The rules for car seats are pretty clear - your baby or young child should be sat in a car seat when you're driving around in a car unless they're 12 years old or at least 135cm tall. That's the law.
But knowing when to move between seat sizes can be more confusing.
If you have a car seat that's approved to the latest R129 i-Size regulations, then the different categories of seats are based on height measurements.
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 birth to around 15 months.
To measure your baby, lie them down (or wait until they're sleeping) and measure from the crown of their head to their toes, with their leg as straight as possible.
Remember, always keep your baby in the seat until it reaches the upper height limit to ensure that your baby is adequately protected in the event of a crash.
Car seats approved to the older (but still legal) R44.04 regulations are based on weight, and are divided into 'groups'. These include:
The sheer number of options for R44 is one of the reasons the newer regulations have less options.
To check the weight of your baby, step on some scales to weigh yourself and then weigh yourself again while holding your baby. Then simply subtract your weight from the combined weight of you and your baby.
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 soon be time to change your car seat.
For example, your child will have outgrown their car seat if the top of their head is higher than the top of the seat.
This makes sense, as 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.
Another sign to look out for is with 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 or 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.
If you've got your child in an extended rearward-facing seat (up to 18kg/105cm), then by the time they start getting towards the upper end of these limits, you may spot their knees are bent and they sit cross-legged.
Bent or folded legs is 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 their inherent flexibility, this is rarely an issue), then the seat is still fine to use.
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. If you're able to keep your child in a car seat until this point, it would provide more protection for them.
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.
Some parents want to move their child from a high-backed booster seat to a backless version as it creates more space in the car and is less obvious as a car seat if your child doesn't like sitting in one.
From 1 March 2017, backless booster seats can now 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.
However, we strongly advise against backless booster seats. Our crash tests demonstrate repeatedly that they do not provide the same level of protection as a high-backed booster in a side-on collision.
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.