Why is child BMI interpreted by percentile?

Children's body composition changes rapidly with age, so the same BMI means different things at different ages. Percentiles compare to peers of the same age and sex.

What are the CDC weight categories?

Underweight: <5th percentile. Healthy weight: 5th–84th. Overweight: 85th–94th. Obesity: ≥95th percentile.

Is BMI an accurate measure of body fat?

It's a screening tool, not a direct measure. Athletes can have high BMI without excess fat; some children with low muscle may have low BMI despite excess fat.

Should I worry about a single reading?

Trend matters more than a single number. Discuss with your pediatrician if your child's percentile is changing rapidly or is at an extreme.

Does this work for kids under 2?

No — under 2, growth is tracked with weight-for-length and head circumference percentiles. Use our Growth Percentile calculator.

Boys and girls have different typical body compositions at every age, so the percentile curves differ.

What should I do if my child is overweight?

Talk to your pediatrician. Focus on family habits — balanced meals, sleep, and physical activity — rather than restrictive diets.

2–20 years Evidence-based Free & private

Child BMI Calculator (Ages 2–20)

Calculate Body Mass Index for children and teens with CDC age- and sex-specific percentile interpretation.

Start calculating Learn about Child BMI Calculator (Ages 2–20)

Happy child next to a colorful percentile gauge with an apple

Understanding this tool

Plain-language explanations grounded in current pediatric evidence.

TL;DR

BMI in children is interpreted as a percentile by age and sex (CDC, ages 2–20). Healthy weight is 5th–84th percentile, overweight 85th–94th, obesity ≥95th, underweight <5th. Trend over time matters more than a single reading.

What it is

The Child BMI Calculator estimates Body Mass Index for ages 2–20 and converts it to an age- and sex-specific percentile using CDC growth references.

How it's calculated

BMI = weight (kg) ÷ height (m)² or BMI = 703 × weight (lb) ÷ height (in)². The percentile compares the child to peers of the same age and sex.

When to use it

Use as a screening tool at well-child visits, when tracking growth trends, or when a clinician is monitoring nutrition.

Who should use it

Parents, pediatricians, school nurses, dietitians, and adolescent-medicine clinicians.

Clinical importance

BMI percentile correlates with body fat in children, but does not measure fat directly. Trend over time matters more than a single reading; muscular or unusually tall children may classify above their actual adiposity.

Example

An 8-year-old boy who is 128 cm and 26 kg has a BMI of ~15.9, near the 50th percentile — a healthy weight.

How to use this calculator

1
Pick your units
Metric (kg/cm) or US (lb/in).
2
Enter age and sex
2–20 years; choose boy or girl — percentiles are sex-specific.
3
Enter height and weight
Use a stadiometer for height if available; remove shoes.
4
Read the percentile and category
Underweight (<5th), Healthy (5th–84th), Overweight (85th–94th), Obesity (≥95th).
5
Track over time
Plot a few readings to see whether the percentile is stable, rising, or falling.

Common mistakes to avoid

Comparing a child's BMI to adult BMI categories — they don't apply.
Treating a single reading as a diagnosis instead of looking at the trend.
Ignoring body composition — muscular athletes may have high BMI without excess fat.
Using this tool for children under 2 — use weight-for-length instead.
Restricting calories for a child without pediatric guidance.

Glossary

BMI: Body Mass Index — weight divided by height squared. A screening proxy for body fat.
BMI-for-age percentile: BMI ranked against same-age, same-sex children using CDC reference curves.
Stadiometer: A wall-mounted height-measurement device used in clinics for accurate height.
Adiposity: Amount of body fat — what BMI is trying to approximate.

Medical references

Frequently asked questions

Answers parents and pediatric professionals ask most often about this tool.

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