An average weight for a 12-year-old girl is 91.5 pounds. However, KidsHealth states there is no set number as to what a child should weigh.
A healthy weight is determined by the height of the girl. Some girls will be smaller because their parents are smaller, notes KidsHealth. Girls will usually begin growing more rapidly at 12 years old. This is also when they begin hitting puberty. They will reach their final height at around 14 years old. Body mass index is the best way to determine if a girl is a healthy weight for her height, but even that should only be used as a guideline.