Why children need a different BMI calculator
Children’s body fat changes as they grow, and it differs between boys and girls. A BMI of 18 can be perfectly healthy for a 12-year-old but high for a 5-year-old. So instead of fixed adult cut-offs, a child’s BMI is converted into a BMI-for-age percentile: where the child sits compared with children of the same age and sex.
BMI-for-age categories
| Percentile | Category |
|---|---|
| Below the 5th | Underweight |
| 5th to below 85th | Healthy weight |
| 85th to below 95th | Overweight |
| 95th or above | Obesity |
| 120% of the 95th or above | Severe obesity |
A result at the 70th percentile means the child’s BMI is higher than 70% of children of the same age and sex in the reference population — it is not a percentage of body fat.
How the child BMI calculator works
First it calculates BMI from height and weight in the usual way. Then it uses the CDC 2000 BMI-for-age reference data and the LMS method to turn that BMI into a z-score and a percentile for the child’s exact age in years and months. It also shows the BMI range and the weight range that would put the child between the 5th and 85th percentiles at their current height.
Measuring kids accurately
- Measure height without shoes, standing straight against a wall, with the head level.
- Weigh in light clothing, ideally at the same time of day each time.
- Enter age in years plus extra months — a few months make a real difference for young children.
Children under 2 are assessed with weight-for-length charts rather than BMI, and children in the UK are usually plotted on the UK90 charts, which can give slightly different percentiles. One result is a snapshot; a health professional looks at how a child’s growth tracks over time before drawing conclusions. For adults aged 20 and over, use the BMI Calculator.