About the BMI Percentile Calculator for Children
Body mass index is a quick screening measure for weight relative to height. For adults, it comes with fixed cut-offs: 25 for overweight, 30 for obesity. For children and teenagers, those cut-offs simply do not work, and this BMI percentile calculator exists because of that.
A healthy child's BMI changes a great deal with age. It falls from infancy until around age five or six — the point called the adiposity rebound — and then rises steadily through adolescence. Boys and girls follow different curves too. So a BMI of 18 is high for a five-year-old and low for a sixteen-year-old.
Instead of a fixed line, a child's BMI is placed on a percentile for their age and sex. This calculator uses the WHO references from 2 to 19 years, and classifies the result using WHO's categories.
How to Use the BMI Percentile Calculator for Children
Choose metric or imperial units and the child's sex.
Enter the age in years, plus any extra months. The calculator covers 2 years to 19 years exactly.
Enter height and weight, measured without shoes and in light clothing if possible.
How the Percentile Is Worked Out
BMI = weight (kg) ÷ height (m)²
z = ((BMI ÷ M)^L − 1) ÷ (L × S)
percentile = the share of a normal distribution below zM is the median BMI for the child's age and sex, S how widely BMIs spread, and L corrects for the right skew in BMI. These three values come from the WHO reference for each month of age. Between whole months, the calculator interpolates; that was tested against WHO's daily data and found to be accurate to about a thousandth of a standard deviation.
The references used are the WHO Child Growth Standards (2006) from 2 to 5 years and the WHO Growth Reference (2007) from 5 to 19 years.
Step-by-Step Example
A boy aged 10, 140 cm tall, weighing 40 kg.
BMI: 40 ÷ 1.40² = 20.4
WHO median for boys of 10 = 16.4
z-score: 1.70
Percentile: 96thFrom age five, WHO classes a z-score above +1 as overweight and above +2 as obesity, so this boy's BMI is in the overweight range.
WHO Categories
From 5 to 19 years:
| z-score | Approximate percentile | Category |
|---|---|---|
| below −3 | below 0.1st | Severe thinness |
| −3 to −2 | 0.1st – 2nd | Thinness |
| −2 to +1 | 2nd – 84th | Healthy weight |
| +1 to +2 | 84th – 98th | Overweight |
| above +2 | above 98th | Obesity |
From 2 to 5 years, the upper lines are one step higher: +1 to +2 is flagged as at risk of overweight, +2 to +3 overweight and above +3 obesity. That reflects how common temporary high BMIs are in young children.
WHO and CDC: Why Two Systems Exist
The United States uses CDC growth charts, which were built from US survey data, and classifies children by percentile lines: the 85th percentile for overweight and the 95th for obesity.
WHO's references were built differently, so a child's percentile is not quite the same on each. The category lines also differ. WHO's overweight line at +1 z is the 84th percentile, close to the CDC's 85th. But WHO's obesity line at +2 z is the 98th percentile, stricter than the CDC's 95th.
This calculator uses WHO throughout and says so. If your child's doctor uses CDC charts, their figures may differ by a few points, and that is expected.
What BMI Can and Cannot Tell You
BMI is a screening measure. It is cheap, quick and useful across a population, but for an individual child it has real limits.
It cannot tell muscle from fat, so an athletic child can have a high BMI without excess fat.
It is affected by the timing of puberty — children who mature early often have a higher BMI for their age for a while.
It says nothing about where fat is carried or about fitness.
So a doctor will look at the trend over time, family history and the child as a whole, not only the category.
Measuring a Child Accurately
Small measuring errors matter more for children than adults, because a child's BMI range is narrower. A centimetre off in height moves BMI by about 1.5% for a ten-year-old.
Height: shoes off, heels, bottom and shoulders against a wall, looking straight ahead. Mark the top of the head with a flat object held level, then measure to the mark.
Weight: light clothing, no shoes, on a hard floor rather than carpet, ideally at the same time of day each time.
Age: use the exact age in years and months. At some ages the median BMI moves noticeably within a year, so rounding a 10-year-11-month-old down to 10 can shift the percentile.
Understanding Your Result
BMI percentile gives the percentile and WHO category.
BMI shows the child's BMI beside the median for their age and sex, which makes clear why adult cut-offs do not apply.
z-score names the reference used.
WHO categories lists the lines for the child's age band.
Worth knowing explains what the category means in practice.
When Should You Use This Calculator?
After a school or clinic measurement, to understand the result.
To follow a child's growth over time, taking measurements the same way.
When an adult BMI calculator gives a confusing answer for a child.
Not to diagnose. Concerns belong with the child's doctor.
Common Mistakes
Using adult BMI cut-offs for children. They do not apply.
Comparing WHO and CDC percentiles directly. The references differ.
Judging on one measurement. The trend matters more.
Putting a growing child on a diet. Family-wide habits work better, and height often catches up.
Forgetting BMI cannot see muscle. Active children may read high.
A child's weight is best discussed with their doctor, who can put the numbers in context. Nothing here is medical advice.