By age — The whole first year, staged

Baby Costs by Age: 0 to 12+ Months, Stage by StageA baby's bill is a curve, not a constant. Pick your stage for its real numbers — changes per day, model month, and the buys and skips that belong to those weeks only.

Advice fails when it ignores the calendar. "Stockpile diapers" is dangerous at two weeks old and smart at five months. "Pouches are fine" is true at six months and expensive at ten. Every stage below carries its own changes-per-day, its own model month, and its own short lists — because that is how the money actually moves.