BMI stands for Body Mass Index which is an estimate of body fat based on a
calculation using height and weight that really applies to adult men and women.
In adults for example, if your BMI is < 18, you are considered
underweight, and if > 25 you are considered overweight.
The following is the actual mathmatical formula to calculate BMI or you can use our calculator to the left:


However, the concept of BMI is extremely difficult to grasp and frankly in many instances misused for
children when tracking risk for overweight and obesity. Unlike adults, a calculated BMI value does not
tell you if your child is already overweight or at risk becoming
overweight. For example, a BMI of 17 maybe considered underweight for
an adult woman, but for a 5 year old girl, she is actually is considered being overweight!
How can this be? Many parents and even health
professionals become very confused when tracking BMI values for the purpose
of obesity prevention and education.
It is my aim in this section to clarify how BMI maybe used in children. In children, BMI percentiles is used rather than an interger value calculated from height and weight. There's an additional stem, and you can use the 2 BMI charts below from the CDC to figure out where your child's BMI is based on his or her age and BMI value.

Why is a BMI value tracking not helpful and in children?
If you
follow the BMI graph, you will find that as children grow, their
height and weight distribution does not have a linear progression as in
adults. This is partially due to continued height growth and increased muscle gain. The graph almost looks like a NIKE swoosh logo, where between
ages 2 to 5 there's a slight dip, and then rapidly accelerates into children's school years. Therefore, in children, we, pediatricians track their BMI %
rather than a calculated BMI #.
To find the %, for a 5 year old girl
with BMI of 17, first follow the age at the bottom (X axis) and then intersect with BMI of 17 (Y axis  on left and right side of the graph). This intersection represented by the blue dot places her on the 3rd red thick line.
Trace this line towards the right, which places her on the 85% for her BMI.
So our 5 year old girl has a 85% for BMI, what does that mean?

source: CDC.gov

Using the classification system for BMI percentiles, now you can find out what 85% means for a given child.
Using the chart to the left, BMI of 85% classifies this 5 year old girl as overweight.

In conclusion, if your child BMI % is greater than 85%, I
would strongly recommend making life style changes first, such as having good
healthy food choices and getting plenty of activities. However if their
BMI % continues to creep up, a visit with your pediatrician then become
highly advised

Let's do another BMI calculation.
This time it is for a 13 year old
boy who's calculated BMI is 25, which is considered at risk for overweight for adults.
As we find his BMI % by intersecting point of his age (xaxis) and his
BMI of 25 (yaxis) we find his BMI to land on the top curve, represented by the red dot.
A BMI of 95% for children is classified not just overweight, but actually obese.
This makes this 13 year old child already obese and I strong encourage aggressive life style changes plus a consultation with your doctor.

