Car seat weight groups and height groups explained

Child car seats are grouped either according to your child's height or your child's weight. They are not selected according to your child's age.

Car seats based on height are i-Size car seats, complying with regulation ECE R129. This is a more stringent regulation, first introduced in 2013, and is applicable to the majority of car seats these days. 

Car seats based on weight are group seats conforming to regulation ECE R44.03 or ECE R44.04, which is an older (although still legal) regulation. 

Keep reading to find out what you need to know about each type of car seat.

Best baby or child car seats - jump straight to our top picks from testing, plus find out the features to look out for and how much you're likely to spend

i-Size car seat height groups at a glance

Here are the height measurements if you're considering an i-Size car seat:

• i-Size baby - 40cm to 87cm (0 to around 15 or 18 months)
• i-Size baby to toddler - 40cm to 105cm (0 months to 4 years)
• i-Size toddler to child - 61cm to 105cm (12 months to 4 years)
• i-Size baby to child - 40cm to 150cm (0 months to 12 years)
• i-Size child - 100cm to 150cm (4 years - 12 years) 

It's worth noting that this is how Which? classifies the height ranges for its reviews. 

Some manufacturers and retailers may choose to name seats differently and the height limits for different seats can also vary between brands. The most important factor to consider is always your child's height.

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What is i-Size and how is it linked to R129?

The idea behind i-Size is that all car seats will fit in all cars. Your car and your car seat must have Isofix, a fitting system that attaches car seats directly to the frame of your car using Isofix connectors. Read our guide to Isofix baby car seats for more information on this.

New cars that come on the market should be i-Size ready but you should always check the list of approved vehicles that fit the car seat you wish to buy and check the fitting in your car.

Technically, i-Size is part of the R129 regulations and all i-Size car seats must comply with these regulations. However, there are rare occasions where R129 seats are not i-Size compatible (because they're installed via the vehicle seatbelt).

Looking to buy a car with i-Size compatibility? See our guide on where to buy a car for the pros and cons of all your car-buying options.

Is an i-Size car seat safer?

One of the big improvements of R129/i-Size regulations is a reduced risk of fitting your child car seat incorrectly. Generally, Isofix seats are simpler to fit than ones that use the vehicle’s seatbelt.

Another key benefit of R129/i-Size is the introduction of a side-impact crash test as part of the approval process. Child car seats approved to the older R44 regulation do not have to pass a side-impact crash test in order to be sold.

Finally, R129 regulations for i-Size infant carrier car seats state that children must remain rearward-facing until they're 15 months, as it's safer. We see more extended rearward-facing car seats approved to R129 regulations that encourage parents to keep their child facing backwards until they're four years old.

We've tested and reviewed i-Size baby, i-Size baby to toddler, i-Size toddler to child and i-Size child seats.

Key differences between the old regulation (R44/04) and R129 (i-Size) child car seats

In a February 2024 online survey of 2,735 parents with a child under five, when asked whether they knew the difference between a car seat approved to Regulation 129 (R129) and one approved to Regulation 44 (R44), we discovered the following:

• 14% knew what it was as it was explained to them by the car seat retailer
• 20% knew what it was because they researched the regulation themselves
• 55% did not know what the difference was and it was not explained to them during the car seat purchase process
• 11% answered 'Don't know' to this question

i-Size baby car seats

Baby car seats are usually the first car seat you'll buy and are rearward-facing only. 

They are smaller and lighter than the car seats you'll use as your child gets older and many are designed so that you can also attach them to a pushchair chassis to form a travel system.

i-Size baby car seats tend to be approved for babies measuring between 40cm or 45cm (birth) to between 75cm to 87cm (12 to 15 months). 

They are installed into your seat by clicking the seat onto an Isofix base, although some also have the option of being installed using the vehicle seatbelt.

Read our i-Size baby car seat reviews.

i-Size toddler car seats

The i-Size toddler car seat group is large and tends to cover a wide range of height options.

