BMI Calculator for Kids
BMI for children and teens is interpreted differently than for adults. This calculator computes BMI and maps it to an approximate CDC percentile based on age and sex, providing a weight status category.
BMI in children and adolescents (ages 2-19) is interpreted differently than in adults because body composition naturally changes with growth and development. Children's body fat varies substantially by age and sex throughout development — boys and girls have different patterns, and the proportion of body fat changes meaningfully each year during growth. Using fixed adult BMI categories (under 25 = healthy, etc.) doesn't work for kids.
Instead, pediatric BMI uses age- and sex-specific percentiles based on the CDC growth charts (which combine data from multiple national health surveys). A child's BMI is plotted on growth charts to determine where they fall compared to other children of the same age and sex. A BMI at the 50th percentile means the child has a higher BMI than 50% of children that age and sex; the 85th percentile means higher than 85% of same-age peers.
This calculator computes BMI from height and weight, then provides an approximate percentile-based weight status category. For precise CDC growth chart percentile placement (which uses smoothed reference curves), use the CDC's online tools or your pediatrician's growth chart tracking. The calculator is a useful screening tool but doesn't replace pediatric medical assessment, which includes growth trajectory over time, family history, and developmental factors.
Inputs
Results
BMI
16.9
Percentile
48th
Category
Healthy Weight
Healthy BMI Range
15.3 - 19.8
Weight Status Categories
Formula
How to use this calculator
- Enter child's age in years (decimal allowed for half-years).
- Select sex (male or female — growth patterns differ).
- Enter height in inches.
- Enter weight in pounds.
- Review BMI value and weight status category.
- For more precise placement: use the CDC's online BMI percentile calculator (cdc.gov) or have pediatrician plot on official growth chart.
- For tracking: pediatricians track BMI percentile over time. Maintaining a consistent percentile range as the child grows is the key healthy pattern. Crossing major percentile lines (e.g., jumping from 50th to 85th) warrants pediatric assessment.
- For overweight/obese children: focus is on slowing weight gain while continuing growth in height, rather than aggressive weight loss. Pediatricians recommend gradual lifestyle changes — improved nutrition, increased activity, reduced screen time, family-based approach.
Worked examples
Healthy-weight school-age child
Age 8, female, 52" tall, 60 lbs. BMI = (60 / 52²) × 703 = 15.6 Percentile: approximately 50th-60th Category: Healthy weight On-track growth. Pediatricians plot multiple data points over time — consistent percentile range as the child grows is the healthy pattern.
Overweight teen
Age 14, male, 64" tall, 165 lbs. BMI = (165 / 64²) × 703 = 28.3 Percentile: approximately 95th+ (obese category) Substantially above healthy range. Pediatric assessment appropriate. Approach: family-based lifestyle changes (not aggressive dieting). Focus on improving nutrition quality, increasing physical activity to 60+ min/day, reducing screen time, and addressing any underlying medical contributors. Pediatricians typically recommend slowing weight gain rather than aggressive loss during growth years.
Underweight concern
Age 6, female, 44" tall, 32 lbs. BMI = (32 / 44²) × 703 = 11.6 Percentile: approximately 3rd-5th Category: Underweight or borderline Below typical range. Could be due to natural variation, recent illness, family genetics (small parents), inadequate nutrition, or underlying medical issues. Pediatric evaluation recommended — including assessment of growth trajectory (has the child always been smaller, or recently lost weight?), eating patterns, and any concerning symptoms. Many small children are simply on a smaller growth trajectory consistent with family genetics. Others have medical conditions worth identifying and treating.
When to use this calculator
Use this calculator at routine intervals for tracking child's growth pattern, or when a parent or pediatrician identifies a need to assess weight status. Pediatric BMI screening is part of standard well-child visits.
For accurate clinical interpretation, use the CDC's official online calculator (which uses precise growth chart curves) or have a pediatrician evaluate. This calculator provides a useful approximation but isn't a clinical substitute for proper pediatric growth assessment.
Pair with: BMI calculator (for adults — note that adult interpretation begins around age 20), calorie calculator (for understanding pediatric calorie needs), and general health-tracking tools.
A few important pediatric weight considerations:
1. **Track trajectory, not single points.** Pediatricians evaluate growth charts over time. Consistent percentile range as the child grows is the healthy pattern. Crossing major percentile lines (jumping from 50th to 85th) warrants assessment, even if absolute BMI is still in "healthy" range.
2. **Slower-is-better for overweight.** For overweight/obese children, the goal is typically to slow weight gain while continuing height growth, rather than aggressive weight loss. Healthy growth requires adequate nutrition; aggressive dieting can stunt growth and create disordered eating patterns.
3. **Family-based approach.** Pediatric weight management research strongly supports family-based interventions — when the whole family adopts healthier eating and activity patterns together, children's outcomes are dramatically better than individual-focused interventions.
4. **Avoid weight-focused language.** Talking to children about weight or body size can contribute to disordered eating. Focus on healthy behaviors (vegetables, water, activity, sleep) rather than weight or appearance.
5. **Screen time matters.** Pediatric obesity research consistently identifies high screen time as a major risk factor — both for sedentary time and for snacking patterns. AAP recommends <2 hours/day of recreational screen time for children 6+.
6. **Underweight requires assessment too.** Below 5th percentile warrants pediatric evaluation — could be benign (small family, normal variation) or could indicate underlying medical issues, inadequate nutrition, or growth disorders.
Common mistakes to avoid
- Applying adult BMI categories to children. Children's healthy BMI ranges vary substantially by age and sex. Use percentile-based interpretation.
- Aggressive weight loss programs for children. Pediatric overweight management focuses on slowing weight gain during height growth, not absolute weight loss. Aggressive dieting can stunt growth.
- Talking to children about weight. Weight-focused language contributes to disordered eating patterns. Focus on healthy behaviors instead.
- Comparing to friends. Children grow at different rates. Individual percentile tracking over time matters more than peer comparison.
- Ignoring percentile changes within "healthy" range. A child crossing from 50th to 85th percentile (still "healthy weight") may indicate trajectory shift worth pediatric assessment.
- Forgetting family-based approach. Family habit changes produce dramatically better child outcomes than individual-focused interventions.
Frequently Asked Questions
Sources & further reading
- BMI Calculator for Child and Teen — U.S. Centers for Disease Control and Prevention
- About Child & Teen BMI — U.S. Centers for Disease Control and Prevention
- Childhood Obesity — Clinical Guidance — American Academy of Pediatrics