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Baby & child.

22 June 2021

How to fit a car seat: 10 essential checks

Checking your baby or child's car seat regularly will help keep them safe. Follow our tips and advice to install your car seat and check the fit effectively.
Hannah Fox

If you still need help choosing, or are looking to buy a new car seat, read our baby and child car seats reviews.

10 essential child car seat checks

Follow the 10 simple steps below to make sure your child's car seat will keep them as protected as possible.

You can also download our free checklist and keep it handy so you can regularly check your car seat, or take it with you when you go to buy a car seat.

There is a file available for download. (1.02 MB). This file is available for download at .

1. Is the seatbelt or harness secure and untwisted?

If your child car seat is secured with a seatbelt, this must be tight so the seat doesn't move. To test this, try pushing down on the seat where your baby's head would be if the seat is rearward facing, and pulling on the harness if it's forward facing – you shouldn't be able to move the seat. The seatbelt mustn't be twisted anywhere around the seat.

2. If the seat is rearward facing, is the handlebar in the correct position as shown in the instructions?

For some rearward-facing child car seats, the handlebar may need to be in a certain position, usually upright or fully back, as it can offer added 360-degree protection if your vehicle turns over in a crash. Look at your child car seat's instruction manual to find out where yours should be.

3. Is the child car seat sitting squarely and is the headrest in the correct position?

The majority of the base of the child car seat should be flat on the seat, sitting squarely and evenly. It shouldn't be riding up, which can sometimes happen if the seatbelt is too short.

For most child car seats the headrest should be removed so it doesn't touch the seat at all. However, some child car seats need the headrest to be attached – check the instructions to find out what is needed for your specific seat.

4. Is the seatbelt following the correct red or blue route guides?

Not having the seatbelt securely fitted into the blue or red route guides will weaken the seat's protection. Check it’s right – red guides for forward-facing seats, blue guides for rearward-facing seats. Seats that are approved to R129 i-Size regulations tend to have green belt guides.

5. Is the metal end of the seatbelt in the right place?

The metal end of the seatbelt (the bit that goes into the buckle) must not be bending around the child car seat in order to fix it into the buckle. This could cause it to fail in a crash. Only the seatbelt should be in contact with the frame of the child car seat. 

You don't tend to experience this with newer cars, as the buckle is recessed into the adults seats so the metal end of the seatbelt doesn't come into contact with the child car seat.

6. For Isofix seats, do the visual indicators show it is fitted correctly?

Visual indicators should show you that the seat is correctly clipped into the car – for example, sections may turn from red to green. Check the instructions to see what this should look like for your seat.

rear facing red car seat top tether

7. Is your Isofix support leg securely on the floor and is the top tether firmly attached?

If you have an Isofix car seat, you will have either a support leg, a top tether or both. With the support leg, you must ensure it sits firmly on the floor, isn't lifting the seat up and isn't resting on underfloor storage, as this could make it less effective in a crash.

The top tether must go over the back of the seat and clip into the dedicated mounting point – be careful not to attach it to luggage hooks.

8. Have you removed any thick clothes?

Thick clothes such as bulky winter jackets will make the harness or impact shield less effective as it's harder to get either tightly fitted to your child, so take off any thick items they're wearing before putting your child in the seat.

little girl strapped into car seat

9. Are the shoulder pads level with your child’s shoulders and is the harness secure?

The shoulder pads of the harness should be as level with your child's shoulders as possible - if they're in a rearward-facing child car seat, the shoulder pads should be dropping down no more than an inch, and in a forward-facing seat, raising no more than an inch. 

The harness shouldn't be too tight or loose – make sure you can get two fingers between the child’s collar bone and the harness, and that there is no slackness.

10. For older children, is the seatbelt across their shoulder and hips?

If you have an older child strapped into a car seat with the seatbelt, this should sit on their shoulder. It shouldn't cut into their neck or sit lower down on their arm. The lap part of the seatbelt should sit across their hips, not their tummy.

View our best baby and child car seats to see which came out top in our tough crash tests.

Common fitting problems and how to fix them

Making sure your car seat is fitted properly is vital to keep your baby or child safe.

Typical fitting issues that could affect the protection your car seat provides include:

  • Twisted seatbelts - Check it's long enough to secure the child car seat, and make sure you can feed the seat belt through the slits in the child car seat without obstructions.
  • Unstable car seat positioning - If fitted properly, your car seat should have minimal forward or sideways movement. When you open the buckle of the adult belt, the car seat should spring upward slightly.
  • Loose harnesses - No bulky clothes that could interfere with the harness and make sure it's tight enough that you can just get two fingers underneath.
  • Not all Isofix points connected or they’re hard to attach - Watch online videos and consult the instruction manual and manufacturer for advice.
  • Seatbelts sitting incorrectly on your child - See our gallery below to find out how your seatbelt should be positioned to be effective in a crash.

