A car seat for your child is one of the most important purchases you’ll make as a parent. It is also a legal necessity. The UK law states: Children must normally use a child car seat until they’re 12 years old or 135 centimetres tall (150cm in ROI), whichever comes first.

There are two sizing systems to choose from when picking a car seat. You can choose a car seat by using your child’s height (I-Size) or weight (R44). As there are different types of child car seats available, take your time when choosing. This guide should help you understand the different types of child car seats, helping you to decide when purchasing your car seat.

Height-based seats – I-Size

They must be rear-facing until your child is over 15 months old. Your child can use a forward-facing child car seat when they’re over 15 months old. You must check the seat to make sure it’s suitable for the height of your child. Only EU-approved height-based child car seats can be used in the UK. These have a label showing a capital ‘E’ in a circle and ‘R129’.

I-Size car seats are considered to be the safest, with improved head and neck protection compared to car seats that meet the basic ECE R44 standard. Both front and side-impact crash test results are a standard and assessed as part of the I-Size. More advanced Q-series crash test dummies are used, which have more sensors than the dummies that are used for the R44 test (P-series). These seats use an ISOFIX, which attaches the car seat directly to your car’s chassis, with an I-Size car seat. ISOFIX car seats are more likely to be correctly fitted in your vehicle. However, many I-Size car seats do still have the option of installing using the vehicle seat belt.

Weight-based seats – R44

Weight based seats are defined by the UK’s R44 Legislation. Only EU-approved weight-based child car seats can be used in the UK. These have a label showing a capital ‘E’ in a circle and ‘ECE R44’. These are defined by a child’s weight, so you can pick a car seat simply based on the child’s weight. Below is a breakdown of the R44 sizing groups:


Child’s weight

Age of child

0 Birth up to 10kg/22lb Birth up to 6 to 9 months
0+ Birth up to 13kg/29lb Birth up to 12 to 15 months
0+ & 1 Birth up to 18kg/40lb Birth up to 4 years
1 9-18kg / 20-40lb 9 months to 4 years
1 & 2 9-25kg / 20-55lb 9 months to 6 years
2 15-25kg / 33-55lb 4 to 6 years
1, 2 & 3 9-36kg / 20-79lb 9 months to 6 years
2 & 3 15-36kg / 33-79lb 4 to 12 years

You may be able to choose from more than one type of seat in the group for your child’s weight.

Avoid borrowing a car seat and we would recommend not buying one second hand. The safety history of the seat can be unknown therefore you can’t guarantee it hasn’t been involved in a previous accident. These seats could be sold without all the correct or necessary parts needed to ensure this fits your vehicle correctly.