BMI Calculator (Children and Adolescents)

Simple, fast and easy

Related links:

Adult BMI (older than 19 years)

Result

BMI: Fill in the fields
Z-Score: Fill in the fields
Classification: Fill in the fields

See also:

Classification Table for the provided data Why child BMI is different? Classification table What is the Z-Score How the calculation is done Fun facts about BMI

Result - BMI and Weight Ranges for the informed Age, Sex, and Height

Run the calculator to view the table.

≠Why is child BMI different from adult BMI?

Constant growth phase: Children and adolescents are in constant development — with rapid changes in height, weight, and body composition.

Classification based on age and sex: BMI in this phase is interpreted using WHO growth charts, which compare the data with reference standards for age and sex.

Use of Z-Score: Instead of fixed ranges, the Z-Score is used, which indicates how much a child's BMI deviates from the expected average for their age.

BMI in adults: At 20 years old, the body reaches physical maturity. Thus, BMI is interpreted with fixed ranges — such as 18.5 to 24.9 for "normal weight".

Child age range: 5 to 19 incomplete years

What is the Z-Score?

The Z-Score is a statistical measure that indicates how much a value deviates from the expected average for a given age and sex, according to WHO standards.

Why is it important in child BMI?

  • Children and adolescents are constantly growing, and BMI varies naturally with age and between boys and girls.
  • The Z-Score allows comparing a child's BMI with an internationally standardized reference.
  • It is the most accurate and WHO-accepted way to classify the nutritional status of young people between 5 and 19 years old.
  • With the Z-Score, it is possible to make a fairer and more accurate nutritional assessment, respecting the growth phases of each child and adolescent.

How to calculate the Z-Score

How to calculate BMI?

Before calculating the Z-Score, it is necessary to obtain the child's BMI with the traditional formula:

\[ BMI = \frac{Weight \, (kg)}{Height^2 \, (m^2)} \]

With this value, we look up the WHO table for the mean and standard deviation corresponding to the informed age and sex.

How is the Z-Score calculated?

The BMI Z-Score is calculated by comparing the child's BMI value with the mean and standard deviation (SD) of the reference population of the same age and sex. The formula is:

\[ Z = \frac{IMC_{observado} - IMC_{média}}{DP} \]

That is, we are measuring how many standard deviations the child's BMI value is above or below the expected average for their age and sex.

Classification Table using Z-Score

# Classification Z-Score BMI/Age
1 Severe thinness Z-Score ≤ -3
2 Thinness -3 ≤ Z-Score < -2
3 Adequate (eutrophy) -2 ≤ Z-Score ≤ +1
4 Overweight +1 < Z-Score ≤ +2
5 Obesity +2 < Z-Score ≤ +3
6 Severe obesity +3 < Z-Score

Fun Facts about BMI in Children and Adolescents

  • Child BMI is interpreted based on percentiles and growth charts.
  • The WHO uses specific age and sex curves to determine nutritional status.
  • Categories include: severe thinness, normal, overweight, and obesity.
  • Unlike adults, child BMI varies with growth and sex.
  • Boys and girls have different patterns of body development.
  • The same BMI can represent different classifications depending on age.
  • The WHO recommends regular assessments of child growth in pediatric consultations.
  • High BMI in children can be an early indicator of risk for future diseases.
  • Good eating habits and physical activity are essential in childhood.
  • Children may experience "growth spurts" where BMI naturally fluctuates.
  • Research indicates that childhood obesity can impact cognitive performance.
  • The use of tablets and cell phones for long periods is associated with increased BMI in adolescents.
  • According to the WHO, childhood overweight has drastically increased in recent decades.