
- Limit sugar-sweetened beverages; encourage water and milk
- Involve children in meal planning and preparation
- Avoid using food as a reward or punishment
Physical Activity by Age
The CDC and American Academy of Pediatrics recommend the following physical activity levels:
- Ages 2–5: Active play throughout the day; aim for at least 3 hours of activity of all intensities per day
- Ages 6–17: At least 60 minutes of moderate-to-vigorous physical activity every day, including aerobic activity, muscle-strengthening exercises, and bone-strengthening activities at least 3 days per week
Screen Time Limits
- Ages 2–5: Limit screen time to 1 hour per day of high-quality programming
- Ages 6 and older: Place consistent limits on screen time; ensure it does not interfere with sleep, physical activity, and other healthy behaviors
- Establish screen-free zones (e.g., bedrooms, dinner table) and screen-free times (e.g., 1 hour before bed)
Sleep Needs by Age
- Ages 1–2: 11–14 hours (including naps)
- Ages 3–5: 10–13 hours (including naps)
- Ages 6–12: 9–12 hours
- Ages 13–18: 8–10 hours
Adequate sleep is essential for healthy growth, metabolism, and weight regulation. Children who consistently get insufficient sleep have a higher risk of obesity.
When to Talk to Your Pediatrician
Rapid Weight Changes
If your child's BMI percentile has shifted significantly over a short period — for example, moving from the 50th percentile to the 85th percentile in one year, or dropping from the 40th to the 10th percentile — this warrants a conversation with your pediatrician. Rapid changes may indicate underlying medical conditions, dietary issues, or psychosocial factors that need to be addressed. Single BMI measurements are less informative than trends over time.
BMI Tracking Over Time
Pediatricians track your child's BMI at each well-child visit and plot it on a growth chart to monitor the trajectory. A child who has been consistently at the 60th percentile and remains there is likely growing normally, even though 60th percentile is above average. What matters most is the pattern. A child whose percentile is steadily increasing (known as upward crossing of percentile lines) may be at risk of developing overweight or obesity. Similarly, a downward trend could signal nutritional deficiencies or other health concerns.
Eating Disorders Awareness
Weight concerns in children and adolescents must be approached with sensitivity. Overemphasis on weight and dieting can contribute to disordered eating behaviors, negative body image, and clinically diagnosable eating disorders such as anorexia nervosa, bulimia nervosa, or binge eating disorder. Signs to watch for include:
- Preoccupation with food, calories, or body size
- Skipping meals, restricting food groups, or secret eating
- Excessive exercise beyond what is age-appropriate
- Rapid or unexplained weight loss
- Social withdrawal or mood changes related to eating
If you notice any of these signs, consult your pediatrician promptly. The focus should always be on health and healthy behaviors rather than a specific number on the scale.
Frequently Asked Questions
BMI percentile for children is a measure that compares your child's BMI (body mass index) to other children of the same age and sex. Because children's body composition changes as they grow, a raw BMI number alone is not meaningful. Instead, the BMI is compared against the CDC growth reference data to determine a percentile ranking. For example, if your child is at the 70th percentile, it means their BMI is higher than 70% of children of the same age and sex in the reference population. Percentiles are used for children and teens aged 2 to 20 years.
The CDC defines four weight status categories for children and adolescents based on BMI-for-age percentiles: Underweight is below the 5th percentile. Healthy Weight is from the 5th percentile to less than the 85th percentile. Overweight is from the 85th percentile to less than the 95th percentile. Obese is at or above the 95th percentile. These categories help identify children who may be at risk for weight-related health problems, but they should always be interpreted by a healthcare provider in the context of the child's overall health, growth pattern, and family history.
BMI is a useful screening tool but is not a direct measure of body fat. It does not distinguish between fat mass and lean mass (muscle, bone). Some children, particularly those who are very muscular or athletic, may have a high BMI without excess body fat. Conversely, a child with a normal BMI could still have an unhealthy body composition. For these reasons, the AAP recommends that BMI percentile be used as one component of a comprehensive health assessment that includes physical examination, dietary evaluation, physical activity assessment, and family health history. If there are concerns, your pediatrician may order additional assessments such as skinfold thickness measurements or blood tests.
The American Academy of Pediatrics recommends that BMI be calculated and plotted at every annual well-child visit beginning at age 2. More frequent monitoring may be recommended if your child's BMI is above the 85th percentile, if there has been a significant change in BMI percentile between visits, or if there are other risk factors such as family history of obesity or type 2 diabetes. Tracking BMI over time is more valuable than any single measurement, as it reveals growth trends and allows early intervention when needed.
The CDC BMI-for-age growth charts are designed for children and adolescents aged 2 through 20 years. For children under age 2, the World Health Organization (WHO) growth standards are used instead, which rely on weight-for-length rather than BMI. After age 20, adult BMI categories with fixed thresholds (18.5, 25, 30) are used because growth is complete and body composition is relatively stable. The age range of 2 to 20 represents the period during which age- and sex-specific growth references are necessary due to ongoing development.
If your child's BMI is at or above the 85th percentile, the most important step is to discuss the results with your child's pediatrician. Avoid putting your child on a restrictive diet without medical guidance, as this can interfere with growth and development and may contribute to disordered eating. Instead, focus on family-wide healthy lifestyle changes: increase fruits and vegetables, reduce sugary beverages, encourage daily physical activity, limit screen time, and ensure adequate sleep. Your pediatrician may recommend additional evaluation or refer you to a pediatric dietitian or specialist if needed. The goal is improved health behaviors, not rapid weight loss.
Medical Disclaimer
This Child BMI Percentile Calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. BMI is a screening tool and is not diagnostic of body fatness or health. This calculator uses an approximation of CDC growth chart data and may differ slightly from clinical calculations. Always consult your child's pediatrician or qualified healthcare provider for a comprehensive assessment of your child's growth and nutritional status. All calculations are performed in your browser — no personal data is stored on our servers or shared with any third party.
References
- Centers for Disease Control and Prevention. CDC Growth Charts: United States. National Center for Health Statistics. 2000. Available at: cdc.gov/growthcharts.
- Kuczmarski RJ, Ogden CL, Guo SS, et al. 2000 CDC Growth Charts for the United States: Methods and Development. Vital Health Stat. 2002;11(246):1-190.
- Hampl SE, Hassink SG, Skinner AC, et al. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity. Pediatrics. 2023;151(2):e2022060640.
- Barlow SE; Expert Committee. Expert Committee Recommendations Regarding the Prevention, Assessment, and Treatment of Child and Adolescent Overweight and Obesity: Summary Report. Pediatrics. 2007;120(Suppl 4):S164-S192.