Child car seats are organised into types, or groups, according to the weight of the infant and it's illegal for your child to be in the wrong type of seat for their weight.
It's also vital that your child is in the correct seat for their size or they could be inadequately protected and possibly at risk from the seat itself if, for example, the harness were to press on the wrong part of their body in an accident.
The official child seat groups are as follows:
Group 0 newborn to 10kg
Group 0+ newborn to 13kg
Group 1 children 9-18kg
Group 2 children 15-25kg
Group 3 children 22-36kg
However, in the UK, car seats that are only in group 2 or group 3 are no longer made, and there is currently just one infant carrier in Group 0 (a type of carry-cot made by Britax). Instead, many seats cover more than one group so you can use them for longer. So, in reality, you can currently choose from the following seat types:
Group 0+ newborn to 13kg (baby seats)
Sometimes called infant carriers, these are rear-facing seats with an integrated three- or five-point harness and a handle so you can carry the baby in the seat outside of the car. Rear-facing seats provide the best protection for a baby's head, neck and spine in a front impact, so avoid moving your baby into a forward-facing Group 1 seat any earlier than necessary.
Group 1 children weighing 9-18kg (toddler seats)
These are chunky, predominantly forward-facing, car seats with an integrated five-point harness. There are some rear-facing models available, providing the benefits of a Group 0+ seat for longer (in a front impact, the child is simply forced against the back of the seat). However, rear-facing models are generally more complicated to install, so there's a higher risk of poor fitment, which could compromise its effectiveness.
Group 0+-1 newborn to 18kg (baby/toddler combination)
Combination seats can be positioned rear-facing for use with babies up to 13kg, then re-configured as a forward-facing seat to use as a regular Group 1 seat for toddlers (9-18kg).
Group 1-2-3 Children weighing 9-25kg (extendable combination seat)
These are slightly bulkier forward-facing seats designed to be used first as a Group 1 seat with five-point integrated harness. As your child grows, the seat back can be extended upwards and the harness removed to create a Group 2-3 seat using the car's own seatbelt to secure the child together with the seat.
Group 2-3 Children weighing 15-36kg (high-backed seat/booster)
Group 2-3 seats come in two forms a high-backed seat (or booster seat), or a separate seat base (booster cushion). Both use the car's seatbelt to secure child and seat.
The high-backed seat puts your child at the right height to use an adult seatbelt without risk of slipping underneath it or being injured by it in an accident. Guides on the shoulder area and seat base also position the seatbelt safely. Some models allow you to remove the seat back to use the seat base on its own.
The separate seat base puts your child at the right height to use the adult seat but, since it doesn't have a back, it provides no support or protection to any part of the head in an accident. Also, it can only help with the positioning of the lap belt, not the diagonal part of the seatbelt.
How to check your child's weight
A set of bathroom scales is a perfectly good way to check your child's weight. For babies or very young children, check the difference between your weight with and without the child in your arms. You need to move up to the next group when your child exceeds the weight limit stated on the seas.
It's also safest to move your child into the next type of seat once their head is noticeably higher than the top of their current seat, otherwise they are not properly benefiting from the side and rear head supports. In extreme cases the seat itself could cause injury in an accident. Some overlap in the weight limits for each seat group makes early upgrades possible for taller children.
Besides choosing the right type of seat for your child, you also want to choose the right seat for your lifestyle.
If you're likely to be regularly swapping the seat between two cars, look for one that is relatively light. Also consider ISOFIX fitments, which allow you to clip it quickly into the car rather than using the car's seatbelts to secure it.
Many babies and toddlers fall asleep in the car, so you might want a seat that can be reclined simply by pulling a handle on the front, leaving your child undisturbed. Many baby seats also come with integrated hoods or removable sunblinds.