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

Child & Teen BMI Calculator – Online and Free

Want to calculate the BMI of children and teens quickly and reliably? Fastools offers a free online child BMI calculator based on WHO charts, providing accurate results, detailed explanations, and monitoring healthy development directly in your browser, with no registration or downloads required.

Simply enter the child's or teen's age, gender, weight, and height, click 'Calculate', and see the result instantly with clear guidance.

Our child and teen BMI tool is ideal for parents, teachers, and health professionals who want to monitor growth in a practical and reliable way.

How to use the Child BMI Calculator?

  • 1️⃣ Enter the child's or teen's age, gender, weight, and height.
  • 2️⃣ Click 'Calculate' to get the BMI instantly.
  • 3️⃣ Check the BMI classification according to WHO charts.

Why use the Fastools calculator?

  • ✅ Completely free and online
  • ✅ Fast and accurate results
  • ✅ Based on official WHO charts
  • ✅ Ideal for parents, teachers, and health professionals
  • ✅ Works directly in your browser, no downloads or registration needed

Benefits of using our Child BMI Calculator

  • 📚 Track the growth and development of children and teens
  • ⚡ Get instant results without hassle
  • 🎯 Helps identify healthy weight patterns or obesity risk
  • 💻 Access from any device: computer, tablet, or smartphone

Can I use the calculator for school or medical monitoring?

Yes! The tool is perfect for monitoring the development of children and teens at home, at school, or in pediatric clinics.

Start calculating children’s and teens’ BMI online, free, and reliably now with Fastools!

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.