If you plan to buy pants online you will want to make sure you buy the correct size. Adult and youth jean sizes can be determined by a person's height, waist and hip measurements. However, if you are buying clothes for a newborn or a young child, you will need to estimate pants sizes using height and weight. With these measurements in mind, you can find out which U.S. standard size the child is by using a child's clothing measurement chart.
Stand the child up on a hard surface. Make sure he does not have shoes on and is standing in his normal posture. With a tape measure, measure his height from his head down to his feet. If the child is a newborn and cannot stand, measure him lying on his back. Record this measurement.
Take any heavy clothing and shoes off of the child and set her on a scale. Record the child's weight.
Convert the child's height and weight measurement to a U.S. standard clothing size using a conversion chart like the one on Overstock.com or other clothing seller (see Resources). Measure clothing by height and weight rather than by age because not all babies and young children are the same size. If the child is taller and/or weighs more than the kid's size XL then you will need to find pants sizes for her by measuring her height, waist and hips.