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How to buy the right child car seat

From booster seats to Isofix: car seats jargon explained

By Lisa Galliers

Article 2 of 4

Isofix, i-Size, buckle crunch, booster seats, bases... Car seats are confusing. Our essential A-Z will guide you through everything you need to know.

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Baby car seat

An infant carrier, or a baby car seat, is the first car seat you'll buy. Also known as a Group 0+ car seat, or i-Size baby car seats. These car seats are rearward-facing only and are designed to cocoon your baby, protecting their head, bones and internal organs from the forces of a crash.

Before you buy your child car seat, make sure it passes our tough crash tests. We test each car seat in a front crash and a side-impact crash, in tests harsher than the standards call for.

Best baby car seatssee which models passed our tough crash tests

Backless booster seats

A backless booster seat provides such minimal protection for your child's head or side in a crash that we do not recommend using them, especially with younger children.  A backless booster seat can seem like a cheap and easy option and they are perfectly legal to use, but the serious risk of injury far outweighs any convenience factors.  

Watch the backless booster seat crash test video to see what could happen.

We'd recommend using a high-backed booster seat. If your current high-backed booster seat can be converted to a booster cushion, we don't recommend doing this.

Belt guides

Belt guides are coloured indicators on each car seat to help you install it correctly when using the car's adult seat belt to attach it to your car.

It's vital you get the belt routing correct, as the belt guiding is designed to direct the force of a crash away from your child's body, to reduce the risk of injury. 

There are two different types, depending on what stage of seat you are fitting: 

Belt guides - blue

Car seats that are installed rearward-facing, using the car's adult seatbelt, have blue belt guides to show you where the seatbelt should be threaded to hold the seat securely. 

Clear instructions must be printed on the side of each car seat seat to help you, and we'd always recommend using this as well as watching any manufacturer videos on how to install your car seat correctly.

Belt guides - red

Forward-facing car seats have red indicators to show you where the adult seat belt should be routed to hold the child and the car seat in place correctly.

There is usually one indicator near the headrest to position the diagonal belt across the shoulder properly, and others near the seat that hold the belt correctly across the lap.


The design of the buckle on all European car seats is guided by regulations. The harness on your car seat must have a single buckle that will only lock when all of the parts are fitted together properly. It cannot be left partially closed.  

The release button must be red, easy to see, and easy to unlock by a single operation. This could be vital in an emergency.

This does mean, however, that some particularly strong and determined toddlers may be able to work out how to open the car seat buckle. 

Our advice would be to get children used to the fact, from an early age, that a car seat is a safety device and not to play with the buckle, although this is easier said than done, we know!


Car seat bases

Do you need a base for your car seat? Some Group 0+ baby car seats come with the option to buy a base. This base is separate and is left semi-permanently installed in your car.

A base will cost extra, but it's much easier to simply click your car seat to the base when you're nipping out in the car, rather than having to belt the car seat into the car each time, and our tests have found that some seats are safer when used with a base. It also means you only have to fit the base once into your car and can then leave it in there. 



Safety tip: Always check your base is installed correctly.

Some bases can be fixed to your car using the adult seat belt, or can be attached via Isofix connectors. Some bases can be attached by either method, which is useful if you currently have an older car (without Isofix) but are thinking of upgrading sometime soon. 

Some bases can be used with both rear-facing Group 0+ car seats and forward-facing or rearward-facing Group 1 seats.

Some i-Size baby car seats can come with bases integrated into the seat, which means they remain permanently installed in your car, so you may need a from-birth pushchair or a baby sling to help when taking your baby out of the car.

Carry handles

Did you know that the carry handles on your infant carrier aren't there simply to make it easy to carry your little one? They are actually a safety feature. 

They contribute to the protection and stability of your child seat during a crash. Read the instruction manual to find out exactly how the carry handles need to be positioned when the car seat is in your car, to make sure you're protecting your baby as much as possible.


Harness – three-point

A three-point harness, commonly found on car seats for infants, has a crotch strap and two shoulder straps that join at a buckle over the child's pelvis.

Harness – five-point

A five-point harness, commonly used on Group 1 seats, helps spread the force of a crash across your toddler's body, which should reduce injury – if it's fitted correctly. This type of harness also helps keep the shoulder straps in place. A five-point harness has a crotch strap, two shoulder straps and two side straps.

Harness – height adjusters

A correctly fitted harness is vital to help keep your child as safe as possible in a crash. Make sure you check it regularly as your child grows. 

When your child starts to grow, lengthen the harness and adjust its height so that it's the right height to hold your child safely and comfortably.

On some seats there are slots in the plastic of the seat shell. You can feed the straps through these slots manually to change their height. Not only is this a faff, it also greatly increases the risk of you putting the harness back incorrectly and therefore reducing the crash protection that the seat should offer.

