Body Mass Index- BMI

Dear Lisa,

My son is 100 pounds. He is 9 years. What is the right weight for him?

“Percentile for Weight”,

Dear “Percentile for Weight”,

Children’s weight and height are plotted on grids to determine their percentiles for age. (1) In order to determine your child’s percentile for weight, his weight is plotted against his age on a growth chart. Physicians and Healthcare providers typically use growth charts to monitor a child's height and weight and screen for potential abnormalities. The purpose of determining your child’s percentile is to compare his weight or height to the established norms for his age.(2)

Growth charts are helpful when a pattern is obtained over time. One isolated measurement of height or weight is not sufficient to make a determination about your child’s health. (2) Growth patterns that move from one percentile to the next, or cross over percentile lines offer more information and are usually the first sign of a problem. For example if a child was consistently at the 75th percentile for weight and then dropped to the 25th percentile, this can be a sign of a medical, social or emotional problem. (1)

A boy who is 100 pounds and 9 years old is above the 100th percentile for weight for his age. What this means is, if you put him in a room with 100 boys his age he would weight more than each individual in the room. Looking at this reading independently would imply that your son is overweight. But this assumption cannot be made without more information.

Your son’s height would need to be taken into consideration. For example there are some very large children whose height and weight are both above the 100th percentile for age. In this scenario the above average percentile for weight is congruent with the above average percentile for height and does not mean that the child is overweight. On the other hand, if your son is in the 25th percentile for height and above the 100th percentile for weight this would be concerning and would warrant further investigation.

It would also be important to know if your son has gradually increased to this point over time. Following a trend over time is more clinically significant than one isolated reading. If your son has always been above the 100th percentile for weight this may also be normal for him. A better way to determine if a child is overweight is to determine the Body Mass Index (BMI) or measure skin fold thickness. (1)

The BMI is also known as the Quetelet’s index is a commonly used tool to assess obesity in children. (3) It can be compared with normal data based on age, sex and race. (1) The BMI does not overestimate body fatness in short individuals or underestimate it in tall individuals.

In order to calculate a child’s BMI the child’s weight in pounds is divided by the height in inches. That number is then divided by the height in inches again. This answer is then multiplied by 703. The answer that you get is your child’s BMI.

When working with Kilograms, the child’s weight in kilograms is divided by the square of the height in meters. (3)

For example: A teenager who is 5 foot 4 inches tall and weighs 140 pounds:
140 (pounds) divided by 64 (inches) = 2.1875
2.1875 divided by 64 (inches) = 0.034
0.034 X 703 = 23.9, rounded to a BMI of 24

Or

140 pounds = 63.5 Kilograms
5 foot 4 inches = 1.625 meters
Meters squared = (1.625 X 1.625) = 2.641
63.5 (Kilograms) divided by 2.641 (Meters squared) = 24.04, rounded to a BMI of 24

Some people prefer to use a BMI calculator. The BMI calculator calculates your child's BMI automatically. All you have to do is input your child’s height and weight and the BMI will be calculated for you.

Once you determine the BMI, the number should then be plotted on a BMI Growth Chart for Children according to their gender. (3) Overweight or Obesity is defined as BMI at or above the 95th percentile. BMI between 85th and 95th percentiles are considered at risk for obesity. (3)

You can also use the following chart as a general guideline to interpret your child’s BMI:

A BMI of
18.5 to 24.9 is considered normal
25 to 29.9 means a person is overweight
30 to 34.9 is “class 1” Obesity
35 to 39.9 is “class 2” Obesity
Above 40 is “class 3” or severe Obesity

Another accurate way of determining if your child is obese is determining skin fold thickness. Skin fold thickness correlates highly with other measures of body fat and is more accurate than physical assessment or weight for height in diagnosing obesity. (1,3) Skin fold thickness can be evaluated by a Dietician or Physician who specializes in obesity.

To access the tools discussed click on the following links:
Girls Growth Chart for Height and Weight

Boys Growth Chart for Height and Weight

Girls BMI Growth Chart

Boys BMI Growth Chart

BMI Calculator

References:
(1)Schwartz M, Charney E, Curry T, Ludwig S. Pediatric Primary Care. A Problem Oriented Approach. 2nd Ed. Littleton, Mass:Year Book Medical Publishers, Inc. 1990:81,59,114.
(2)Betz C, Hunsberger M, Wright S. Family-Centered Nursing Care of Children. 2nd ed. Philadelphia, PA:W.B.Saunders Company. 1994:72.
(3)Someshwar J, Someshwar S, Perkins K. The Obese Adolescent. Pediatric Annals. 2006.35(3):181-186.


Lisa-ann Kelly R.N., P.N.P.,C.
Certified Pediatric Nurse Practitioner

Pediatric Advice For Healthy Kids