Five signs your child is too big for their car seat

The rules for car seats are pretty clear – your baby or child should be in one when riding in a car unless they're 12 years old or at least 135cm tall. That's the law.

But knowing when to move between child car seat sizes can be more confusing.

Read on for more advice on keeping your baby or child safe in their car seat, or head straight to our car seat reviews to pick a Best Buy in your next size up.

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1. Reaching a height limit

If you have a car seat that's approved to R129 i-Size regulations, then the different categories of seats are based on height measurements. You can tell if your car seat is R129 by checking the orange label on the bottom of the seat – it will look like the image below.

Confusingly, there aren't set height groups like you get with weight-based seats (more on this below). You'll need to check the measurements of each seat to work out how long it will last your child, and they may be categorised as either a baby, toddler or child i-Size seat.

Most i-Size baby car seats will last from 40cm to 85cm, which is from birth to around 15 months.

But the height limits can differ between manufacturers. Maxi-Cosi infant carriers often only go up to 75cm, which is around 12 months old, while Cybex car seats are approved up to 87cm, which is closer to 18 months old.

To measure your baby, lie them down (or wait until they're sleeping) and measure from the crown of their head to their toes, keeping their legs as straight as possible.

Always keep your baby in the seat until they reach the upper height limit to ensure they're adequately protected in the event of a crash.

What about older children?

UK law states that once your child is 135cm tall they no longer need to use a child car seat and can be restrained with just the vehicle three-point seat belt.

However, the laws are different in Ireland and some other European countries, such as France and Germany.

Children need to be 150cm/4ft 11in to stop using a car seat in these countries, and many safety experts agree that it's safer to keep your child in a car seat until they're 150cm. If you're able to keep your child in a car seat until this point, it would provide more protection for them.

Read our guide to child car seat laws in the UK and abroad to find out more.

More questions? Take a look at our i-Size child car seats explained

2. Reaching the weight limit

Car seats approved to the older (but still legal) R44.04 regulations are based on weight, and are divided into 'groups'. 

These include:

• Group 0 – Newborn to 10kg (around 6 months)
• Group 0+ – Newborn to 13kg (around 12 to 15 months old)
• Group 0+/1 – Newborn to 18kg (around four years old)
• Group 0+/1/2 – Newborn to 25kg (around six or seven years old)
• Group 0+/1/2/3 – Newborn to 36kg (around 12 years old)
• Group 1 – 9kg to 18kg (nine months to four years old)
• Group 1/2 – 9kg to 25kg (nine months to six or seven years old)
• Group 1/2/3 – 9kg to 36kg (nine months to 12 years old)
• Group 2/3 – 15kg to 36kg (four years to 12 years old)
• Group 3 – 22kg to 36kg (125cm or around five or six to 12 years old).

To check the weight of your baby, step on some scales by yourself then pick up your baby and weigh yourself again. Subtract your weight from the combined weight of you and your baby to get their measurement.

Car seat weight groups explained: what you need to know

3. Their head is higher than the top of the seat

We're not expecting parents to be measuring or weighing their children every week, but you can look out for visual clues that it might be time to change your car seat soon.

For example, your child will have outgrown their car seat if the top of their head is higher than the top of the seat.

You want the car seat to protect your child's head, so if it's sticking out of the top it won't be protected.

The best child car seats in our tough crash tests

4. The shoulder straps are in the wrong position

Another sign to look out for is the position of the harness shoulder straps.

The safest position for the top of the straps will depend on whether your child is rear or forward-facing. We'd always recommend keeping your child rear-facing for as long as possible, ideally until they reach four years old, as research shows it's the safest way for babies and young children to travel.

• In a rear-facing car seat, the point at which the straps meet the back of the car seat should be at or just below the top of the child's shoulders.
• In a forward-facing seat, the straps should be at or just above the level of the child's shoulders.
• If your child is forward-facing and the shoulder straps have to come up and over their shoulders, this could be a sign they're ready to move to the next size up car seat.

You're less likely to spot signs that your child has outgrown their seat via the harness, but you can use other methods such as head height to guide you instead.

Don't forget to check first that you've adjusted the height of the shoulder straps to their highest setting (if that's an option) before moving up a seat.

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5. They've reached the age limit

Most children will hit the height measurement before the age limit, so by the time your child is 12 years old, there's no need for them to sit in a high-back booster car seat.

This may not be the case if your child has a disability (more on this below) or is especially small – so use parental judgement if you're worried.

What about extended rearward-facing seats?

If you've got your child in an extended rearward-facing seat (up to 18kg/105cm), you may spot their knees are bent and they sit cross-legged by the time they're near the upper end of these limits.

But bent or folded legs are not a 'sign' that they've outgrown the seat or that it needs to be turned forward facing (if this is an option).

As long as your child still feels comfortable to bend or cross their legs while sitting rearward-facing (and due to kids' flexibility, this is rarely an issue), then the seat is still fine to use.

Some extended rearward-facing seats have more space when rearward-facing than others, so it's worth reading our car seat reviews to see which ones are best.

See the best rear-facing baby car seats for older children

What age can my child use a backless booster seat?

Moving your child from a high-backed booster seat to a backless version can create more space in the car – and it looks less obviously like a car seat if your child doesn't like sitting in one.

Since 1 March 2017, backless booster seats can only be used with children taller than 125cm and weighing more than 22kg (around five years old).

However, backless booster seats made and bought before this date are still legal and approved for use by children weighing between 15kg (about four years old) and 36kg (about 12 years old). They can also still be sold by shops.

So if you own a backless booster seat bought before this date you can still use it for a child within this weight range.

But we strongly advise against backless booster seats. Our crash tests repeatedly demonstrate that they do not provide the same level of protection as a high-backed booster in a side-on collision.

Booster seats - are you breaking the law?

Children with disabilities or medical conditions

The same rules on car seats apply for children with disabilities or medical conditions, so you'll need to keep an eye on weight or height to ensure they're sat in the correct size of seat.

A doctor can issue an exemption certificate if a child is unable to use a restraint or seat belt because of their condition.

For more information go to child seat laws in the UK and abroad.