BMI for Children
Unlike adults, children's body composition changes significantly as they grow. A BMI of 22 might be perfectly healthy for a 15-year-old but overweight for a 7-year-old. That's why pediatric BMI assessment uses age- and sex-specific percentiles rather than fixed cutoff values.
The BMI formula is the same as for adults (weight ÷ height²), but the result is interpreted by comparing it to reference data from the CDC growth charts, which are based on surveys of children's growth patterns.
Understanding Percentiles
| Percentile Range | Weight Status | What It Means |
|---|---|---|
| Below 5th percentile | Underweight | BMI is lower than 95% of same-age, same-sex peers |
| 5th to <85th percentile | Healthy Weight | BMI is in the normal range for the child's age and sex |
| 85th to <95th percentile | Overweight | BMI is higher than 85% but lower than 95% of peers |
| 95th percentile or above | Obese | BMI is higher than 95% of same-age, same-sex peers |
BMI-for-Age Percentile Chart
Differences Between Boys and Girls
Boys and girls have different body compositions and growth patterns, which is why separate CDC growth charts exist for each sex:
- Boys tend to have slightly higher lean body mass and lower body fat percentage
- Girls begin puberty earlier on average and accumulate more body fat during adolescence
- A BMI that places a boy at the 60th percentile might place a same-age girl at the 55th or 65th percentile
What to Do About Results
If Underweight (<5th percentile)
- Rule out medical causes (celiac disease, thyroid disorders)
- Ensure adequate caloric and nutrient intake
- Monitor growth trajectory over time
If Overweight (85th–94th percentile)
- Focus on healthy eating habits, not restrictive dieting
- Encourage 60+ minutes of physical activity daily
- Limit screen time to <2 hours/day
- Goal: slow weight gain to allow height to "catch up"
If Obese (≥95th percentile)
- Consult pediatrician for comprehensive evaluation
- Screen for comorbidities (type 2 diabetes, dyslipidemia, hypertension)
- Family-based lifestyle intervention programs
- Never put a growing child on a weight-loss diet without medical supervision
Limitations
- BMI does not directly measure body fat in children any more than in adults
- Muscular children may have higher BMI without excess fat
- CDC charts are based on US data from the 1960s–1990s; may not perfectly reflect all ethnic groups
- WHO growth standards are preferred for children under 2 years
- A single BMI measurement is less informative than tracking trends over time
Frequently Asked Questions
What age range does this calculator cover?
This calculator is designed for children and adolescents aged 2–19 years. For children under 2, weight-for-length charts (WHO standards) are used instead of BMI. For adults (20+), use our standard BMI calculator.
My child is very athletic. Can BMI be misleading?
Yes. Muscular children (especially teen athletes) may have a higher BMI due to muscle mass rather than excess fat. In such cases, additional assessments like skinfold measurements or waist circumference can provide a more complete picture.
How often should I check my child's BMI?
The AAP recommends annual BMI screening starting at age 2. Your pediatrician will plot BMI on the growth chart at well-child visits. More frequent monitoring may be recommended if there are concerns about growth patterns.