How to calculate BMI?
BMI stands for Body Mass Index, which can be easily calculated with just body mass and height:
- Get your child's body mass and height.
- Use the BMI formula: BMI = body mass / height²
- Substitute the values and calculate.
Weight and height must be in SI units (kilograms and meters). You can switch between Metric (kg, cm) and Imperial/American (lbs, ft/in) units directly in the calculator.
What is BMI classification for children?
Child BMI charts differ from adult BMI classifications. For children aged 2–20, BMI is interpreted using age- and sex-specific CDC growth charts. The CDC defines four weight-status categories:
- Underweight — BMI below the 5th percentile
- Healthy weight — BMI at the 5th up to the 85th percentile
- Overweight — BMI at the 85th up to the 95th percentile
- Obese — BMI at the 95th percentile or above
This differs from adult norms because children's bodies change rapidly during growth. A BMI value of 18 may be perfectly healthy for a 6-year-old but underweight for a 16-year-old.
Why is the child's BMI chart useful?
The pediatric growth chart quickly alerts us to any signs of developmental disorders. Key reasons to use it:
- It accounts for age and sex, making it far more accurate than adult BMI for growing children.
- A sudden change in percentile lines (crossing two or more channels) can be an early warning sign — visit a doctor if this occurs.
- It helps track growth trends over time, not just a single snapshot.
- Athletic children may have higher BMI due to muscle mass — a doctor can interpret this in context.
The BMI percentile calculator in practice
This BMI percentile calculator covers children and teens aged 2 to 20 years. It uses the CDC LMS (Lambda-Mu-Sigma) method with Box-Cox transformation to convert your child's BMI into a percentile on the growth chart.
For example, a BMI at the 60th percentile means 60% of children of the same age and sex have a lower BMI — this child is in the "healthy weight" zone.
How to interpret BMI chart for teens/kids?
When reading the BMI percentile chart:
- < 5th percentile: The child may be underweight. Consult a healthcare provider about nutrition and potential underlying conditions.
- 5th – 84th percentile: Healthy weight. Continue regular check-ups and maintain balanced diet and physical activity.
- 85th – 94th percentile: Overweight. Consider dietary adjustments and increased physical activity. Talk to a pediatrician.
- ≥ 95th percentile: Obese. Medical evaluation is recommended to assess health risks and develop a management plan.
During puberty, teens can naturally gain weight quickly — especially girls. An athletic child's BMI might also appear elevated due to muscle mass. Always interpret results in consultation with a healthcare provider.
FAQs
- Can I use this for children under 2?
- No — for infants and toddlers under 2 years, WHO weight-for-length charts are recommended instead of BMI.
- Is BMI the same for boys and girls?
- No. The CDC maintains separate growth charts for boys and girls because their body composition changes at different rates, especially during puberty. Our calculator automatically applies the correct chart based on the selected sex.
- How often should I check my child's BMI?
- Most pediatricians recommend checking BMI annually during routine well-child visits. Tracking the trend over time is more informative than any single measurement.
- What if my child's percentile changed dramatically?
- A sudden crossing of two or more percentile channels is a clinical red flag. Consult your pediatrician promptly if you notice this on the growth chart.