When to Use a Booster Seat: UK Safety Guide for Kids

When you’re thinking about switching your child from a car seat to a booster seat, a raised seat that positions a child so the adult seatbelt fits properly. Also known as a high-back booster, it’s not just about age—it’s about height, weight, and safety standards set by UK law. Many parents assume that once a child turns five, they’re ready for a booster. But that’s not always the safest choice. In fact, keeping your child in a five-point harness car seat until they outgrow it—often past age five—is the best way to reduce injury risk in a crash.

The UK car seat laws, the legal requirements for child restraints in vehicles across the United Kingdom say a child must use a booster seat once they’re 15kg (about 33 lbs) or 125cm tall, whichever comes first. But here’s the catch: those are minimums, not recommendations. The safest path? Stay in a harness seat until your child hits the top weight or height limit of that seat—often around 18kg or 105cm. That could mean your 5-year-old still needs a harness, even if they look big for their age. A child car safety, the practice of using approved restraints to protect children during vehicle travel isn’t about rushing to the next stage. It’s about giving them the most protection possible, for as long as possible.

Booster seats come in two main types: high-back and backless. High-back boosters are better for younger kids because they guide the seatbelt across the shoulder and chest correctly, especially if your car’s seat doesn’t have a headrest. Backless ones are fine if your car has a tall, supportive headrest—but only if your child’s ears are below the top of the headrest. If you’re unsure, the high-back version is the safer bet. And don’t forget: no booster seat is safe unless the seatbelt lies flat across the collarbone and hips—not the neck or stomach. A poorly fitted belt can cause serious injury in a crash.

If you’re wondering whether your 5-year-old is ready, ask yourself these three things: Are they tall enough that the seatbelt fits right without the booster? Do they sit still without slouching or twisting? And can they stay seated properly for the whole journey? If any answer is no, they’re not ready. Kids who wiggle, lean, or play with the belt defeat the whole purpose. That’s why many UK parents keep their kids in harness seats until age six or seven—and that’s perfectly normal.

There’s no rush. Every extra month in a five-point harness gives your child more protection. The booster seat isn’t a milestone—it’s a safety upgrade, and you get to decide when the time is right. Below, you’ll find real advice from UK parents who’ve been there, clear breakdowns of the law, and what experts say about when to make the switch. No guesswork. Just what works.