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Baby & child.

Updated: 28 Mar 2022

Car seat weight groups explained

Car seat groups explained – everything from what a Group 0, 1, 2 or 3 car seat is, to what kind of car seat your baby or five year old child should be in.
Which?Editorial team
Child-car-seat 2

Child car seats are bought in two ways, either according to your child's weight or according to your child's height. They are not selected according to your child's age.

Car seats based on weight are group seats conforming to regulation ECE R44.03 or ECE R44.04, which is explained below. 

Whereas car seats based on height are i-Size car seats, complying with regulation ECE R129. Go to i-Size child car seats explained to find out more about height-based i-Size car seats.

Read more on child car seat laws in the UK and child car seat laws around the world.

Child car seat weight groups at a glance

Below we've rounded up all of the available weight groups for child car seats:

  • Group 0/0+  – Newborn to 13kg (0 to 15 months)
  • Group 0+/1 – Newborn to 18kg (0 to around 4 years)
  • Group 0+/1/2 – Newborn to 25kg (0 to around 7 years)
  • Group 0+/1/2/3 – Newborn to 36kg (0 to around 12 years)
  • Group 1 – 9kg to 18kg (9 months to 4 years)
  • Group 1/2/3 – 9kg to 36kg (9 months to 12 years)
  • Group 2/3 – 15kg to 36kg (4 years to 12 years)
  • Group 3 – 22kg to 36kg (from 5 or 6 years and more than 125cm tall)

It's important to bear in mind that moving babies and younger children up to the next seat up before they reach the maximum weight or height for their seat could lead to more severe injuries in a crash. 

Babies and children can vary wildly in terms of size, so its better to use height or weight as a guide for what car seat they should be in, rather than age.

Always make sure your car seat fits your car and your child, and get it fitted by a trained expert before you buy.

Car seats that span several groups, for example Group 1/2/3, may seem like great value for money as they last for longer than single group seats, but our testing has uncovered issues with some of them.

Here is the range of child car seat groups and the corresponding weight ranges in a chart format:

Baby car seats (Group 0+)

Baby car seats (group 0+) are usually the first car seat you'll buy. They are smaller and lighter than the car seats you'll use as your child gets older. 

They are also rearward facing – so your baby will be travelling with his or her back to you when you're driving. 

Research shows it's safest for babies to travel rearward-facing for as long as possible.

Group 0+ car seats will last your baby from birth until he or she is around 12 to 15 months old, with a maximum weight of 13kg for most baby car seats.

Some group 0+ baby car seats are designed so that you can also attach them to a pushchair chassis to form a travel system.  

This means that you can take your baby from car to pushchair without having to disturb them too much. 

Our advice is to avoid keeping babies, particularly very young ones, in car seats any longer than absolutely necessary and never for sleeping inside the house.

Group 0+ car seat reviews

Joie brand spin child car seat

Group 0+/1 car seats

Group 0+/1 car seats will last your baby from birth to around four years old. Your baby will initially travel in the seat rearward facing and you can change the seat to face forwards. 

Some seats allow this from 9kg (approximately nine months) but we do not recommend you turn your baby forward-facing too soon. 

You should leave your baby in a rear-facing child car seat until they are least 15 months old. 

Read more about extended rear-facing child car seats

Many Group 0+/1 car seats let your child to travel rearward facing until up to four years old.

Unlike a baby car seat (Group 0+) a Group 0+/1 car seat will stay fixed in your vehicle. You won't be able to take it out and pop it onto a travel system chassis. 

Because they are larger than a Group 0+ car seat this type of seat may take up more space in your car.

Group 0+/1 car seat reviews

Diono Radian child car seat

Group 0+/1/2 car seats

These are designed to last a child from birth to 25kg (approximately six years old). But we believe children change too much from birth to 25kg for one seat shell to adequately provide the protection they need at each stage. 

We've tested some very poor Group 0+/1/2 child car seats, which fail to position the adult seat belt correctly on the child and could injure their neck and internal organs in a crash.

Group 0+/1/2 car seat reviews

child car seat group 1

Group 1 car seats

Group 1 car seats are for toddlers weighing between 9kg and 18kg. That's from about nine months old up to around four-and-a-half years old. 

Many car seats on sale that are Group 1 are forward facing, although you can find some that let your child travel rearward facing until he or she is up to four years old.

Group 1 car seat reviews.

child in high back booster seat

Group 2/3 car seats

If your child is ready to move up from a Group 1 seat the next seat up is a Group 2/3 high-backed booster seat.  

Group 2/3 car seats are high-backed booster seats. 

A group 2/3 car seat will last your child from 15-36kg. That's from about 3 years to 12 years.

Most group 2/3 child car seats don't have a harness. Instead you'll secure your child and the seat using the car's adult seatbelt. 

Some models can still be installed using Isofix to keep them securely attached to the car. 