We’d always recommend checking the instruction manual and contacting the car seat manufacturer if you’re having issues fitting your car seat correctly.

You can also take it to a car seat checking event, where a car seat expert can get hands on with your car and your car seat.

Nine signs your car seat is fitted wrongly

Our gallery below highlights some of the common problems that show a child car seat is wrong for your child or your car.

1. Adult seatbelt under the child's arm

When a child is seated in a booster car seat, the adult seatbelt should come across the child's shoulder and lap.

In the pictures below the seatbelt is under the armpit. In a car crash the seatbelt would fail to reduce movement in the upper body, which could lead to injury.

adult seat belt under arm

2. Incorrect position of adult seatbelt at neck

In the pictures below, the position of the adult seatbelt indicates the child car seat is unsuitable. The seatbelt should come across the child's shoulder and chest, but the seatbelt cuts across the neck.

The belt should be positioned across the collarbone, which is stronger, to reduce the risk of injuries.

seat belt on neck

3. Adult seatbelt not on shoulder

A suitable child car seat would place the adult seatbelt across the child's shoulder, rather than over the upper arm (as in the first and third image), or not touching the shoulder at all (as in the second image).

These seatbelt positions reveal these child car seats are unsuitable because the child may not be held in properly by the seatbelt in a crash.

4. Child car seat harness too loose

A child car seat harness should fit snuggly across your child's body, with space for two fingers between the harness and the child.

Some child car seats have harnesses to act as restraints. For a harness to work well, it needs to fit snugly across your child's body – you should be able to fit just two fingers between the harness and your child's body.

In our images, there's too much space between the child and the harness, leaving room for the shoulders to slip out and the child would be thrown too far forwards in a collision.

child car seat harness too loose

5. Incorrect position of adult seatbelt across the tummy

With a suitable child car seat, the adult seatbelt should come across the child's lap. This is because a child's pelvis is not as strong as an adult's and can't absorb the force of a crash.

If the seatbelt sits across the child's tummy, as in the images below, the seatbelt could crush the soft internal organs, including the stomach and liver, and cause serious internal injuries.

Incorrect position of adult seatbelt across the tummy

6. Adult seatbelt too slack

A child could be thrown out of the seat in a crash if the seatbelt or harness isn’t correctly adjusted. It's important that the adult seatbelt is tightly fitted so that it securely keeps the child and car seat in place.

In the first image below, the seatbelt being slack across the lap could mean too much force is applied to the chest by the seat belt, crushing the lungs. Or the child car seat could move in a collision.

In the second image, the belt is so slack across the child's shoulder that it wouldn't stop her moving forward during a crash.

Always make sure the seatbelt is properly clicked into its buckle and that harnesses are secured tightly.

Adult seat belt too slack

7. Adult seatbelt twisted

If a seatbelt is twisted, it won't react as it should in a collision and it may in fact fail to work altogether. As you can see from our images below, sometimes seatbelt twisting can be obvious or subtle.

When fitting your car seat, make sure that the seatbelts run smoothly and securely through any fittings. The same applies if the car seat uses the seatbelt to hold your child in place.

twisted seat belt

8. Poor positioning of adult seatbelt

Sometimes the problems with fitting a child car seat can be quite difficult to notice. In these two examples, there are different problems with the fitting of the seats that commonly occur.

In the first picture below, you can see the car seat is unsuitable for the child because of the position of the adult seatbelt. Although the belt may have been correctly routed, for this child it wouldn't give any protection as it does not secure her over the strongest parts of her body (her shoulder, across her chest and across her lap). She's simply too small for this kind of seat.

In the second picture, the problem with the seatbelt mainly occurs because the seatbelt comes over the arm rest of the child car seat (just before it's buckled). This part of the seatbelt should be underneath the arm rest to better secure the child in the best position to protect her.

Poor positioning of adult seat belt

9. Buckle crunch

Positioning the car’s seatbelt buckle hard up against the child seat frame can cause the buckle to fail under crash conditions.

Only the seatbelt webbing should be in contact with the frame of the child car seat. If the buckle of the adult belt lies across the frame of the child seat, pressure on the buckle (in an accident, or even under sharp braking) could cause the buckle to fail.

If it fails, the buckle is likely to open, allowing your baby or child to be projected out of the seat, completely unrestrained.

buckle crunch

For more expert safety advice, see our guide to car seat accessories.