Child BMI 2026: How to Calculate and Interpret with the WHO Percentile Table
Child BMI is calculated with the same mathematical formula as adult BMI (weight ÷ height²), but its interpretation is completely different. For children and adolescents aged 2 to 19 years, the BMI result is not evaluated using fixed ranges — it is compared with population growth curves organized by percentiles, specific for each age group and sex.
This means that the same BMI value may be "normal" for a 5-year-old child but indicate overweight in a 10-year-old. Understanding this mechanism is essential for parents, caregivers and health professionals.
Use our Child BMI Calculator to get the correct percentile and classification according to the WHO curves.
Why is Child BMI Different from Adult BMI?
During childhood and adolescence, body composition changes dynamically and non-linearly:
- Infants and young children (0-5 years): Very accelerated and variable growth; body composition changes quickly
- Pre-pubertal (6-9 years): More stable growth, with the beginning of pre-pubertal fat deposition
- Adolescence (10-19 years): Intense pubertal development with hormonal changes that affect weight, height and body composition differently in boys and girls
Therefore, the WHO and CDC (Centers for Disease Control) tables establish growth curves that show the distribution of BMI in healthy reference populations. A child's BMI is compared to this distribution to determine what "percentile" they fall into.
The Child BMI Formula (Same as Adult)
BMI = Weight (kg) ÷ [Height (m)]²
Example: 8 year old boy, 30 kg, 1.30 m:
- BMI = 30 ÷ (1.30 × 1.30) = 30 ÷ 1.69 = 17.75 kg/m²
But knowing that the BMI is 17.75 doesn't say anything without consulting the percentile table by age and sex.
What are Percentiles and How to Interpret them?
The percentile indicates where the child is relative to a reference population of healthy children of the same age and sex.
- 50th percentile = median value: half of healthy children are above, half are below
- 85th percentile = the child has a BMI higher than 85% of children of the same age and sex
- 5th percentile = the child has a BMI lower than 95% of children of the same age and sex
Child BMI Classification Table (WHO/CDC)
| Percentile | Classification |
|---|---|
| Below P3 | Severe thinness |
| P3 to P15 | Thinness |
| P15 to P85 | Adequate (normal) weight |
| P85 to P97 | Overweight |
| Above P97 | Obesity |
BMI by Age Tables: Reference Values (WHO)
Boys — BMI at 50th Percentile and Cutoff Percentiles by Age
| Age | P3 (thinness) | P15 (lower limit) | P50 (median) | P85 (overweight) | P97 (obesity) |
|---|---|---|---|---|---|
| 2 years | 13.4 | 14.0 | 15.4 | 16.9 | 18.6 |
| 4 years | 12.8 | 13.5 | 14.9 | 16.4 | 18.2 |
| 6 years | 12.4 | 13.1 | 14.7 | 16.6 | 19.2 |
| 8 years | 12.4 | 13.2 | 15.1 | 17.7 | 21.3 |
| 10 years | 12.8 | 13.8 | 16.0 | 19.2 | 23.2 |
| 12 years | 13.6 | 14.8 | 17.3 | 20.9 | 25.3 |
| 14 years | 14.8 | 16.1 | 18.9 | 22.8 | 27.4 |
| 16 years | 16.1 | 17.5 | 20.3 | 24.4 | 29.2 |
| 18 years old | 17.3 | 18.8 | 21.5 | 25.8 | 30.7 |
Girls — BMI at 50th Percentile and Cutoff Percentiles by Age
| Age | P3 (thinness) | P15 (lower limit) | P50 (median) | P85 (overweight) | P97 (obesity) |
|---|---|---|---|---|---|
| 2 years | 13.1 | 13.8 | 15.2 | 16.8 | 18.7 |
| 4 years | 12.6 | 13.3 | 14.7 | 16.5 | 18.7 |
| 6 years | 12.2 | 13.0 | 14.7 | 16.8 | 19.7 |
| 8 years | 12.3 | 13.2 | 15.2 | 18.0 | 22.0 |
| 10 years | 12.8 | 13.9 | 16.4 | 19.9 | 24.6 |
| 12 years | 13.7 | 15.0 | 18.0 | 22.1 | 27.4 |
| 14 years | 15.0 | 16.5 | 19.7 | 24.1 | 29.5 |
| 16 years | 16.1 | 17.6 | 20.8 | 25.3 | 30.8 |
| 18 years old | 16.7 | 18.3 | 21.5 | 26.0 | 31.9 |
Practical Examples of Interpretation
Example 1: 10 year old girl, 42 kg, 1.40 m
- BMI = 42 ÷ (1.40)² = 42 ÷ 1.96 = 21.4 kg/m²
- Comparing with table of 10 year old girls:
- P85 = 19.9 | P97 = 24.6
- BMI of 21.4 is between P85 and P97
- Classification: Overweight (nutritional monitoring recommended)
Example 2: 8 year old boy, 28 kg, 1.30 m
- BMI = 28 ÷ (1.30)² = 28 ÷ 1.69 = 16.6 kg/m²
- Comparing with table of 8 year old boys:
- P50 = 15.1 | P85 = 17.7
- BMI of 16.6 is between P50 and P85
- Classification: Appropriate weight ✅
Example 3: 6 year old girl, 18 kg, 1.14 m
- BMI = 18 ÷ (1.14)² = 18 ÷ 1.2996 = 13.85 kg/m²
- Comparing with table of 6 year old girls:
- P3 = 12.2 | P15 = 13.0 | P50 = 14.7
- BMI of 13.85 is between P15 and P50
- Classification: Adequate weight (tendency towards thinness) — monitor
Childhood Obesity in Brazil: A Public Health Problem
According to data from IBGE and the Ministry of Health, childhood obesity has grown alarmingly in Brazil:
- 1 in 3 Brazilian children aged 5 to 9 are overweight or obese
- The percentage of children with obesity has tripled in the last 3 decades
- Obese teenagers have an 80% chance of becoming obese adults
Consequences of Childhood Obesity
- Type 2 prediabetes and insulin resistance in children
- High blood pressure in childhood
- Orthopedic problems (knees, spine)
- Sleep apnea
- Impact on self-esteem and mental health
- Precocious puberty
What to do if the BMI indicates overweight or thinness?
