The first thing you must do to calculate your baby BMI, or body mass index, is convert your baby's weight to kilograms. To properly do this, take your child's weight as a decimal and divide it by 2.2. For example, your baby may weigh 17 pounds, 4 ounces. You will use the number 17.25 and divide it by 2.2 to end up with 7.84 kilograms. Write this number down and then move on to converting your baby’s height.
Once your baby’s weight has been converted to kilograms, you can convert her height into meters. To do this, you will need to multiply her height in inches by .0254. For example, if your baby is 17.5 inches, you will multiply 17.5 by .0254. This will mean your baby is .4445 meters. Your resulting number may be longer, but you only need four numbers after the decimal point—rounding is not necessary.
After finding the weight in kilograms and height in meters, the next step is to multiply the height in meters by itself. In the example above, you would multiply .4445 by .4445, and the resulting number would be .1975 meters, which is your baby’s height squared. The baby BMI is then found by dividing the weight by the height squared. If the weight of 7.84 kilograms is divided by .1975 meters, then the baby BMI would be about 39.69.
Once you have the BMI number, you can then look it up on the body mass index by age chart for either your baby boy or girl. This will give you a very good idea whether your baby’s growth is on track or if the infant is over- or underweight. Remember that every baby is going to grow at a different pace, and a number which may be a little out of range is something to talk with you doctor with, but not panic over. Calculating a baby BMI is a task that can be done at home on a regular basis as you continue to track your infant's growth.
Children's BMIs are different than adults, something to keep in mind as your child grows. Generally, a specific formula is used for children and adolescents from 2 to 20 years of age. As children grow, their amount of body fat will change.