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Baby & child.

Updated: 28 Feb 2022

I-Size child car seats explained

What are i-Size child car seats? How are they different from other car seats? Are they safer for my child? Here's all you need to know.
Hannah Fox
Boy in i-Size child car seat

All car seats are approved to a regulation, meaning they must pass certain tests before they can go on sale. 

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

In this guide we focus on car seats based on height, which are called i-Size car seats, explained in more detail below. To learn more about weight-based groupings such as groups 0+ and 1/2/3 car seats, go to car seat weight group explained.

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 - Newborn to 85cm (0 to around 15 or 18 months)
  • i-Size baby and toddler - Newborn to 105cm (0 to 4 years)
  • i-Size toddler and child - 61cm to 105cm (15 months to 4 years)
  • i-Size child - 100cm to 135cm (4 years - 12 years)
Sleeping baby in child car seat

What is i-Size?

The new European standard for child car seats, known as i-Size, forms part of the UN regulation ECE R129.

The idea behind i-Size is that all car seats will eventually 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. At the moment, even if your car has the anchor points, an Isofix car seat won't necessarily fit: i-Size aims to change this. Find out how Isofix car seats get on in our crash tests.

More and more new cars that come on the market should be i-Size ready. However at the moment there are very few available. For now, you can buy an i-Size seat, but you'll need to check the list of approved vehicles that fit the seat and check the fitting in your car.

Is an i-Size car seat safer?

One of the big improvements that R129/i-Size should bring about 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 seat belt.

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. 

All car seats reviewed by Which?, regardless of which rules they are approved too, are put through a frontal impact crash test and a side impact crash test.

We've tested and reviewed i-Size baby, i-Size baby to toddler, i-Size child and i-Size toddler to child seats. See how safe they and compare the results of our tested models with our baby and child car seat reviews.

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

R44/04R129 (i-Size)
Frontal and rear-impact tests but NO side-impact testsFrontal, rear and side-impact tests
Seat can fixed to car using seat belt or isofixIsofix-only fixing
Baby must be rear-facing until 9kg (approx 10 months)Baby must stay rear-facing until 15 months
Classification of seat based on weightClassification of seat based on height

When to stop using a rear facing car seat

i-Size car seats, which are part of  R129,  makes it mandatory to keep your baby rearward-facing until they are at least 15 months old.

Rearward-facing travel is advisable until this age or longer because of the relative size of babies’ heads compared with their bodies, and the weakness of their neck muscles. In a frontal impact, in a forward-facing seat, the forces placed on a baby’s neck by their head can be too great to withstand. But in a frontal impact in a rear-facing seat, the child is pushed further into the seat, supporting the head and back and limiting the movement of the head on the neck.

Some i-Size/R129 seats can be used rearward-facing until your baby reaches 105cm, which is around four years of age.

Check out our guide to the best rear-facing baby car seats, and read more on child car seat laws in the UK and more specifically the law on booster seats.

How are i-Size car seat groups measured?

The weight of your child will no longer be the main reason to swap your child to the next-stage car seat, which is how the old system of weight group classification works. Instead your child's height determines which seat they need. This can vary from seat to seat. Your child will outgrow the seat once they exceed the maximum height stated on the car seat’s label.  

If you're not sure how tall your child is, look at the labels on their clothes. You’ll probably find a height band printed somewhere on them. But you'll need to measure your child properly to make sure you are buying the right seat for their size. Some child car seat retailers have height charts in store.

I-size logo

The difference between i-Size and Isofix

i-Size/R129 currently requires the use of Isofix to fit your car seat, not seat belts. However, many i-Size car seats do still have the option of installing using the vehicle seat belt. 

Isofix is designed to make installing your car seat quick and easy. We know from our research that parents and retailers regularly make errors when fitting seats into the car. Using an Isofix child car seat removes the potential to make many of these mistakes.

Isofix is a child car seat attachment system which uses metal bar connectors built into the chassis of the car to connect to the child car seat. The connectors are often hidden within the car's seat padding.

Once the connectors are clicked together with these anchorage points, the child car seat is secured by a third point, either a support leg which comes built-in into the seat or seat base, or a top tether (a strap that attaches to a mount somewhere behind the rear seat). Both of these work to stop the car seat tipping forward in an accident.

i-Size car seats fit all i-size-ready vehicles. 

Do I need to buy a new car seat?

At the moment, i-Size/R129 is running in parallel with the old regulation, ECE R44/04. Originally this was expected to continue until at least 2018, but as the date that ECE R44/04 is totally phased out is not fixed, or confirmed, and car seats are still being approved to the older regulation, we can't see this happening any time soon.

This means that while there are some i-Size seats on the market, you don't have to buy a new car seat, unless you want to.  

All car seats display a label that tells you they comply with a relevant regulation. As more and more i-Size/R129 seats come on the market, you'll find car seats that display either the i-size/ECE R129 label or the ECE R44/04.

I-size child car sear concord-reverso with mother and girl

How do I know if a car seat is i-Size?

You'll see the logo above. Many of the models of i-Size seat will also have an 'i' in their name or include the word 'i-Size' in their name, and lots claim compliance to i-Size or R129 in the marketing bumph as a selling point. 

Why has i-Size/R129 been created?

The old regulation has been in place since the 1980s. Child car seats – and cars – have come a long way since then in terms of the safety they offer children. The new i-Size tests are more up to date.

For a long time Which? has been putting pressure on the industry to improve their car seats by carrying out tests that are far more stringent than the old standard. The new i-Size test has incorporated some of the aspects Which? has been testing for years, including a side-impact test. Read more about how Which? makes child car seats safer.

Side-impact accidents make up around 25% of collisions, but the old regulation doesn't test how a child car seat and its occupant will react in this type of accident.

The new tests also use more technologically advanced dummies, known as the Q-series, which behave far more like real people in a crash scenario, and have more sensors in them than the previous crash test dummies used.

At the moment the main i-Size/R129 seats you'll see for sale are baby car seats and toddler car seats that can be used from birth up to 105cm (approximately four years old), but we're now starting to see more R129 compliant car seats for older children up to age 12.