🧒 Child BMI Calculator
A child's BMI means nothing against the adult bands. It has to be read as a percentile for their exact age and sex, because healthy BMI changes enormously between two and eighteen.
Why a child's BMI is read as a percentile and never as a number
A child's BMI is calculated exactly as an adult's, and then interpreted in a completely different way. An adult BMI of 18 is underweight; for a six-year-old it is slightly above average. Children's bodies change shape as they grow, so the only meaningful question is how a child compares with other children of the same age and sex — which is what a percentile answers.
BMI = weight (kg) / height (m)2
The arithmetic is the same as the adult BMI calculator. What changes is the reference: instead of fixed cut-offs at 18.5 and 25, the result is placed on a growth curve for that exact age in months and that sex.
The four categories
| Percentile | Category |
|---|---|
| Below the 5th | Underweight |
| 5th to below 85th | Healthy weight |
| 85th to below 95th | Overweight |
| 95th and above | Obesity |
A percentile is a ranking, not a score. A child on the 70th percentile has a higher BMI than 70% of children their age and sex, and that is squarely within the healthy range. Parents often read a high-sounding percentile as a problem when anything from the 5th to the 85th is simply normal variation.
Healthy BMI moves with age, and it dips first
The median BMI for a boy is about 16.6 at age two, falls to roughly 15.3 by age five, and then climbs steadily to around 21 by eighteen. That dip is normal: toddlers lose their baby roundness as they stretch upward, and the leanest point of a typical childhood arrives around age five or six. Applying the adult threshold of 25 to a five-year-old would classify almost every child as healthy no matter how much they weighed, which is precisely why paediatric assessment uses curves.
| Age | Boys | Girls |
|---|---|---|
| 2 years | 16.6 | 16.4 |
| 5 years | 15.3 | 15.2 |
| 8 years | 15.8 | 15.8 |
| 11 years | 17.2 | 17.4 |
| 14 years | 19.1 | 19.3 |
| 17 years | 21.1 | 20.6 |
The figures in this tool are interpolated from the published CDC growth reference and are approximate. For the official percentile, use the CDC's own Child and Teen BMI Calculator or ask your paediatrician, who will plot the point on the actual chart. Nothing here is a diagnosis.
The trend matters far more than the reading
A single percentile is a snapshot; a child's growth is a trajectory. A child who has tracked along the 60th percentile for five years is growing consistently, which is reassuring whatever the number. A child who has moved from the 50th to the 88th in eighteen months has changed trajectory, and that change is worth discussing even though 88 is only just outside the healthy band.
This is why clinicians plot repeated measurements rather than calculating one BMI. Crossing percentile lines in either direction is the signal. Staying on a line, high or low, usually is not.
What BMI cannot see in a child
BMI measures weight against height and nothing else. It cannot distinguish muscle from fat, which matters for athletic adolescents who may land in the overweight band while carrying very little fat. It does not account for the timing of puberty, so an early-developing eleven-year-old and a late-developing one are compared against the same curve despite being at different stages. It also says nothing about frame size, body composition or fitness.
For a rough composition estimate on an older teenager the body fat calculator adds tape measurements, and the child height predictor puts the growth itself in context. Neither replaces a clinical assessment.
What actually helps, if the number is high
For most growing children the goal is not weight loss but slowing weight gain while height catches up. A child who holds their weight steady through a growth spurt moves down the percentiles without ever having dieted. Deliberate calorie restriction in children is a decision for a clinician, not a parent with a calculator, because growth and puberty both have real nutritional requirements.
The changes with the best evidence are unglamorous and apply to the whole household rather than one child: meals at a table instead of a screen, water instead of sweetened drinks, sleep at a consistent hour, and activity that is play rather than exercise. Singling out one child's eating tends to backfire, and language about weight in front of children is strongly associated with later disordered eating.
A worked example
A boy aged ten, 1.42 m and 34.9 kg, has a BMI of 17.3. That sits between the 50th and 85th percentile for his age — comfortably healthy, slightly above the median of 16.6. An adult reading of 17.3 would be flagged as underweight, which shows how badly adult thresholds mislead when applied to children. If the same BMI belonged to a seventeen-year-old, it would fall near the 15th percentile: the identical number, two very different readings.
If this is one step in a larger decision, the BMI calculator is usually the next question, and the height calculator is worth a look while you are here.
Frequently Asked Questions (FAQs)
Why can't I use adult BMI categories for my child?
Because healthy BMI changes with age. The median falls from about 16.6 at age two to around 15.3 at five, then rises to roughly 21 by eighteen. Adult cut-offs of 18.5 and 25 would misclassify almost every child, which is why ages 2 to 19 are assessed against growth curves instead.
What is a healthy BMI percentile for a child?
The 5th up to the 85th percentile is the healthy weight range. Below the 5th is underweight, 85th to 95th is overweight and 95th or above is obesity. A child on the 70th percentile is within the normal range, not approaching a problem.
Is a high percentile always a concern?
Not on its own. A child who has tracked the same percentile for years is growing consistently. What clinicians look for is a change in trajectory — crossing percentile lines over months — rather than any single reading.
Does BMI work for athletic children?
Poorly, for the same reason it struggles with athletic adults: muscle weighs more than fat, so a muscular adolescent can land in the overweight band with very little body fat. If a child is active and visibly lean, the number is probably describing muscle.
Should I put my child on a diet if the BMI is high?
Not without clinical advice. For most growing children the aim is to slow weight gain while height catches up, not to lose weight, because growth and puberty have real nutritional requirements. Household-wide habits work better than singling out one child.
How accurate are the percentiles in this calculator?
They are interpolated from the published CDC growth reference and are approximate — good enough to orient you, not a diagnosis. The official percentile comes from the CDC's own calculator or your paediatrician, who plots the measurement on the real chart.
At what age does this apply?
Ages 2 to 19. Below two, growth is assessed with weight-for-length charts instead, and BMI is not used. From twenty onwards the adult categories apply, which is why they take over at the end of the childhood curves.