Stage 3: Booster Seat


What is a booster seat? Child passengers ride facing the front of the vehicle in a belt-positioning booster seat together with a lap/shoulder seat belt. Boosters may also referred to as Stage 3.



Why do children ride in booster seats? For children who are old enough, heavy enough, and mature enough to no longer need a 5-point harness, the booster seat positions the adult seat belt on the strong parts of the body. Children this size need a booster seat because they are still too small to safely fit the adult seat belt.

Cars, car seats, and booster seats are designed to best protect passengers in the most common and most frequently fatal types of crashes: frontal or front offset (front corner) crashes.

Best practice is to keep a child in a booster seat until they are tall enough to safely ride in just the adult seat belt, and to not move a child into a booster seat until they are old enough, heavy enough, and mature enough to safely use a booster seat instead of a 5-point harness.

If your child has outgrown their 5-point harness by standing height, torso height, or weight, but is not heavy enough or mature enough for a booster, look for a taller, higher-weight harnessed seat.

Bare minimums to move into a booster seat or move out of a booster seat depend on the province, in addition to how the seat is labeled. Some require higher weight, height, or age minimums than others to begin use. Some children are ready at age 5, many at age 6, and most at age 7.

Height is most important for proper fit of the adult seat belt and kids may start to fit the adult seat belt around 4’9″ (145 cm).


Who MUST ride in a booster seat? Children who are too small to safely fit the adult seat belt, but have outgrown a 5-point harness must use a booster seat. Children who are not yet old enough, heavy enough, or mature enough to safely use a booster seat must continue to use a higher capacity 5-point harness forward facing.

Who MAY use a booster seat?  Provincial rules and individual seat requirements vary, but generally anyone who is 40 lbs (18 kg) or more, and/or over four years of age, may use a booster seat.

Almost all kids in almost all seats can safely ride forward facing with a 5-point harness until at least age 5, and we strongly recommend children remain harnessed until at least age 5-6. Is your child ready to use a booster seat? Reading this may help you to decide.



Booster seat modes are available on different types of seats.

Combination: Combination car seats are also called child/booster seats or stage 2/3 seats. This is because they combine two modes: forward-facing with a 5-point harness, and later on a belt-positioning booster. Sometimes these are called harnessed boosters, but that term refers to two very different mechanisms and is not a term we use. See our favourites here.

3-in-1: 3-in-1s might be called a multi-mode seat, all-in-ones, infant/child/booster seats, or stage 1/2/3 seats. They tend to have three modes, including rear facing, forward facing, and booster. Some manufacturers differentiate between rear facing for infants and rear facing for toddlers, and may also differentiate between high back boosters and backless boosters. Phew – that’s a lot to wade through, we know. We group our favourites together with convertibles and you can see them here.

Dedicated Booster Seats: Dedicated boosters don’t have any other mode and there is no 5-point harness. Sometimes they are called Stage 3 seats. Booster seats may have a high back, sometimes abbreviated as HBB, or may be called backless or no back boosters. Sometimes they are called low back boosters, or LBBs, but there is no back, and so we do not use this term. A long time ago they were called booster cushions. See our favourites here.

Booster seats with UAS: Some booster seats, and 3-in-1s or combination seats in booster mode, offer the option to install with lower anchors, or UAS. In this case the lower anchors are securing the booster seat to the car for convenience and stability when climbing in and out, but it is the seat belt that restrains the child. Using lower anchors is not required, and is not possible in all seating positions in all vehicles.


No really, how?

Booster seats rely on the adult seat belt to safely restrain a child, and to do its job, the seat belt must fit correctly on the strongest parts of the child’s body. The child must be mature enough to stay properly seated for the whole ride.

Booster seats must always be used with a lap/shoulder seat belt. The lap belt should be low and flat across the upper thighs/pelvis, never riding up on the belly; the shoulder belt should sit across the collarbone and touching the chest, not riding up on the neck or slipping off the shoulder.

Children new to boosters should start with a high back booster. High back boosters give smaller, inexperienced booster riders a better sense of security because it feels similar to a car seat, provides head support when sleeping or tired, usually provides superior shoulder belt fit, and is required in vehicles without adequate head support.

How the seat belt fits is most important when shopping for a booster seat. Take your booster-ready child shopping with you to see what fits in your car, and what your child finds comfortable and can buckle most easily. Teach them how to properly route the seat belt, and how to tighten it to snug it up and remove slack. If they are wearing a jacket, teach them to unzip it, and to make sure that the lap belt is under the jacket and touching their thighs/pelvis.


I have questions.

How do I get help to know if my child is ready for a booster seat? How do I find out about what booster seat to buy? How do I find someone to help me install my seat?

Please ask us. Or ask another CPST near you. We tend to get to questions on our Facebook page more quickly than anywhere else, but if you don’t do social media please reach out via email to

We would be delighted to help you choose an appropriate seat that fits your family’s needs.