Child BMI Calculator

How the Child And Teen BMI Calculator works

Child BMI calculator: why a percentile is needed

The adult cut-offs of 18.5, 25 and 30 mean nothing for children. A healthy four-year-old has a BMI around 16, which would be underweight for an adult; a healthy sixteen-year-old boy sits near 21. Children's BMI dips after infancy, reaches its lowest point around age five or six, then rises through adolescence, and boys and girls follow different curves. So a child's BMI is compared with children of the same sex and age, and the result is expressed as a percentile: the share of children of that age and sex with a lower BMI.

The CDC method used here

The calculator uses the CDC 2000 growth chart reference, the same tables paediatricians in the United States and many other countries plot on. For every month of age and each sex the CDC publishes three parameters — the LMS values — that describe the skew, median and spread of BMI at that age. The child's BMI is converted to a z-score with those parameters and the z-score to a percentile. The tables are vendored into the page, so nothing is looked up online, and ages between the tabulated months are interpolated.

Child BMI categories

The CDC and American Academy of Pediatrics define categories by percentile, not by BMI value.

The calculator also shows the median BMI for the child's age and sex, so you can see how far from typical the figure is in the units you measured.

Age matters to the month

Because the reference curves move with age, the same height and weight can give different percentiles a few months apart, and the difference is largest in the early years and at the start of puberty. Enter the age in whole years and add months if you know them; a birthday last week and a birthday eleven months ago are almost a year apart on the chart.

Reading a child's result

One percentile is a snapshot. Paediatricians watch the trend: a child tracking steadily along the 75th percentile is growing normally, while a child crossing from the 50th to the 90th over a year deserves a conversation even though both numbers are under 95. Puberty moves children across percentiles temporarily, growth spurts drop BMI before weight catches up, and very athletic teenagers carry muscle the index reads as fat. A high or low result is a reason to talk to a GP or paediatrician, who will plot it alongside earlier measurements; it is never a diagnosis on its own, and it is not a number to share with the child as a verdict.

Outside the 2 to 20 range

Under two years, weight-for-length is used instead of BMI, on the WHO charts. From twenty, the adult categories apply; use the main BMI calculator, or the pages for women and men.

Child BMI worked examples

The formula: BMI = kg ÷ m²; z = ((BMI ÷ M)^L − 1) ÷ (L × S) using the CDC L, M and S values for the child's age and sex; percentile = the normal distribution's share below z.

First example. A boy aged 8 years, 128 cm and 26 kg, has a BMI of 15.9. The median for his age is 15.8, which puts him at the 52nd percentile, a healthy weight.

Second example. A girl aged 12, 150 cm and 50 kg, has a BMI of 22.2 against a median of 18.1. That is the 87th percentile, in the overweight band of 85th to below 95th.

How many children fall in each band? In the reference population 5% are below the 5th percentile, 80% are in the healthy band, 10% overweight and 5% obese.

BMI at the 5th, 85th and 95th percentiles by age (CDC 2000)

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