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Isofix child car seats

What are Isofix child car seats?

By Olivia Howes

Article 3 of 3

Learn all you need to know about Isofix, the child car seat fitting system that comes as standard in all new cars and lots of older models, too.

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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. In Spring 2014 we asked 42 major stores from all the major baby product retailers to fit two car seats. Most made mistakes, some of them worrying ones.

90% of stores we investigated failed to fit our test car seats correctly.

Using an Isofix child car seat removes the potential to make many of these mistakes.

To find the Isofix car seats Which? recommends, visit our Best Buy car seats.

What is Isofix?

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.

Top tether

A top tether is a fabric strap that is used to secure the car seat to a tether anchor point in the car. It helps to absorb some of the force of a crash and stops the car seat pivoting forwards.

You do need to be careful that the anchor point you find is definitely meant for a top tether, and is not a headrest stem or luggage tether fixing. These are not meant for use with a child car seat and are not designed to withstand the forces of a crash. All cars manufactured since November 2012 have to have top tether anchor points but some much older cars will have them, too.

Isofix support leg

A support leg braces the car seat against the floor of the car, preventing the seat from tipping forward in a crash. You must make sure the support leg is correctly adjusted for the height of the seat. You'll usually find there is a visual or audible indicator that lets you know when the leg is set to the right length.

Look out for underfloor storage compartments in your car. The lid of an underfloor storage compartment will not be strong enough to support a car seat in a crash. Ask your car manufacturer what it recommends before using an Isofix seat with a support leg.


Isofit applies to group 2/3 seats only. They use the Isofix anchorage points in the car's back seat to tether the seat to the car but the adult seat belt is still used to secure your child. 

The main advantage of this is that you don't have to remember to strap the child car seat in when your child isn't in it. It's very important to do this if you don't have an Isofit seat as an unsecured seat becomes a very heavy projectile in an accident. 

Isofix connectors on a group 2/3 seat are only really there to help keep the seat in place when it's not being used, rather than as a load path for the crash forces.

Find out how Isofix seats score in our child car seats reviews.

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Do all Isofix seats fit in Isofix cars?

Ah if only life were that simple! No, unless you have an i-Size seat and i-Size car (for more information on i-Size please read our i-Size advice guide) then you will still need to check whether a car seat fits your car (and any other cars it might be used in). Most of the main manufacturers have a fit finder, which is a good starting point, but it's still really important to actually try out the car seat in your car before you buy.

Visit our guide on how to fit a child car seat for tips and checklists.

Do all cars have Isofix?

No. The first car to have Isofix was the Volkswagen Golf IV in 1997.

Since 2004 it has become more widely offered and now all new cars have Isofix on at least some of the back seats. Top tether anchor points have come as standard since the end of 2012.