Formula
What is BMI?
BMI (Body Mass Index) is a single number that combines a person's weight and height to give a quick estimate of how their body mass compares to their stature. It's calculated as weight in kilograms divided by the square of height in meters, so the units are kg/m². The World Health Organization uses BMI as a screening tool worldwide.
Why BMI for kids needs its own chart
Children are not just small adults. They're still growing, their proportions of muscle, fat, and water change rapidly, and their bodies pass through growth spurts that can shift BMI quickly. This makes the fixed adult ranges (underweight, normal, overweight, obese) inappropriate for children. Pediatricians instead use age- and sex-specific BMI percentiles built from large population surveys.
The BMI percentile chart for kids
Doctors use a percentile chart to compare a child's BMI against thousands of other children the same age and sex. The 50th percentile is the median - the child is heavier than half their peers and lighter than the other half. A child in the 5th–85th percentile range is considered healthy. Below the 5th is underweight, 85th–95th is overweight, and 95th and above is classified as obese. The chart for boys and girls differs because their growth patterns diverge as they approach puberty.
BMI Prime and percentiles, side by side
BMI Prime is the BMI divided by 25, so a value of 1.0 sits at the upper edge of the adult healthy range. We show it for completeness, but for kids the percentile is the meaningful number - a healthy 7-year-old can have a BMI Prime well below 1.0 and still be perfectly normal for their age.
How to use this calculator
Pick the child's sex (boy or girl), enter their age in years, then their height in centimeters and weight in kilograms. The calculator instantly returns the BMI, BMI Prime, and the percentile they sit at on the CDC reference curves. The category chip (Underweight / Healthy / Overweight / Obese) is based on the standard CDC cut-offs (5th, 85th, 95th percentile). The recommended age range is 2 to 10.
Differences between BMI for boys and girls
At very young ages, boys and girls have nearly identical BMI curves. As they approach 10 and head toward puberty, the curves start to diverge - boys tend to gain more muscle mass, girls tend to gain more body fat as part of normal development. That's why a BMI of 18 at age 10 represents a slightly different percentile for a boy than for a girl, and why our calculator asks for sex. Note that for accurate adolescent assessment, we plan to add a dedicated BMI calculator for teens.
Frequently asked questions
A child's BMI is considered healthy when it falls between the 5th and 85th percentile for their age and sex on the CDC growth chart. The absolute BMI number changes year to year, which is why pediatricians use percentiles instead of fixed cut-offs.
Because children are still growing. Body composition, proportions, and growth speed change with age, and the curves for boys and girls diverge as they approach puberty. The percentile chart adjusts for all of that.
Not necessarily. As children grow taller and gain weight at the same time, their BMI can stay roughly steady or even drop slightly during a growth spurt. What matters is whether they're tracking along a percentile curve. A sudden jump up or drop down across percentiles is what warrants a pediatrician visit.
No. BMI percentile is a screening tool. It can flag children who may benefit from a clinical assessment, but it doesn't diagnose anything on its own. If you have concerns, talk to a pediatrician.
Sources
- CDC BMI percentile calculator for children and teens— Centers for Disease Control and Prevention
- WHO Child Growth Standards— World Health Organization
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