Children’s Weight & BMI


How can I tell if my child is overweight or obese?

Why Weight Ireland recommends the use of Body Mass Index (BMI) to screen for overweight and obesity in children and teens aged 2 through 18 years. Although BMI is used to screen for overweight and obesity in children and teens, BMI is not a diagnostic tool.

Calculating Children’s BMI

The formula of calculating the BMI for children is the same as for adult people. But the meaning is different.

Use the below charts to calculate your child’s current BMI and once you get your child’s BMI you can further check the charts further below to see what your child’s percentile is. Body Mass Index (BMI) is a number calculated from a child’s weight and height. BMI is a reliable indicator of body fatness for most children and teens.

What is a BMI percentile?

After BMI is calculated for children and teens, the BMI number is plotted on the Why Weight Ireland BMI growth charts below (for either girls or boys) to obtain a percentile ranking. Percentiles are the most commonly used indicator to assess the size and growth patterns of individual children in Ireland. The percentile indicates the relative position of the child’s BMI number among children of the same sex and age. The growth charts show the weight status categories used with children and teens (underweight, healthy weight, overweight, and obese).

How is BMI used with children and teens?

BMI is used as a screening tool to identify possible weight problems for children. Why Weight Ireland recommends the use of BMI to screen for overweight and obesity in children beginning at 2 years old.

For children, BMI is used to screen for obesity, overweight, healthy weight, or underweight. However, BMI is not a diagnostic tool. For example, a child may have a high BMI for age and sex, but to determine if excess fat is a problem, a health care provider would need to perform further assessments. These assessments might include skinfold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.

Is BMI interpreted the same way for children and teens as it is for adults?

Although the BMI number is calculated the same way for children and adults, the criteria used to interpret the meaning of the BMI number for children and teens are different from those used for adults. For children and teens, BMI age and sex specific percentiles are used for two reasons:

  • The amount of body fat changes with age.
  • The amount of body fat differs between girls and boys.

The Why Weight Ireland BMI growth charts below take into account these differences and allow translation of a BMI number into a percentile for a child’s sex and age.

The adult calculator provides only the BMI number and not the BMI age- and sex-specific percentile that is used to interpret BMI and determine the weight category for children and teens. It is not appropriate to use the BMI categories for adults to interpret BMI numbers for children and teens.

These shows the spreading of the Body Mass Index with age as reference.


Boy's BMI Chart - Children's Obesity


Girl's BMI ChartThe charts have the age from left to right and the BMI from bottom to top.

You see Percentil-Lines in the chart. P50 means, that 50% of the children have a higher and 50% have a lower BMI. P3 means, that 3% of the children have a lower BMI.

Overweight is from P90 and higher. This means, 10% of the children are overweight. Underweight is lower than P3. In the charts above the areas are coloured in red. Normal weight is with green colour.

Why can’t healthy weight ranges be provided for children and teens?

Healthy weight ranges cannot be provided for children and teens for the following reasons:

  • Healthy weight ranges change with each month of age for each sex.
  • Healthy weight ranges change as height increases.

It is important to practice what we preach and show our children the importance of food, the benefits of and possible issues of food, what’s healthy and what’s not. If you are using our Food & Exercise Diary, why not let your child help you, show him/her how your calories work, let them help in the recipe creation so they can learn about different foods and what effect/benefits they can have on your body. If you haven’t had a chance to use our diary, why not take a 2 Day, no obligation, free trial. Just click the Free Trial button on the home page to begin.

If you are located near our studio in Tinahely, Co. Wicklow and you are worried about your child and you would like to discuss the issue please feel free to contact Hannah to organise a consultation 

Click here to learn about the importance of vitamins & your children. 

Please contact your local GP if you are worried about your child’s weight

Childhood Obesity

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