If your child is overweight or obese:
- Consult your pediatrician: The diagnosis must be confirmed with a complete clinical evaluation
- Nutritional assessment: Referral to a pediatric nutritionist
- Gradual dietary changes: Reduce ultra-processed foods, increase fruits, vegetables and lean proteins
- Physical activity: At least 60 minutes of moderate to vigorous activity per day (WHO)
- Limit screen time: Maximum 1h/day for children aged 3-6, 2h/day for older children
If the Child is Underweight:
- Urgent medical consultation: Severe thinness (below P3) requires immediate evaluation
- Evaluate causes: Absorption problems, parasites, chronic diseases or eating disorders
- Supervised calorie increase: With specific nutritional guidance for the cause
Differences Between the WHO and CDC Tables
There are two reference tables widely used in Brazil:
| Table | Recommended For | Focus |
|---|---|---|
| WHO (WHO) | Children aged 0 to 5 years and 5 to 19 years | Global international standard; includes breastfed children |
| CDC (USA) | Children aged 2 to 20 | Most used in North America |
The Brazilian Ministry of Health recommends the WHO table as the primary reference for Brazilian children. The WHO uses a sample of healthy children from different countries and ideal growth conditions.
When is Child BMI Not Enough?
Just like adult BMI, childhood BMI has limitations:
- Does not differentiate between muscle and fat: Very active children with high muscle mass can have a high BMI without excess fat
- Pubertal maturation: During puberty, girls and boys accumulate fat in different ways. BMI does not capture these nuances with complete accuracy
- Ethnic origin: Children from different ethnic origins have naturally different body composition
Therefore, the pediatrician may additionally request the skinfold (measurement of subcutaneous fat) or bioimpedance for a more accurate assessment of body composition.
Frequently Asked Questions (FAQ)
1. From what age can I use BMI to assess a child's weight? The WHO recommends using BMI as a screening tool from 2 years of age. For babies under 2 years of age, the Weight-for-Height (W/A) index and the Weight-for-Age (W/A) index are used in the WHO growth curves.
2. My son has a BMI of 17, is this normal for his age? It depends on age and sex. A BMI of 17 may be normal (adequate weight) for a 6-year-old boy (P85 = 16.6) but indicate overweight for a 4-year-old girl (P50 = 14.7, P97 = 18.7). Always compare with the percentile tables by age and sex, or use our calculator.
3. Is childhood BMI used to diagnose obesity? Pediatric BMI is a tracking tool (triage), not a definitive diagnosis. A BMI above P97 statistically indicates obesity, but the medical diagnosis of obesity also considers family history, laboratory tests and complete clinical evaluation.
4. How often should I calculate my child's BMI? In well-child visits (routine consultations with the pediatrician), the doctor monitors weight, height and BMI regularly — at least once a year for older children and more frequently in the first years of life. Parents don't need to calculate daily at home.
5. Does a child with a high BMI but without the appearance of overweight need attention? Yes. Children with high visceral adiposity (internal fat) may not appear overweight externally, but still present metabolic risks. BMI is just a screening — the pediatrician evaluates the complete picture.
6. What is "juvenile adiposity" and how does it affect BMI? Juvenile adiposity is a natural physiological increase in BMI that occurs between the ages of 5 and 7 in many children, called "adiposity rebound". Children who experience this rebound before the age of 5 have a greater risk of obesity in adulthood. The pediatrician monitors this development on the growth curves.
7. Can I use the adult BMI chart to assess my 17 year old teenager? No. The WHO tables for adolescents go up to age 19 and should be used for the entire 10 to 19 age range. Using adult charts for adolescents underestimates the risk of overweight and is not recommended by pediatric guidelines.
8. Is high childhood BMI related to parental weight? Yes, there is a genetic component. Children of obese parents have a greater genetic predisposition to obesity. However, the family environment (eating habits, sedentary lifestyle, sleep) has an equal or greater impact than genetics on the development of childhood obesity. Both are modifiable with early intervention.
Calculate Your Child's BMI Now
Our calculator uses WHO growth curves to provide the correct percentile and classification for each child, considering age and sex.
Access the Child BMI Calculator — result with percentile, classification and specific guidelines by age group.
Related calculators:
- BMI Calculator (Adult) — BMI for people over 18
- BMI Calculator for Seniors — BMI adjusted to 65+
- Calorie Calculator (TDEE) — daily calorie need by age and activity