When it comes to assessing the body, there are a few different approaches that are taken to evaluate someone's health status. Some of these include body fat percentage, body weight, lean body mass and BMI, also called body mass index. Body mass index is used to determine the amount of fat someone has in regards to his given height and weight. When it comes to children, the way to calculate this is the same as it is with adults--plug some key numbers into an equation and do the math.
Step 1
Weigh the child. Have the child step on a scale first thing in the morning on an empty stomach and after using the bathroom. This will give you the most accurate reading. You can also have her wear as little clothes as possible to keep the weight more precise.
Step 2
Measure the child's height. Have the child stand up against a wall with his back to it. He can also stand inside an open doorway. Take a pencil and place a little tick mark on the wall at the top of his head. Get out a tape measure and determine his height in inches.
Step 3
Use the equation. The equation for BMI is 703 multiplied by body weight divided by height squared. Here's an example using a girl that is 4-foot-2 and weighs 51 pounds:
(703 x 51) / (50 x 50) = a BMI of 14.34.
Step 4
Use the metric system equation. In this case, take the weight in kilograms and divide it by height in meters squared. To figure out weight, divide by 2.2, and to determine height, multiply by .0254, as this is the equivalent of an inch. Here's an example using a boy that is 4-foot-6 and weighs 75 pounds:
75/2.2 = 34kg
54 x .0254 = 1.37
34/ (1.37 x 1.37) = a BMI of 18.1
Step 5
Use a computer calculator. The Centers for Disease Control has a calculator that can be used online to figure out body mass index. Just punch in the child's information that is requested and the BMI will come up in a few seconds (References).
Step 6
Compare your results. When it comes to children, there are percentiles that determine their status based on their age and gender, which differs from that of adults. If a child is in the less than 5th percentile, they are considered underweight, if they are in the 5th to less than 85th percentile, they are considered normal weight, if they are in the 85th to less than 95th, they are considered overweight and if they are at the 95th percentile or above, they would be considered obese (see References).
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