Industry standard for baby apparel is to use age as the size, but it is important to measure your child as it is possible to have an older child that still needs a smaller size, or conversely a younger child that needs to size up. If your child falls into two different sizes between weight and length then it is best to size up to ensure a better fit. Please note that baby apparel usually accommodates extra room for a diaper. Measurements are represented with a diaper on.