Child BMI Calculator with WHO Percentiles
[BMI](/bmi-calculator) (Body Mass Index) in children is interpreted differently than adults—it's compared to age and sex-specific percentiles because children's bodies change dramatically during growth. This calculator computes your child's BMI and compares it against WHO growth standards for ages 2-18.
When to use this calculator
- Your pediatrician mentioned your child's BMI and you want to understand it.
- You want to know if your child is overweight according to WHO standards.
- You need to calculate BMI percentile for a health checkup.
- Your child's percentile has changed and you want to track it.
- You want to compare your child's growth against official WHO charts.
Example: 7-year-old boy, 25 kg, 122 cm
- BMI: 25 / (1.22)² = 16.8.
- Percentile: ~P50-P85 (normal).
- Classification: normal weight.
How it works
1 min readBMI in children vs adults
In adults, BMI has fixed values (<18.5 underweight, 18.5–25 normal, >25 overweight). In children, BMI changes with age due to growth, which is why age and sex-specific percentiles are used instead.
Classification by BMI percentile
| Percentile | Classification |
|---|---|
| < 5 | Underweight |
| 5–84 | Normal weight |
| 85–96 | Overweight |
| ≥ 97 | Obese |
BMI formula
BMI = weight (kg) / height (m)²Childhood overweight and obesity in Argentina
Argentina has a prevalence of childhood overweight/obesity of approximately 41% (ENNYS 2018–2019), one of the highest rates in Latin America.
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Frequently asked questions
What is a healthy BMI for a child?
There's no single 'ideal' BMI number for children—it depends on age and sex. A healthy BMI falls between the 5th and 85th percentile on WHO growth charts. Ask your pediatrician what's healthy for your child's age.
How do you calculate BMI for children?
BMI is calculated the same way for kids as adults: weight (kg) ÷ height (m)². The difference is that the result is compared to age and sex-specific percentile charts instead of fixed adult ranges.
What BMI percentile is considered overweight for kids?
BMI between the 85th–96th percentile is overweight. The 97th percentile or higher is obese. Less than the 5th percentile is underweight. These percentiles are age and sex-adjusted.
Why do doctors use percentiles instead of fixed BMI numbers for children?
Children's bodies change dramatically during growth. A BMI of 20 might be healthy for a 10-year-old boy but underweight for a 16-year-old girl. Percentiles account for these normal changes, making them a much better tool.
Can a child's high BMI be from muscle instead of fat?
Possibly. Very active, athletic kids may have high BMI from muscle mass. However, in most children with elevated BMI, the extra weight is fat. Your pediatrician can assess body composition beyond BMI.
What should I do if my child is overweight?
Don't put kids on 'diets'—they need nutrition to grow. Focus on: more physical activity, fewer processed foods and sugary drinks, appropriate portion sizes, and family-wide healthy habits. Always consult your pediatrician or a registered dietitian.
When should I worry about my child's weight?
If your child's BMI percentile is consistently above the 85th percentile, or if they've had a significant increase in percentile over time, discuss it with your pediatrician. They can determine if action is needed.
Does BMI work the same way for teenagers?
Yes—BMI percentiles based on age and sex still apply to teenagers up to age 19. However, teenage years bring significant body changes, so there's naturally more variation. The WHO growth standards account for this.
Are WHO growth charts accurate for all children?
WHO growth charts are population averages and work well for screening. However, genetic factors mean some children naturally run leaner or heavier. Always consider growth trends and family patterns alongside BMI.
At what age should I start tracking my child's BMI?
BMI is reliable from about age 2 onward. Before age 2, pediatricians use different growth metrics (length, weight gain patterns). The calculator uses WHO standards for ages 2–18.