Quick answer
Height and weight matter, but the real decision combines state law, product limits, and seat belt fit.
There is no single national number
Parents often search for one booster seat height and weight requirement. The problem is that requirements come from several places. State laws may use age, height, or weight. Booster manufacturers set minimum and maximum product limits. Safety guidance focuses on belt fit. You need all three.
The 4'9" benchmark
A common height benchmark is 4 feet 9 inches, or 57 inches. Several state laws or agency recommendations use it, and AAP/HealthyChildren also points to 4'9" as a common point where vehicle belts may start fitting. But 4'9" is not a guarantee. Vehicle seat shape and belt geometry can change the answer.
Weight matters differently
Weight is most important when checking car seat and booster labels. A child leaving a forward-facing harness should have outgrown that harness by height or weight. A child entering a booster must fit within the booster model's allowed range. There is no one universal booster weight that replaces the manual.
Use this practical order
First check your state law. Second check the forward-facing harness limits. Third check the booster label. Fourth do the seat belt fit test. If those answers conflict, do not use the weakest answer as the final answer. Use the safest setup that is legal and fits the child.
Before you decide
Do not make this decision from one number on a search result. Check the child restraint label, the car seat manual, the vehicle manual, and your state law. Then test the setup with the child actually sitting in the vehicle. A booster or seat belt that looks fine in theory can fit poorly in a specific back seat because belt anchors, seat depth, buckle stalks, and head restraints vary by vehicle.
If this is a legal question, start with your state page. If this is a safety question, start with the core guide linked below. The safest answer is usually the one that satisfies the law, matches the product instructions, and keeps the belt or harness positioned correctly for the full ride.
When to get hands-on help
Get help from a certified Child Passenger Safety Technician if the belt crosses the neck or face, the lap belt sits on the stomach, the booster slides around, the child cannot stay positioned, the vehicle only has a lap belt in the seating position you want to use, or you are unsure whether a forward-facing seat has been outgrown. A short in-person check can catch problems that are hard to see from a product photo or a general article.
Helpful next steps
Sources used for this topic
This article follows the same source hierarchy as the core guides: NHTSA, CDC, AAP/HealthyChildren, IIHS, federal standards, and state law pages. Check the linked core guides for the source list and state-specific notes.