Worldwide
Child BMI calculator
Body mass index works differently for children and teenagers than it does for adults.
For a child or teenager the BMI number on its own does not give a weight category.
For a grown-up, a BMI of 22 is a healthy weight whatever their age. For a child, the same BMI can be perfectly healthy at one age and a concern at another, because children are still growing and their body composition shifts year by year. This tool gives you the BMI number from a child's height and weight, but the number on its own is not a category. It has to be read against a growth chart that compares the child with others of the same age and sex, which is called a BMI-for-age percentile. Below the calculator explains how to do that, gives the percentile bands health agencies use, and points you to the official charts. The maths of the BMI itself is the same as for adults; what changes is how you interpret it.
How it works
- Enter the child's sex and age in years, then their height and weight in your preferred units.
- The calculator works out BMI as weight in kilograms divided by height in metres squared, the same formula used for adults.
- It returns that BMI value for you to look up, and reminds you that for a child the number means nothing without the age-and-sex context.
- Take the BMI to a BMI-for-age growth chart for the child's sex to find the percentile, which is what actually tells you whether the weight is healthy.
BMI = weight (kg) / height (m)^2, then read against a BMI-for-age percentile chart
The BMI calculation is identical to the adult one: divide weight in kilograms by the square of height in metres. The difference is entirely in interpretation. Rather than comparing the result with fixed adult thresholds, you plot it on a growth chart for the child's exact age and sex to find the percentile, and the percentile band is what classifies the weight.
- weight
- the child's weight in kilograms
- height
- the child's height in metres
- percentile
- the child's rank against others of the same age and sex
CDC BMI-for-age percentile bands
| Below 5th percentile | Underweight | screen for cause with a clinician |
| 5th to below 85th | Healthy weight | the target range |
| 85th to below 95th | Overweight | monitor and review habits |
| 95th percentile or above | Obesity | discuss a plan with a doctor |
Worked example
A ten-year-old weighing 32 kg and standing 140 cm tall: BMI is 32 divided by 1.4 squared, which is 32 divided by 1.96, or about 16.3. That single figure is meaningless until you plot it on the BMI-for-age chart for a ten-year-old; on the CDC chart a BMI near 16 for that age sits comfortably inside the healthy band, but the same 16.3 would read very differently for a fifteen-year-old.
Key facts
- A child's BMI number means nothing without the age-and-sex percentile that goes with it.
- The BMI calculation is the same as for adults; only the interpretation changes.
- CDC charts cover ages 2 to 19; WHO charts are used widely outside the United States.
- The percentile bands are screening tools, not diagnoses, and a clinician reads them in context.
Tips
- Measure height without shoes and weight in light clothing for the most accurate BMI.
- Take the BMI value to the CDC or WHO BMI-for-age chart for your child's sex, or ask your doctor to plot it.
- Track BMI over time rather than reading a single point, since the trend on the growth chart matters more than one figure.
- For a child under two, use weight-for-length guidance and speak to a health visitor instead.
Frequently asked questions
Why can't I just use the adult BMI categories?+
The adult cut-offs (under 18.5 underweight, 25 overweight, 30 obese) are fixed numbers that ignore age. Children's healthy BMI rises and dips as they grow, so a fixed cut-off would wrongly flag many healthy children and miss others. That is why paediatric BMI is read as a percentile against children of the same age and sex.
What is a BMI-for-age percentile?+
It ranks a child's BMI against a reference population of the same age and sex. A child on the 60th percentile has a BMI higher than 60 percent of children like them. The widely used reference in the United States is the CDC growth chart for ages 2 to 19; the World Health Organization publishes charts used in many other countries.
What do the percentile bands mean?+
On the CDC system, below the 5th percentile is classed as underweight, the 5th to below the 85th is a healthy weight, the 85th to below the 95th is overweight, and the 95th percentile or above is obesity. These are screening bands, not a diagnosis, and a doctor weighs them alongside growth history and overall health.
Does this tool give the percentile?+
No, and that is deliberate. The percentile needs the official age-and-sex reference tables, and a screening tool should not stand in for a clinician reading a growth chart. This calculator gives you the accurate BMI value to take to that chart or to your child's doctor, nurse or health visitor.
What about children under two?+
BMI-for-age charts start at age 2. For a baby or toddler under two, growth is tracked with weight-for-length charts instead, and any concern should go to a health visitor or doctor rather than a BMI figure.
Things to watch
- This tool gives a BMI value only; it does not calculate a percentile or diagnose a weight category for a child.
- BMI does not measure body fat directly and can mislead for very muscular or unusually built children.
- Any concern about a child's growth or weight should go to a doctor, nurse or health visitor, not a calculator.
Sources
- BMI for children and teens (BMI-for-age percentiles) · Centers for Disease Control and Prevention
- Growth reference data for 5 to 19 years · World Health Organization
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with the official sources listed before making decisions.
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