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Growth Monitor

How we calculate Body Mass Index (BMI)


Body mass index or BMI is a person's weight in kilograms divided by the square of their height in metres. It is a way of measuring how much body fat a person has i.e. whether a person is under- or over-weight.

In children and adolescents, BMI changes with age and is different in males and females. Like growth charts, BMI charts allow us to understand whether a child’s BMI is ‘normal’ by comparing it with other children of the same age and sex.

The different ‘cut-offs’ for overweight and underweight BMIs are very well established in adults. For example, a BMI greater than 25 is considered overweight and one greater than 30, very overweight. In children, this is more complicated because the normal BMI ranges depend on age and sex.

ISO-BMI is a way of adjusting a child’s BMI for age and sex. In other words, it converts a child’s BMI to an adult equivalent value which we can compare to established cut offs.  

If you enter a weight, GrowthMonitor will automatically calculate your child's ISO-BMI and compare to the different cut-offs for under and over-weight. These are shown below:

  • 16-17 - UNDERWEIGHT
  • 17-25 - NORMAL
  • 25-30 - OVERWEIGHT
  • Over 30 - VERY OVERWEIGHT 

If your child falls into one of the overweight or underweight categories, you could consider asking your GP for advice. Overweight or underweight children may have or may be at risk of health problems. Overweight children are more likely to be overweight as adults. Overweight adults are at greater risk of serious diseases such as diabetes or heart disease. For more advice about underweight and overweight children see the links below:


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