Seat belt or ISOfix?
A car seat is installed using the adult seat belt or the car's ISOfix points. ISOfix is the international standard for attachment points for child seats in cars. These two metal brackets which are located 28cm apart at the base of the vehicle seat's backrest mean that the car seat or base can be clicked into the car without using the seat belt.
ISOfix was first introduced in 1997, but it took another ten years or so for it to become standard in most cars. The two cars that were used for the photographs on this page are a Ford Focus from 2008 which does not have ISOfix points and a 2003 Toyota Estima which does.
ISOfix points are approved for a combined child and seat weight of 33kg.
R44/04 ISOfix car seats all weigh about 15kg, which means they all have a child limit of 18kg. In i-Size seats up to 105cm the weight limit is set by deducting the weight of the seat from the 33kg total approved weight, so a seat that is lighter than 15kg will have a higher weight limit.
Because ISOfix seats have an 18kg limit, car seats with a five-point harness up to 25kg are always belt fitted. Rear facing seats up to 25kg have tether straps which are attached to the runner rails of the front seat, you don't need special points in your car to use one of those, they will fit in every car. But forward facing seats with a 25kg weight limit (there are currently only three models available in the UK), require a top tether point in the car, and a few forward facing seats up to 18kg do too. These small metal brackets with an anchor symbol are either located on the back of the vehicle seat's backrest, or the floor of the boot. All cars from 2013 onwards have top tether points, and you will find them in some older cars too.