It's all about the weight and height

When was the last time you weighed or measured your child? Do it today and you might be surprised by how quickly they are growing. It is dangerous to use a car seat that does not suit your child.

Remember also that one car seat can have multiple installation methods depending on the weight or height of your child. So you need to know at all times what the weight or height of your child is. For example, the group 1/2/3 Cosatto hubbub seat lasts from 9 - 36 kgs and has a 5-point harness. But the harness can only be used to 18kgs. After this the harness needs to be taken out and the child must use the vehicle 3-point seatbelt. If you use the harness beyond 18kgs you are risking that the car seat won’t perform in a car crash. Always read the user manual.

You should ignore age recommendations and go by weight and height. All manufacturers and retailers who use age give it as an approximation. It’s never a definite cut off point. This is because two children at the age of for example 4 can have quite different weights and heights. Read on to find out more.

Car seats are categorised by manufacturers by one or more of the following criteria: weight, height and/or age of the child and assigned to the corresponding car seat group.

Weight is categorised as shown below (this is the older R44 regulation which is still in force):

GroupChild weight [kg]Notes
0up to 10 kg

You will often see corresponding age brackets for each weight and group category.

You should ignore this age recommendation. Read below why.

0+up to 13 kg
19 - 18 kg
215 - 25 kg
322 - 36 kg

For height, the new regulation (R129) doesn’t provide height brackets to use in groups. Instead car seat manufacturers developed car seats for different heights. We have summarised the most common groupings below.

GroupChild height [cm](given as a range; check user manual what your car seat allows, e.g. 45 - 75cm)Type of car seatNotes
There is no group classification in the new regulation.40 - 87 cm
(check user manual for actual range)

First 15 months of life MUST be rear-facing.

This means you cannot turn your child forward before they are 15 months old.

You should continue to rear-face your child for as long as possible.

40 - 105 cm
(check user manual for actual range)
Infant / Toddler
60 - 105 cm
(check user manual for actual range)
100 - 135 cm
(check user manual for actual range)

NOTE: although the new regulation emphasises height, all seats are still tested to a weight limit which your child cannot exceed when using a car seat. So always know your child’s weight AND height. Check user manual for allowed child weight and height. Car Seat Jungle provides a free library of car seats with their manuals for easy reference.

Why is weight and height so important? And why is there a move towards height in new laws? When car seats are crash tested, strict weight and, for the new regulation (R129) height measurements are used to determine whether the seats are safe. If you have a child that weighs 20kg in a car seat that is designed for children up to 18kg, then it’s likely that the car seat won’t perform in an accident. Similarly, if you have a child that is too short for a car seat, the straps are likely not to hold your child in a crash. So knowing the weight and height of your child is really important.

The move towards height is meant to make choosing a new car seat easier for parents and carers. Knowing a child’s height is just an additional reference point which can help in making the right car seat choice.

Let’s take a look at a few i-size examples:

ManufacturerCar seat modelManufacturer recommended ageChild weightChild height
Maxi CosiAxissFix AirFrom approx. 4 months up to 4 yearsUp to 19kgFrom 61 to 105 cm
CybexSirona Q i-SizeUp to approx. 4 years Up to 18 kgFrom 45 to 105 cm
Britax RömerDualfix M i-SizeFrom 3 months - 4 yearsUp to 18 kgFrom 61 to 105 cm

As you can see, there is some age variation between manufacturers. In the i-size seat examples above Maxi-Cosi recommend their car seat to be used from approximately 4 months, while Britax Römer say 3 months. Also, note that age is more often than not given as an approximation.

The age reference was adopted as an easy way to choose a car seat because you always know what exact age your child is and you may not know their exact height or weight. But if you want to use car seats correctly, which really just means that you want to make sure your child is safe, you need to go by their weight (for non i-Size compliant seats) and their height (for i-size seats) of the child. Why? Because car seats are manufactured and tested against weight or height and not age. Two children at the same age can have very different weights or heights. The below growth charts are used by healthcare professionals to monitor boys’ and girls’ weight gain. If we look at 4 years old boys’ range it sits between 12 and 23 kg with a median of over 16kg. For girls, the range is between 11 and 24 kg with a median of 16kg. So a small girl aged 4 could weigh 11kg and a boy also aged 4 could weigh as much as 23 kg.

These are pretty big ranges! Combine this with the fact that car seats are tested for weight or height, using age as an indicator not only doesn’t make sense but can be dangerous. That’s why you should simply weigh and/or measure your child to ensure you have the correct car seat.