Baby to toddler seats are usually approved from 40cm to 105cm (0 months to 4 years), and many are also extended rearward-facing for some if not all of this period, which is the safest way to travel.

Look for swivel car seats that can turn either 180 or 360 degrees in this size group, as they give you the freedom to turn the car seat to face towards the vehicle door, which makes it much easier to get them into and out of the seat. 

Some toddler seats go from 61cm to 105cm (12 months to 4 years). To help distinguish them from the 'from birth' models, we've called this sub-group i-Size toddler to child.

Almost all seats will be installed using Isofix connectors on the seat and either a support leg or a top tether strap.

Read our i-Size toddler car seat reviews.

i-Size baby to child car seats

This type of seat is similar to the multi-group seats found in R44 car seats because it covers a wide span of heights. 

This is typically 40cm (birth) to at least 115cm (around six years old) but can be as high as 125cm (eight years) or even 145cm (10 years).

These seats are installed using Isofix and either a support leg or top tether strap.

A multi-group child car seat may seem 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. But our testing has found that a seat that covers more than one group doesn't always protect as well as it should throughout all three groups.

Read our i-Size baby to child car seat reviews.

i-Size child car seats

An i-Size child car seat is typically approved for children measuring between 100cm and 150cm, which is around four to 12 years old. 

Although UK law states that children must be in a car seat until they're 135cm or 12 years old (whichever comes first), many i-Size baby-to-child seats can be used up to 150cm, as that is the law in some European countries.

i-Size child car seats are high-backed booster seats, with your child restrained using the vehicle seatbelt but with the seat clicked into the Isofix points.

Read our i-Size child car seat reviews.

Child car seat weight groups at a glance

Below we've rounded up all of the available weight groups for child car seats approved to R44 regulations:

• 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)

Baby car seats (Group 0+)

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

Read our group 0+ car seat reviews

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 from 9kg (approximately nine months). 

We do not recommend you turn your baby forward-facing as early as this, and if you choose a group 0+/1 car seat, you should keep them rearward-facing for as long as possible.

Read our group 0+/1 car seat reviews.

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 seatbelt correctly on the child and could injure their neck and internal organs in a crash.

Read our group 0+/1/2 car seat reviews.

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.

Read our group 1 car seat reviews.

Group 2/3 car seats

A group 2/3 high-backed booster seat will last your child from 15-36kg. That's from about three 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.

Read our group 2/3 car seat reviews.

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

Group 1/2/3 seats are used from nine months to 12 years (9-36kg), while group 0+/1/2/3 are from birth to 12 years.

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. 

As previously mentioned, our car seat experts think you're probably better off buying a dedicated baby car seat, then a toddler seat and then a child car seat, rather than choosing a multi-group or combination seat.

Read our group 1/2/3 car seat reviews and multi-group car seat reviews.

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.

The rules for using booster seats changed in March 2017. 

• 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 between 15kg and 36kg. 
• Both seats are legal to use as long as you follow what the label says. 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.

Find out more about booster seat rules in our guide to car seats for older children: high-back vs backless booster seats.

What are the rules for selling and buying R44 car seats now?

In September 2023, the European Union banned the sale of new R44 car seats and completely replaced the R44 regulation so that R129 is the only legal regulation.

Since the United Kingdom left the European Union, it is no longer beholden to the rules of the EU. This means that the UK can continue selling models approved to these older regulations.

At Which? we believe car seats approved to R129 regulations are a better option, and for the last year or so, have only been testing these types of seats, and have not carried out testing on any R44 seats. 

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 buy a new seat until you've had the maximum usage out of your current seat. 

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

As you can see in the image above, it's time to swap this boy's car seat. A child will have outgrown a toddler or 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 height or weight of each group are more vulnerable to injuries. This is why we don't advise you swap too soon. 

The recommended heights and weights for each car seat group often overlap, so let your child reach the top limit for one group rather than swapping when they reach the bottom limit of the next.

If you're ready to update your child's seat, see our round-up of the best child car seats.