Need to calculate BMI for children or adolescents quickly and reliably?
Fastools provides a free online pediatric BMI calculator
based on WHO growth charts, delivering accurate results, detailed explanations,
and support for healthy development monitoring—directly in your browser, with no registration or downloads.
Simply enter the child’s or adolescent’s age, sex, weight, and height, click ‘Calculate’,
and instantly view the result with clear guidance.
Our child and adolescent BMI tool is ideal for parents, teachers,
and healthcare professionals who want to monitor growth in a practical and reliable way.
How to use the pediatric BMI calculator?
1️⃣ Enter the child’s or adolescent’s age, sex, weight, and height.
2️⃣ Click ‘Calculate’ to get the BMI instantly.
3️⃣ Review the BMI classification according to WHO reference tables.
Why use the Fastools calculator?
✅ Completely free and online
✅ Fast and accurate results
✅ Based on official WHO reference tables
✅ Ideal for parents, teachers, and health professionals
✅ Works directly in your browser—no downloads or sign-up required
Benefits of using our pediatric BMI calculator
📚 Track growth and development in children and adolescents
⚡ Get instant results—no complications
🎯 Helps identify healthy weight patterns or obesity risk
💻 Access from any device: computer, tablet, or smartphone
Can I use this calculator for school or clinical monitoring?
Yes! This tool is perfect for monitoring child and adolescent development
at home, in schools, or in pediatric clinics.
Start calculating BMI for children and adolescents online, free and reliably,
with Fastools today!
What is BMI (Body Mass Index)?
BMI—Body Mass Index is an indicator used to assess the relationship between
a person’s weight and height. It is widely used by healthcare
professionals as an initial screening tool to identify potential nutritional risks.
In children and adolescents, BMI is not interpreted in isolation, as it is in adults.
This is because the body is constantly growing and undergoes natural changes with age and sex.
Therefore, pediatric BMI is analyzed using
World Health Organization (WHO) growth charts.
When BMI is combined with age and sex, it allows classification of the child’s nutritional status,
supporting healthy growth monitoring and early detection of conditions such as
underweight, overweight, or obesity.
Interpretation of BMI and Weight Values by Nutritional Classification
The result table presents estimated values for weight (in kg) and
Body Mass Index (BMI), according to different nutritional
classification ranges for children and adolescents aged 5 to 19 years,
based on the provided age, sex, and height.
Values outside the displayed ranges may indicate critical stages of
malnutrition or severe obesity. Professional clinical assessment is recommended.
Each column represents a nutritional status classification,
ranging from severe thinness to severe obesity,
based on the WHO Z-score. Intermediate ranges represent the
healthy (eutrophic) interval.
Weight: the child’s estimated weight in each range, based on the given height.
BMI: index value corresponding to the nutritional classification.
How to interpret:
Compare the child’s/adolescent’s current BMI or weight with the values
in the table to identify whether they fall under underweight, healthy weight,
overweight, or obesity. This analysis supports nutritional
assessment and healthy development monitoring.
≠ Why is pediatric BMI different from adult BMI?
Continuous growth phase: Children and adolescents are in
constant development—with rapid changes in height, weight, and
body composition.
Age- and sex-based classification: BMI during this stage is
interpreted using WHO growth charts, which compare individual data
against reference standards for age and sex.
Z-score usage: Instead of fixed ranges, the Z-score is used,
indicating how much a child’s BMI deviates from the expected average for their age.
The same BMI value may be considered “healthy” for one age, but “overweight” or
“underweight” for another.
BMI in adults: By age 20, the body reaches physical maturity.
Thus, BMI is interpreted using fixed ranges—e.g., 18.5 to 24.9 for
“normal weight”.
The pediatric age range covers children and adolescents from 5 to under 20 years.
What is the Z-score?
The Z-score is a statistical measure indicating
how many standard deviations a value deviates from the mean
expected for a given age and sex, according to standards from the
WHO.
Why is it important for pediatric BMI?
Children and adolescents are constantly growing, and
BMI naturally varies with age and between boys and girls.
The Z-score allows comparing a child’s BMI against an
internationally standardized reference.
It is the most accurate method endorsed by the WHO for
classifying the nutritional status of children and adolescents aged 5 to 19 years.
Using the Z-score enables a fairer and more accurate nutritional assessment,
respecting each child’s and adolescent’s unique growth phase.
A Z-score of 0 means the child is at the average. A value of +2, for
example, indicates the BMI is two standard deviations above the mean.
How to calculate the Z-score
How to calculate BMI?
Before calculating the Z-score, you must first obtain the child’s
BMI using the standard formula:
Using this value, we look up the corresponding mean and standard deviation
in the WHO reference tables for the given age and sex.
How is the Z-score calculated?
The BMI Z-score is calculated by comparing the child’s BMI
with the mean and standard deviation (SD) of the reference population
for the same age and sex. The formula is:
\[ Z = \frac{IMC_{observado} - IMC_{média}}{DP} \]
Statistical Z-score Formula
In other words, we are measuring how many standard deviations the child’s BMI
lies above or below the expected average for their age and sex.
Classification Table Based on Z-score
Classification of pediatric nutritional status based on BMI-for-age Z-score,
according to WHO standards.
#
Classification
BMI-for-age Z-score
1
Severe thinness
Z-score ≤ -3
2
Thinness
-3 ≤ Z-score < -2
3
Normal weight (eutrophy)
-2 ≤ Z-score ≤ +1
4
Overweight
+1 < Z-score ≤ +2
5
Obesity
+2 < Z-score ≤ +3
6
Severe obesity
+3 < Z-score
Interesting Facts about Pediatric BMI
🧠 Classification
Pediatric BMI is interpreted based on percentiles and growth reference curves.
The WHO uses age- and sex-specific growth curves to determine nutritional status.
Categories include: severe thinness, normal weight,
overweight, and obesity.
⚖️ Differences
Unlike in adults, pediatric BMI varies with growth and differs by sex.
Boys and girls follow distinct patterns of physical development.
The same BMI value may correspond to different classifications depending on age.
🩺 Recommendations
The WHO recommends regular growth monitoring during pediatric check-ups.
High BMI in children may serve as an early indicator of future disease risk.
Healthy eating habits and physical activity are essential during childhood.
🎯 Miscellaneous
Children may experience growth spurts, during which BMI naturally fluctuates.
Research suggests childhood obesity may negatively impact cognitive performance.
Prolonged use of tablets and smartphones is associated with increased BMI in adolescents.
According to the WHO, childhood overweight has risen dramatically over the past decades.