Child BMI Calculator
Works out a child's body mass index from weight and height and bands the result by age and sex to show whether it sits in a healthy range.
Recent
Formula
How the calculation works
Enter the child's weight in kilograms and height in centimetres as measured without shoes on a flat surface.
Enter the age in years and select the sex, because the healthy BMI range shifts with both age and sex right through childhood.
The tool converts height to metres, divides weight by the square of that height for the BMI, and bands it for the age and sex you gave.
Common mistakes
- Using adult BMI cut-offs for a child, when children are graded on age and sex specific charts and not the fixed adult numbers.
- Measuring height with shoes on or weight just after a heavy meal, which shifts the BMI enough to move the band.
When to use it
- Use it between growth check-ups to see whether a change in weight is keeping pace with a change in height for your child's age.
- Use it to prepare for a doctor's visit, since having the BMI alongside the age makes the growth discussion quicker and clearer.
Worked example
A 7-year-old boy weighing 24 kg and 122 cm tall has a BMI of 16.1, which for his age and sex falls in the healthy range.
Common questions
Is a BMI above the range always a health problem?
No, a single BMI above the range is a signal to look further, and a doctor will weigh up the growth trend, diet and activity before drawing any conclusion.
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