Car Seat Fitting Guide

When you buckle in your most precious cargo, your children, you want to be totally certain of one thing: that they are safe and sound. Luckily, there are plenty of simple steps you can take to minimise the risk of fitting errors and ensure your seat is safe as can be.
Whether you’re using a car seat belt or an ISOFIX system, follow along as we share our top tips — and the most important things to think about — when fitting your child’s car seat.

Fitting a car seat with a seat belt

Fitting a child car seat with a 3-point adult seat belt is the most traditional way of securing a car seat. However, ISOFIX seats (which we’ll cover below) are growing in popularity thanks to the safety and security they offer.

Although it may look a bit daunting, reading the car seat instruction booklet is an essential first step before securing your child’s car seat with a seat belt. We also recommend familiarising yourself with the child safety section of your car manual, which outlines additional information about how to best keep your child safe in the car.

Maxi-Cosi Rock i-Size Car Seat
Maxi-Cosi Rock Car Seat (Essential Black)

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Once you’ve read the instruction manual, you can refer to these tips to make the installation a bit easier:

When placing the car seat in the vehicle, check that it sits squarely and evenly on the base of the seat.

Ensure the seat belt passes through the correct route guides on the car seat. (If you’re installing a rear-facing seat, it must go through the blue guides. If your seat is forward-facing, it will go through the red guides).

Once you’ve taken the top part of the seat belt around the back, pull to tighten the seat belt to make sure it’s secure the whole way around.

Take a moment to check the entire seat belt; there should be no slack or twists at any point.

Activate any lock-off devices and tension levers or buttons. These systems will help the seat belt fit more tightly, and prevent it from slipping.

Ensure the seat moves no more than one inch in any direction.

Check that the carry handle is in the right position (which varies for different car seats) and that the seat belt buckle doesn’t bend around the frame of the car seat.

As with any car seat, it’s best to carefully follow the manufacturer’s instructions, and seek professional help if you’re ever unsure about anything.

Fitting a car seat with ISOFIX

i-Size car seats are designed to be fitted to your vehicle using a standardised system called ISOFIX. This system works by attaching the car seat onto small metal bars, or anchor points, which can typically be found where the back seat and seat cushion meet.

Most newer cars are equipped with ISOFIX connectors, and this system is generally considered the safest fitting option. Best of all, this type of car seat fitting makes the installation process significantly easier.

Cybex Sirona S i-Size Car Seat & Base
Cybex Sirona S Car Seat (Manhattan Grey)

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After you’ve located the ISOFIX prongs on the back of the car seat, you simply need to plug the connectors into the corresponding anchor points in your car. You’ll hear an audible “click” when they’re connected, and many seats also come with an indicator light that turns green when secured correctly.

On some seats, a third anchor point is needed (such as a supporting leg which extends from the seat down to the car’s floor for certain baby car seats). If you’re using a child seat with a supporting leg, ensure it’s correctly adjusted, so the seat is braced against the floor.

The key thing to remember is that ISOFIX was introduced to make fitting a car seat much easier, and to lower the risk of installing it incorrectly. That being said, ISOFIX bases aren’t compatible with every vehicle, so it’s best to check suitability with your manufacturer.

Common Car Seat Fitting Issues

According to a 2018 study, nearly two-thirds of car seats are incorrectly fitted. No matter how many safety features your car seat is equipped with, if it’s not fitted correctly, it simply isn’t safe.

Here are some of the most common car seat fitting issues to look out for:

1. The car seat harness is too loose

Too much space between your child and the harness increases the risk of their shoulders slipping out in the event of a collision. How can you tell if the harness is fitted correctly? You should be able to fit only two fingers between the harness and your child’s chest.

2. The seat belt is too slack

If the seat belt isn’t correctly adjusted, your child is at greater risk of being injured in a crash. Always make sure the seat belt is tightly fitted and properly clipped in its buckle to ensure both your child and the car seat are secure.

3. The seat belt is sitting incorrectly on your child

Whether it ends up below your child’s arm, cutting across their neck, or sitting on their tummy, there are numerous ways the seat belt could be positioned incorrectly. Never try to guess how a seat belt should be positioned. Instead, make sure you follow the car seat manufacturer’s instructions carefully.

4. Buckle crunch

If the seat belt buckle rests against the frame of the car seat, pressure on the buckle could potentially cause it to fail (also known as buckle crunch). Only the material part of the seat belt should ever touch the car seat frame.

5. The seat belt is twisted

A twisted seat belt won’t react as it’s designed to in a collision, and it may even fail to work altogether. To avoid this problem, make sure the seat belt runs smoothly through every fitting.

Car Seat Fitting Checklist

To simplify the installation process and provide you with added peace of mind, follow this handy checklist when you’re fitting your child’s car seat:
1. Is the seat belt untwisted?
2. If the seat is rear-facing, does the handlebar need to be adjusted?
3. Is the car seat sitting evenly on the seat of the car?
4. Is the seat belt securely fitted into the correct red or blue route guides?
5. Does the car seat move no more than one inch in any direction if you wiggle the base?
6. Is the seat belt buckle in the right place?
7. Do the visual indicators show the seat is fitted correctly? (If using an ISOFIX seat)
8. Have you removed your child’s jacket or other bulky clothing?
9. Is the harness snug and free of twists?
10. Are the harness shoulder pads at shoulder level? (For younger children)
11. Is the seat belt sitting across your child’s body correctly? (For older children)

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About the author: Lucy

Lucy is our youngest regular contributor at 24 and has recently welcomed her first child, Charlotte, only 3 months ago. As a single mum living in the Scottish capital of Edinburgh, she's a huge fan of photography, cycling, and all things DIY - when she has time, of course.

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