Check school backpack weight against guidelines.
School backpack guidance is expressed as a share of the child's body weight rather than an absolute limit, because a load that is trivial for a fifteen-year-old is substantial for an eight-year-old. Ten percent is the commonly cited ceiling, with some guidance allowing up to fifteen for older students carrying a well-fitted pack. Dividing pack weight by body weight gives the figure to compare, and the excess tells you how much has to come out.
Relative load
Pack share = backpack weight / body weight x 100
Guideline maximum
Safe maximum = body weight x guideline percent / 100; excess = pack weight - safe maximum
General guidance only, not medical or safety advice. Persistent pain, injury or a specific health condition needs assessment by a qualified professional.
Loads beyond roughly ten to fifteen percent of body weight measurably change posture and gait in children, increasing forward trunk lean. The lower figure gives margin for poorly fitted packs.
Considerably. Both straps used, the pack sitting against the upper back rather than below the waist, and heavy items packed closest to the spine all reduce the strain a given weight causes.