What is ISOFIX?

ISOFIX is an international standard system that allows you to fit a car seat directly into a vehicle without using a seat belt. Designed in the 1990s it provides a quick, easy and safe way of installing a child's chair.


Also known by the very catchy title of the International Standards Organisation FIX, it didn’t become widely used until 2006, when the law changed and required all new cars to be manufactured with ISOFIX fittings.

Keeping in line with the recent i-Size law on child car seats, manufacturers will be making more and more ISOFIX car seats and eventually (although no one is sure on the exact date) there will no longer be non-ISOFIX infant car seats.

It’s worth noting that the ISOFIX law covers Group 0+ and Group 1 chairs, which is from birth to 4 years. While Group 2/3 (4-12 years) car seats are governed by a system called ISOFIT (we've got more on this below).

How does ISOFIX work?

ISOFIX car seats have two arms or connectors on the back of the seat that hook, or anchor onto small metal bars fixed to the frame of your vehicle.

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You’ll find the points where the back seat meets the seat cushion. By pushing the seat backwards onto these anchor points, the seat locks directly onto the chassis of your vehicle.

Is there just one type of ISOFIX?

No. Confusingly, there are different ISOFIX systems in circulation. Some connect with two points and some connect with three and have additional tethers. This can make it difficult to find the right car seat that’ll fit your car.

However, most major manufacturers have websites where you can check your car’s compatibility, so we’d recommend you check out the maker’s suitability guide before you buy.

Is there much difference between the systems?

A little, all systems use metals connectors to lock the seat in place and all ISOFIX car seats should have the logo (above) in the manual or on the seat itself. However, the most recent systems have additional fixing points above or behind the seat, known as top tethers.

A top tether is a belt that secures the child seat to a tether point behind the rear car seat, usually at the back of the rear seat, boot floor, or on the ceiling of the car. From 2013 top tether points became mandatory in all new vehicle models.

Many new seats (particularly infant chairs) also come with an in-car base that uses a supporting leg, which is secured on the floor of the car.

These additional fixings prevent tilting and rotation (without these, in a crash situation the car seat may sink into the seat cushion or lurch forward, which is potentially damaging to your child’s neck and spine).

What’s the difference between ISOFIX and ISOFIT?

Both systems connect your child’s chair to the frame of your vehicle using metal connectors, but ISOFIX applies to all Group 0/0+ and Group 1 car seats, while ISOFIT applies to Group 2/3 chairs.

With ISOFIT you secure your little one in the seat using car’s 3-point seat belt instead of the chair’s 5-point harness.

How does ISOFIX differ from i-Size?

Firstly, ISOFIX is an international standard, which is voluntary and i-Size is a mandatory law, which means you don't have to use ISOFIX, but you will have to follow the i-Size law if it applies to you and your family.

Both cover Group 0+ and Group 1 child car seats, but ISOFIX is a way of fitting a child seat into a car, whereas i-Size is a law the regulates how you fit the seat into the car, the position children must face when sitting in the car the age range the child must be to sit in the seat, among other things.

The i-Size law also requires car seats to have:

  • Added side-impact protection
  • Be tested with new Q type crash dummies
  • ISOFIX connectors

The three approved forms of ISOFIX car seats:

  • Universal – Uses either a top tether or foot prop, in addition to the connectors, to fix the seat to an additional anchorage point for forward-facing seats. Universal ISOFIX car seats should fit into most newer cars.
  • Semi Universal – Uses a top tether, foot prop and connectors but is not compatible with all cars. Can be used with forward and rearward-facing (i.e lie-flat) car seats.
  • Vehicle Specific - Uses only the anchor point to secure the seat. This method is vehicle specific and is only compatible with certain vehicles. Always check the suitability of the make/model of the chair on the manufacturer’s website.

There are also several different size classes of ISOFIX (see image above) seat that take into account the different sizes of car - a full height forward facing car seat might not fit into the back of a Mini, so you’d need to shop for a reduced height seat.

The positive points of ISOFIX

ISOFIX scores highly in the safety stakes. By plugging your car seat onto your vehicle you’re reducing the chance of fitting it incorrectly.

A study of Group 1 seats, conducted by the German Insurance Institute (GDV), found that 96% of seats fitted using ISOFIX were installed properly. In comparison, the same study found that only 30% of car seats were fitted correctly when the adult seat belt was used. This is because the seat belt anchor points vary in size, length and position from car to car, so creating a seat that fits perfectly in every car is virtually impossible.

ISOFIX is also a quick and simple way of getting the car seat in and out and makes light work of using a travel system.

The downsides of ISOFIX

Not all ISOFIX car seats will fit in every car, so do your research before buying. This poses problems for two-car families who swap car seats between cars.

And ISOFIX car seats don’t come cheap - you may end up paying double the price of a non-ISOFIX model.

If you’re about to buy a car seat for you baby, toddler or child, our buyer’s guide to car seats will take you step by step through finding the right one.


We've got more car seat safety information here...


Hazelann WilliamsFormer Reviews Editor

Having been a journalist for 15 years – and Reviews Editor at MadeForMums for five of those – Hazelann has accrued a lot of experience testing and reviewing every baby product imaginable.