If you're concerned about how heavy your child's backpack is when full, it's worth weighing it to make sure it's of a safe weight. Your child shouldn't be carrying a backpack that weighs any more than 15% of his or her body weight, and even that might be too heavy for some kids. If your child is struggling under the weight of the school backpack, think of some strategies to minimize it, such as leaving books for afternoon classes in the locker and collecting them during lunch break.