Child and teen BMI calculator
A child's BMI cannot be read against the adult numbers. A healthy BMI changes month by month while a child grows, so this calculator places them on the WHO 2007 growth curves for their exact age and sex and tells you the percentile. That is a position among children like them, not a verdict about them.
How to read the chart
The curved lines are percentiles. Each one traces the BMI that a given share of children of that sex sit below at every age, and they slope upward because BMI naturally rises through childhood and adolescence. Your child is the dot.
The shaded band runs from the 3rd percentile to the 85th, which is the healthy range the WHO defines for this age group. It is wide because healthy children genuinely vary a lot. Sitting near the edge of it is not in itself a problem; a pattern of moving steadily across it over a year is the thing a doctor would want to look at.
What the percentile bands mean
| Percentile | WHO classification | What it says |
|---|---|---|
| Below 3rd | Thinness | Lower than most children of the same age and sex. Worth raising with a doctor, especially if it is a change. |
| 3rd to 85th | Healthy range | The broad middle. Most healthy children are somewhere in here, near either edge included. |
| 85th to 97th | Overweight | Higher than most. Usually a reason to look at the pattern over time rather than to act on one reading. |
| Above 97th | Obesity | Worth a conversation with your doctor about whether anything needs to change. |
Those labels are the WHO's, and they describe a position on a distribution. They are not a description of a child's health, which depends on a great deal this calculation knows nothing about.
What this calculation cannot see
Everything that BMI cannot see in an adult, it also cannot see in a child, and a couple of things besides. It does not know how much of the weight is muscle, which matters for a child who trains seriously. It does not know where they are in puberty, and a growth spurt moves height and weight at different times, so a percentile can swing for entirely normal reasons. It does not know their build or their family.
There is also a choice of reference. This page uses the WHO 2007 reference, which is used across most of the world. The CDC 2000 charts remain standard in United States clinical practice and will place the same child at a somewhat different percentile. Neither is wrong; they describe different reference populations.
If the number concerns you
Take it to your doctor, health visitor or school nurse rather than acting on it directly. They can put it next to your child's height history, their stage of growth and everything else they know, which is the context this page does not have. Restricting a child's diet without that advice can do harm, and so can making a child conscious of a number on a chart.
Common questions
Why can't I use a normal BMI calculator for my child?
Because a healthy BMI changes as a child grows. An adult's healthy band sits between 18.5 and 25 at any age, but a perfectly healthy seven year old often has a BMI around 15, and the same number in a fifteen year old would be low. Adult cut points applied to a child will label healthy children as underweight and will miss others entirely. Children are placed on a growth reference for their exact age and sex instead.
What is a percentile?
It is your child's position among children of the same age and sex. The 60th percentile means that out of a hundred children of that age and sex in the reference population, about sixty would have a lower BMI. It is a description of where they sit, not a score and not a diagnosis.
What counts as a healthy percentile?
Using the WHO 2007 reference, the healthy range for ages 5 to 19 runs from the 3rd percentile up to the 85th. Above that is classed as overweight and above the 97th as obesity, while below the 3rd is classed as thinness. The shaded band on the chart is that healthy range, and it is wide on purpose: healthy children come in a broad range of shapes.
Which growth reference does this use?
The WHO 2007 growth reference for school-aged children and adolescents, covering 5 years 1 month to 19 years. It is the same reference the app uses, so you will get the same answer in both. The United States also has the CDC 2000 charts, which are still standard in American clinical practice and will give a somewhat different percentile for the same child.
My child's percentile changed a lot since last time. Should I worry?
Not on its own. A single reading moves with how recently they ate, how they stood on the scale, and simple measurement error, and children genuinely grow in uneven bursts. What paediatricians look at is the pattern across several measurements over months. If a real trend concerns you, that is a conversation with your doctor or health visitor, not with a calculator.
How do I talk to my child about this number?
Most guidance is that you often do not need to. A percentile is information for the adults, and research on childhood weight consistently finds that focusing a child's attention on their own weight carries its own risks. If something needs to change, changes that involve the whole family, around food and activity rather than around a number, are what is usually recommended. Your doctor can help you decide whether anything needs to change at all.
Does this work for children under 5?
No. This calculator covers 5 years 1 month to 19 years, which is what the WHO 2007 reference covers. Under 5 there is a separate WHO Child Growth Standard with different measures, and growth at that age is usually followed by a health visitor or paediatrician rather than by a calculator.
Sources
Every figure above comes from one of these. Check them.
- de Onis M, Onyango AW, Borghi E, Siyam A, Nishida C, Siekmann J (2007). Development of a WHO growth reference for school-aged children and adolescents. Bulletin of the World Health Organization 85(9), 660-667
- World Health Organization. Growth reference data for 5 to 19 years: BMI-for-age
- Cole TJ, Green PJ (1992). Smoothing reference centile curves: the LMS method and penalized likelihood. Statistics in Medicine 11(10), 1305-1319
- NICE guideline NG246 (2025). Overweight and obesity management
One reading is a dot, several are a pattern
The app keeps each measurement on the same curves, so the shape over months is visible without you plotting anything.
Free, no account, and it works offline. 100,000+ downloads on Google Play.
This is not medical advice. Body Numbers is a calculator. It does not diagnose, treat or monitor any condition, and no number it produces should replace a conversation with a doctor or a dietitian. If a result worries you, or if you are pregnant, unwell, or under eighteen, speak to a health professional.
