Child BMI Calculator — Percentile for Age and Sex
For a child, BMI is only half of the measurement. The arithmetic is the adult one — weight divided by height squared — but the result has to be plotted on a growth chart for the child’s sex and age in months before it means anything. That is why the calculator below returns the raw value and stops there.
Calculated BMI
16.5kg/m²
This number on its own does not tell you whether the child is at a healthy weight — in children and teens, BMI only means something once it is plotted on a growth chart for age and sex. Take this value to the pediatrician, who reads it on the WHO growth chart (birth to age 2) or the CDC BMI-for-age chart (age 2 and up).
How we got this number
BMI = weight ÷ height² = 20 ÷ 1.10² = 16.5 kg/m²
Which chart, and from what age
The chart changes as the child grows, and using the wrong one is the most common mistake made at home:
- Under 2 years old: no BMI. The CDC recommends the WHO weight-for-length charts for this age, according to the pediatric obesity chapter of StatPearls.
- From 2 through 19: BMI-for-age, on separate curves for boys and for girls.
Age is entered in months, not years, because the curve rises steeply in early childhood. A 2-year-old and a 2-year-and-9-month-old are two different points on the same line, even though both get rounded to “two” in conversation.
The percentile bands used in the United States
| BMI-for-age percentile | Category |
|---|---|
| Below the 5th | Underweight |
| 5th to below the 85th | Healthy weight |
| 85th to below the 95th | Overweight |
| 95th and above | Obesity |
Above the 95th percentile the classification is refined further, and the refinement is expressed as a share of that same line rather than as new percentiles: class 1 runs from the 95th percentile up to 120% of it, class 2 from 120% to 140%, and class 3 at 140% and above. Both tables come from StatPearls, published by the National Library of Medicine.
Plotting, in practice
- Take the BMI from the calculator, without rounding it first.
- Pick the chart that matches the child’s sex.
- Find the age in months along the bottom axis and the BMI up the side.
- Read which percentile curve the point lands on or between.
A pediatrician does this at well-child visits and, more importantly, does it repeatedly. One point is a dot; four points across two years is a growth pattern, and the pattern is what changes clinical decisions.
Why the adult table never works here
Applying the 18.5 to 24.9 adult band to a 7-year-old would flag almost every healthy child as underweight, since a healthy BMI at that age often sits near 16. The adult range on the BMI calculator begins at 20 years old. Between 13 and 19, everything on this page still holds, with the wrinkle that puberty stretches the curve — see the BMI calculator for teens.
This content is informational and does not replace evaluation by a health professional.
Frequently asked questions
Why does this page not label the result healthy or overweight?
Because the label depends on a chart this page cannot reproduce safely. The same BMI of 17 is a healthy weight for a 10-year-old and above the 95th percentile for a 4-year-old, and only the plotted point settles it.
What do I need before plotting the number?
Three things: the BMI, the child's sex, and the age in months rather than years. A child of 4 years 1 month and one of 4 years 10 months sit at visibly different places on the curve.
My child's percentile dropped without any weight loss. Why?
Percentiles compare a child to peers of the same age, and that peer group changes every month. Standing still on the scale while getting older moves the point down the chart on its own.
Is there a chart for babies under 2?
Yes, but it is not a BMI chart. For that age the CDC recommends the WHO weight-for-length charts, which are also specific to age and sex.
Does a high percentile mean my child needs a diet?
No. Restricting a growing child's food on the strength of a single plotted point is its own risk. A high point is a reason to bring the growth record to the pediatrician, who looks at the whole curve.
Sources
- Body Mass Index — StatPearls, National Library of Medicine (NCBI Bookshelf)
- Pediatric Obesity — StatPearls, National Library of Medicine (NCBI Bookshelf)
Written by Lucas Silva · Reviewed by Lucas Silva · Updated on August 13, 2026