Some parents worry about moving their child from a Group 1 seat, which uses a harness, to a Group 2/3 child car seat using a seat belt, however there are a few seats available which have extended harness use, or other methods to help a child transition from a harness to a belted seat. 

Group 2/3 car seat reviews.

Multi-group or Group 1/2/3 car seats

A multi-group child car seat spans more than one group, so it should last your child longer than a single-group seat. Some parents may find them appealing, as it means only having to choose and pay for one child car seat, which could last from birth or 12 months up to 12 years of age. 

You might see multi-group seats called combination seats, and some retailers list them as either Group 1/2/3 seats, which are used from nine months to 12 years (9-36kg) or Group 0+/1/2/3 which is birth to 12 years.

Car seats approved to the newer R129 regulations that are also multi-group are rarer, although you may find some retailers define extended rear-facing car seats (that take your child from birth to four years) as multi-group. One i-Size multi-group model that we've recently tested is the Axxkid One+ car seat which is suitable for babies measuring 40cm to 125cm, or birth to around six years old.

Multi-group car seats allow you to secure your baby in a five-point safety harness initially, then the seat converts to a high-backed booster seat when your child is older and bigger. 

These car seats can appear to be a more economical way of buying a child car seat, as they will last your child for longer, but our testing has found that a seat which covers more than one group doesn't protect as well as it should throughout all three groups. 

Children change too much for one seat shell to adequately provide the protection they need at each stage.

Our car seat experts think you're probably better off buying a dedicated baby car seat, then a toddler seat and then a Group 2/3 child car seat, rather than choosing a multi-group, or combination, seat.

Some multi-group seats we've tested don't score highly for a number of reasons, including failing to position the adult seatbelt correctly on the child, or being difficult to convert between the groups, or failing to sufficiently protect the passenger when used in a particular group or mode. 

A good car seat should be able to protect your child well enough in both front and side crashes, as you can't tell what type of accident you may be involved in. 

And as we expect parents to use a child car seat in all the modes and groups it's designed to cover, we limit the score to the worst-case scenario, even it the seat has scored well in other modes.

Group 1/2/3 car seat reviews and Multi-group car seat reviews.

booster seat group 3

Backless booster seats aka group 3 car seats

Some Group 2/3 car seats convert to a group 3 seat, which is a backless booster seat, also known as a booster cushion. You can also buy booster cushions separately.

Booster seats raise the child's body to a height suitable for use with the adult seatbelt. 

A booster cushion satisfies the legal requirement for children up to 1.35 metres (approximately 4ft 6in) and they're cheap (about £6-£30), but we don't recommend them, especially for younger children.

That's because booster cushions are not as safe as a high-backed booster seat with a full-length back and 'wings', which provide extra protection for the head and chest in a side-impact crash.

It's worth paying a bit more for extra protection.

We've tested one Group 3 car seat, read our review of the MiFold Grab and Go.

dad measuring the height of a child

From 1 March 2017 the rules for using booster seats have changed. 

  • Any newly approved backless booster seat on sale will be limited for use only with children taller than 125cm and weighing more than 22kg.  
  • But you might still see other backless booster seats on sale approved for use for children of between 15kg and 36kg. 
  • Both seats are legal to use as long as you are following what it says on the label. So always make sure you check the label before you buy. 
  • If you already own a backless booster seat you can still use it, as long as you're following the age/height on the label.

More about booster seat rules

Isofix connectors

Which car seat groups have Isofix?

You can find Isofix child car seats in all car seat groups from Group 0+ up to Group 2/3 car seats. 

You'll often see Isofix in Group 2/3 car seats referred to as Isofit. That's because there is a difference. 

In Group 2/3 car seats, even though the car seat is attached to the car using the same connectors, your child is kept in the car by the car's seat belt. In the lower car seat groups, the Isofix car seat is attached to the car using Isofix connectors but your child is kept in the seat by a harness that's attached to the car seat.  

Are Isofix baby car seats safer?

How do you know when it's time to change car seats?

It's better and safer to keep your child in the lowest group seat for as long as possible, rather than moving up groups too early. It's also more economical, too, as you won't have to splash out on a new seat until you've had the maximum usage out of your current seat. 

Do check what the upper weight or height limits are for your car seat and keep an eye on your baby or child's weight and height, to make sure they aren't out-growing the seat too quickly.

Older child booster seat
As you can see in the image of the little boy above, it's time to swap his car seat. A child will have outgrown a Group 1, 2 or 3 child car seat if his or her eyes are level with the top of the seat when sitting in it.

Don’t forget to check if they’re too tall for the car seat harness – the top of it should sit 2cm above their shoulder – even when it’s adjusted to the highest position.

In most cases, a child will reach the weight limit of a child car seat before becoming too tall for it, but all babies and children are different. 

Children at the bottom weight of each group are more vulnerable to injuries, this is why we don't advise you swap too soon. 

The recommended weights and heights for each group overlap, so we advise you to let your child reach the top limit for one group rather than swapping when they reach the bottom weight of the next.