A baby's weight and height percentiles indicate how he relates to other babies of the same gender and age. For example, a three-month-old baby boy in the 40th percentile is heavier than 40 percent of other boys that are three months old. Boys and girls are measured on different charts because their development stages are different. To figure percentile, measure the baby's weight, height and head circumference. Understanding how a baby's percentile is figured is important to monitor his healthy development.
Determine your baby's weight. Pediatric scales are the most accurate in weighing a baby. Weigh your baby nude. If you do not have a pediatric scale, you can use a regular bathroom scale. Step on the scale yourself and document your weight. Afterwards, pick up your naked baby and step back on the scale. Write this number down. Subtract the first weight from the second weight and you will have an accurate weight for your baby.
Figure your baby's height. Lie your baby down on a flat surface. Mark with a pencil where the head is. Gently stretch out your baby's legs and mark where the heels come to. Measure the length in inches or centimeters from one mark to the next.
Calculate your baby's head circumference, which is the largest part of the baby's head. Place the end of the tape measure above the eyebrows and the ears and continue on around the head. Document in centimeters or inches where the tape measure comes together.
Record the results in an online growth calculator. A number of free online services will calculate your baby's height and weight percentile. They will ask for age, sex, height, weight and head circumference. These calculators include conversion charts from pounds to kilograms and inches to centimeters. Input your baby's measurements and select "Calculate," "Get Results" or similar button on the calculator you are using.
Consult with your baby's pediatrician if any of the results measure less than the 5th percentile or more than the 95th percentile. A pediatrician can confirm whether the baby's growth is within a normal range.