How do I know when to transition my child to booster mode?

Children should use the built-in harness system for as long as possible until they reach the maximum forward-facing weight or height for the car seat (22-65 lb and up to 49 inches). At that point, transition to booster mode, which supports children from 40-100 lb, 43-57 inches tall, and 4+ years old. A booster is no longer needed once the vehicle seat belt fits properly, typically when the child reaches 4 feet 9 inches tall and is between ages 8-12.