Introduction
This child BMI calculator helps you find out if a child's weight is healthy for their age and height. BMI stands for body mass index. It is a number based on how much a child weighs and how tall they are. For kids and teens, BMI is compared with age- and sex-specific CDC growth charts to find a percentile, which the guideline applies from age 2 to 18.1 This calculator accepts ages 2 to 20. The percentile shows how a child's BMI compares to other kids of the same age and sex.
Adults and children use BMI differently. An adult BMI calculator uses fixed cutoffs, so a BMI of 25 always means the same thing regardless of age. But a child's body changes a lot as they grow, so a "healthy" BMI number shifts with age. That is why BMI-for-age percentiles matter more than the raw number alone.
To use this calculator, enter the child's sex, age, height, and weight. You can use pounds and inches or kilograms and centimeters. The tool will calculate the child's BMI, percentile, and z-score. It will also show a step-by-step breakdown of the math, a color-coded percentile gauge, and a BMI-for-age growth chart. Results are grouped into standard CDC weight categories: underweight, healthy weight, overweight, or obese.
Note: This calculator is for informational purposes only. It is not a medical diagnosis. Always talk to a pediatrician or healthcare provider about your child's growth and health.
How to Use Our Child BMI Calculator
Enter your child's sex, age, height, and weight below. The calculator will show their BMI, percentile, weight category, healthy weight range, a percentile gauge, and a growth chart.
Sex: Choose whether the child is male or female. The CDC uses different growth charts for boys and girls, so this choice affects the results.
Age in Years and Months: Pick the child's age in years (2 to 20) and months (0 to 11). This calculator works for children and teens between ages 2 and 20. If you select 20 years, months will lock to 0.
Unit System: Pick Imperial to enter height in feet and inches and weight in pounds. Pick Metric to enter height in centimeters and weight in kilograms. Switching units will clear any height and weight you already typed in.
Height: If using Imperial, enter the feet in one box and the remaining inches in the next box. If using Metric, enter the full height in centimeters.
Weight: Enter the child's weight in pounds if using Imperial or in kilograms if using Metric.
Calculate BMI: Press the Calculate BMI button to see the results. You can also press Reset / Start Over to clear all fields and start fresh.
What Is a Child BMI Calculator?
A child BMI calculator finds out if a child's weight is healthy for their height, age, and sex. BMI stands for body mass index. It is a number that compares how much a child weighs to how tall they are. For kids and teens aged 2 to 20, BMI works differently than it does for adults. A child's BMI is compared to other children of the same age and sex using percentile charts from the Centers for Disease Control and Prevention (CDC).
How BMI-for-Age Percentiles Work
A percentile tells you where a child's BMI falls compared to other kids. For example, a child at the 70th percentile has a BMI higher than 70 out of 100 children the same age and sex. The CDC uses these percentile ranges to group weight status:
- Underweight: Below the 5th percentile
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obese: 95th percentile or above
Those bands come from the pediatric obesity guideline. It defines overweight as a BMI at or above the 85th percentile and below the 95th, and obesity as a BMI at or above the 95th percentile.1
Children who fall at or above the 95th percentile are further grouped into obesity classes. These classes use the percent of the 95th percentile BMI to measure how far above the obesity threshold a child is. Class II starts at 120% of the 95th percentile BMI — the guideline's threshold for severe obesity — and Class III at 140%, the level used in bariatric-surgery eligibility criteria.1 The calculator converts BMI into a z-score, which is then mapped to a percentile using a standard normal distribution.
Why Child BMI Matters
Kids grow at different speeds. A BMI number alone does not tell the full story. That is why doctors use age- and sex-specific percentiles instead of a single cutoff number like they do for adults. Tracking BMI over time helps parents and doctors spot trends early. A BMI that is too high or too low can be a sign that a child may need changes in nutrition, activity, or medical care.
What BMI Does Not Tell You
BMI is a screening tool, not a diagnosis. It cannot tell the difference between muscle, bone, and fat. A very active child with strong muscles may have a higher BMI without having extra body fat. A doctor can use other tests and exams alongside BMI to get a clearer picture of a child's health. Always talk to a pediatrician before making changes based on a BMI result.