What Is i-Size? – Car Seat Regulations Explained

Britax Dualfix i-Size installed in car

ECE R44 vs. ECE R129 (i-Size)

Child car seats must undergo accident simulation testing to determine whether or not a seat is safe enough to be sold. There are currently two regulations; ECE R44/04 and since 2015, ECE R129 (also known as i-Size). These are the safety standards that car seats are rated against.

‘i-Size’ is the most recent safety regulation controlling the manufacture of children’s car seats (ECE R129). It was introduced in July 2013. The intention was to make child car seats easier to fit, provide better protection from side impacts and keep children rearward-facing for longer.

R129, i-Size car seats are the new standard of car seat safety and force the industry to adhere to the highest of safety standards. R44/04 is an older and frankly, out of date set of safety standards. It’s being phased out by a new wave of i-Size car seats.

R44/04 vs. R129 (i-Size) Comparison Table

There are key differences between the regulations. Below we have condensed the differences between the two regulations into a table to make it simpler to understand.

ECE R44/04

ECE R129 (i-Size)

Classification based on child’s weight (in kg)

Classification based on child’s height (in cm)

Categorised by groups (Group 0+, 1, 2, 3)

                                         Child must rear-face to a minimum of 15 months

Child can be forward-facing from 9kg

                                               Rear, front and side impact testing

Can be used for seatbelt or ISOFIX seats

                                          Q dummy used in crash tests with 32 sensors

Rear and front impact testing

                                          Can be used for seatbelt or ISOFIX seats

P dummy used in crash tests with 4 sensors



Our Best-Selling i-Size Seats

The Main Aims of i-Size

Regulation ECE R129, also referred to as i-Size, is an evolving regulation.  Above you will see the differences between it and the previous regulation ECE R44/04. I-Size was developed in order to improve child seat safety and below you will find the main aims of these regulations.

At a glance, the main aims of this regulation include:

  • Better Compatibility – as all I-Size car seats will fit all I-Size seating positions in any certified vehicles.
  • Easier Installation – reducing the risk of misuse and improper installation due to seats being secured by ISOFIX.
  • Promote Rear-Facing – it is mandatory to keep I-Size seats rear-facing until the child is a minimum of 15 months old.
  • Make Classifications Easier – by measuring children according to their height rather than their weight.
  • Make Seats Safer – as they are now subjected to side-impact testing

As you can see, there are many reasons why we love i-Size at In Car Safety Centre, not least because of the requirement for rear facing. So we hope you will too.

Are i-Size car seats safer?

Absolutely. In addition to their increased compatibility, easier installation and rear-facing setups, they are side-impact tested.

Believe it or not, side-impact testing is not a legal requirement for R44 car seats (although some manufactures do side-impact tests anyway). With i-Size seats, you can rest assured that all seats pass rigorous side-impact tests before being awarded the R129 mark of approval.

Is i-Size compatible with my car?

The benefit of i-Size approved seating positions is that i-Size car seats are automatically compatible. There’s no need to even check! Checking compatibility is nevertheless recommended.

If your car does not have the latest i-Size regulated adult passenger seats, don’t worry! You can still use i-Size children’s car seats in a standard ISOFIX car, however, you must always check the vehicle compatibility list first. As our compatibility guide suggests, we always recommend coming to visit us in-store so we can correctly fit your car seat and check it is fully compatible.

Are i-Size seats easy to install?

Yes – more so than most car seats. Currently, they must be installed via the ISOFIX mounting system which creates a strong, robust connection to the bodywork. This makes the seat extra-sturdy and mitigates the risk of incorrect installation in the vehicle.

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

To see if your car seat is i-Size, check for this label:

i-size label

The label is usually found underneath or the back, of the plastic elements of the seat. This label shouldn’t be removed for any reason. It will always state the word ‘i-Size’.

Which i-Size car seat do you recommend?

There are now many seats that are ‘approved’ to the new i-Size (R-129) Regulation. The correct seat will be the one that is the most suitable for your child, fits the vehicle(s) it is to be used in and is ‘approved’ for use in those vehicles. Your child is unique, seats have differences, physically checking suitability is by far the best way of ensuring your child as safe as is possible to achieve.

Are i-Size seats a legal requirement?

Currently, R44/04 and R129 co-exist, but R129 will soon be the legal standard (although a date is still to be confirmed). However, we believe there is no sense in waiting for the law to catch up with the research and safety data, all of which conclusively demonstrate the effectiveness of seats manufactured to this new standard

What does the law say, about i-Size?

  • In the United Kingdom, children must be secured with a suitable restraint up until 12 years of age or 135cm in height.
  • However, elsewhere in Europe, including the Republic of Ireland the minimum height is 150cm.
  • Your Child must travel rearward facing until at least 15 month old.
  • Your child may not use a booster cushion (i.e. a cushion without a back) until they weigh 23kgs

The In-Car Safety Centre will always advise parents to max out the height of the seat their child is currently in, before moving them to the next stage of seat. Of course, if the child is too heavy, they should be moved also. This is because a seat that is too big can be just as ineffective as one that is too small.

Our Motto – Fit Right, Sit Tight, Max out the Height!

First Right, Sit Tight, Max out the Height

Above you will see our company motto. This is to be used as an easy reminder for choosing the right car seat and knowing when to change it.

  1. “Fit Right” refers to how the car seat is fitted into your car and whether or not it is compatible.
  2. “Sit Tight” refers to how well the seat is installed, and how securely your child is in it.
  3. “Max Out the Height” means that there is no rush to move to your next car seat, and you should keep your child in their current seat until they have maxed out the height/weight.

Follow these steps to make sure your child is in the right seat. If you are unsure, contact us or visit any of our centres for assistance.

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The Main Aims of I Size R4404 vs R129 I Size


Remember – stay safe 🙂