ISOFIX is a method of attaching a child seat to a vehicle. It was invented over 20 years ago, and designed to remove the use of the vehicle seat belt.
To use an ISOFIX child seat, you must have ISOFIX points in your car. These are two brackets which are in the vehicle seat - usually the rear seats, but sometimes they are in the front.
An ISOFIX child seat then has two corresponding arms, which click onto the ISOFIX points.
ISOFIX child seats also use an anti rotation device - either a support leg or a top tether. In a crash, these help to reduce movement, and absorb energy.
ISOFIX seats which use support legs are required to have a vehicle compatibility list, as they do not fit with some vehicles. You can generally find these lists on the child seat manufacturers website.
The support leg lowers down to the floor of the vehicle, and has an indicator to show you when it is fitted.
Some ISOFIX child seats use a top tether to keep the seat stable in a frontal impact.
A top tether is a strap that is attached to the child seat, and it hooks onto the top tether point in the car. The top tether point can be anywhere in the vehicle, and may not be marked clearly - so it is always best to find out where the tether points are by checking your vehicle handbook.
Top tether child seats generally do not need a vehicle fitting list, as an ISOFIX and top tether seat is automatically compatible with the ISOFIX and top tether point in the vehicle. However it is still useful to try the seat before you buy it, to ensure you are happy with how it fits into your car.
There are some exceptions to this, such as seats which fit using ISOFIX, Top Tether and the vehicle seat belt - in these cases you must still check the vehicle compatibility list from the child seat manufacturer.
Floor Storage Compartments
If your vehicle has floor storage compartments, it can be very difficult to find an ISOFIX seat with a support leg that is approved.
Some vehicle manufacturers provide a filler for the storage box, so you can use a child seat with a support leg. It is extremely important to only use approved fillers and avoid DIY fillers.
Using an unapproved filler method has no guarantee of working and absorbing energy.
Poorly positioned ISOFIX
On some older vehicles, the ISOFIX can be positioned very low in the vehicle seat cushion. This makes it extremely difficult to access the ISOFIX, and can give a very poor fit for the child seat.
In this case it is worth trying a few different ISOFIX seats, as some may fit better than others. It may also be better to opt for a seat belt fitted child seat.