Look out for seats where the headrest and the harness are connected, so pulling the headrest up automatically adjusts the harness height to fit correctly, which means less risk of getting it wrong. 

Check your car seat fits correctly with our free downloadable guide: 10 simple car seat fitting checks

Harness - one-pull tightening

Getting a wriggling child into a harness and tightening the straps properly can be a bit of a challenge. Look out for a one-pull strap which allows you to pull both shoulders of the harness tight at the same time.

You'll need to release the straps with a button or handle before you can loosen them.

Harness - padding

Soft shoulder pads stop the straps from digging into the child's shoulders and rubbing against their face. They can also help to spread the forces of a crash across the chest. Check your seat's instructions to make sure these accessories are in the right place when using the car seat.

Look for harness pads that are easily removable to help with cleaning them up after any sick-related incidents.

Head rests

On Group 1 and Group 2/3 seats, look for padded headrests that can be adjusted as your child grows. Remember, it's UK law that a child needs to be in a child car seat until they reach 12 years of age or 135cm tall, so check that the seat you choose has a headrest that will adjust enough to last that long.

Some high-backed booster seats also have side padding that can be adjusted if your child grows broader as well as taller.



Most new cars built since 2002 should have Isofix connectors fitted as standard. These connectors are designed to make installing your car seat quick and easy. 

Isofix in your car can be identified by two slots beside each rear seat. Some slots are easily identifiable, and quite accessible, but others are a bit hidden, so check your car manual or have a look in the back if you're unsure. 

On Isofix car seats there are two metal connectors that fit into these slots and directly attach your car seat to the chassis of your car.

Isofix car seatssee our guide for more essential information


i-Size is part of the new European child car seat regulations (ECE R129), the start of which was introduced in 2013 and came into force in the UK in April 2015. 

It's mandatory with i-Size (R129) seats to keep your baby rearward-facing until they are at least 15 months old. The car seats are categorised by height, rather than the weight categorisation that the current grouping system uses. 

All i-Size car seats use Isofix to connect to your car, and undergo more testing than seats approved under the R44.04 regulation (the older regulation). 

How we test child car seatssee why and how we make our child car seat tests tougher.


Newborn accessories

Baby car seats, or infant carriers as they're also known, are the first car seat you'll buy. They can come with seat inserts or head-huggers especially designed for newborn babies, to make sure they will be supported and fit into the car seat properly.

If your car seat comes with one of these accessories, make sure you use it according to the instructions. If this accessory is missing when your seat arrives, contact the retailer before using the car seat and ask for a replacement.

Without these accessories, it's highly likely that the harness won't fit tightly enough around your newborn, and it's possibly that a wriggly baby could free themselves from the harness.  


Support leg 

Rear-facing Isofix child car seats need three anchor points.

The seat you choose will need either a top tether that attaches to a mount behind the rear seat, or a support leg that reaches down to the floor of your car.

If your car doesn't have a top tether point, you'll need a child car seat with a support leg.

A support leg extends from your child's car seat to the floor of the car and prevents the seat from tipping forwards in a collision. A support leg is easier to use than a top tether (see below).

But if your car has underfloor storage compartments in the rear, you may not be able to use a car seat or a base that has a support leg, and will need a top tether instead.


Top tether 

A top tether is a strap that extends from your car seat and fixes to a point in your car that's usually somewhere behind your car's back seat or sometimes near the boot.

Like a support leg, this prevents your child car seat from rotating forwards in a crash, and can be used if you can't use a seat with a support leg, because your car has underfloor storage compartments in the back.

The choice of third fixing point – top tether or support leg (see above) – will depend on your car, so check your car manual or contact your car manufacturer before choosing a seat.

Travel system-compatible car seats

Some baby car seats can be popped onto the chassis of a pushchair, usually with adaptors, so that you can easily transfer your child from pushchair to car without disturbing them too much. This arrangement is called a travel system.

It's vital to remember that a baby car seat should only be used for transporting your baby in the car. it's not a place for him or her to sleep in for a long period. Experts currently advise no longer than 90 minutes in a baby carrier, but this reduces down to no longer then 30 minutes for newborn babies in the first four weeks of their life.

The car seat will either:

  • click on over the existing pushchair's seat – common with pushchairs that have forward-facing seats
  • attach to the chassis instead of the pushchair's seat – common on pushchairs with reversible seat units.

Sometimes the car seat attaches directly to the pushchair, or you may need to buy adaptors to make it fit.

Many leading brands of child car seats, such as Britax, Joie or Maxi-Cosi, can be used with a wide range of pushchairs.



Safety tip: avoid using a baby car seat, in the first four weeks of a baby's life, for periods of more than 30 minutes.

Quickly find the best travel system for your baby in our 10 best travel systems