How to choose a car seat: i-Size, R129, rear-facing and ISOFIX

A car seat is one of the most important things you'll buy for your child, and also one of the most confusing: weight groups, i-Size, R129, ISOFIX, rear-facing, extended rear-facing. This guide cuts through the jargon. We've taken the legal points from GOV.UK and the advice from the NHS and RoSPA, and linked to each source at the bottom so you can check for yourself.

The law in a nutshell

According to GOV.UK, children must normally use a child car seat until they're 12 years old or 135 centimetres tall, whichever comes first. Children over 12, or taller than 135cm, must wear a seat belt. RoSPA adds that the driver is responsible for making sure children under 14 are restrained correctly.

You can choose a seat based on your child's height or their weight:

• Height-based seats must be rear-facing until your child is over 15 months old. Only approved height-based seats can be used in the UK, and they carry a label showing a capital "E" in a circle and "R129".
• Weight-based seats carry a label showing a capital "E" in a circle and "ECE R44". The seat your child can use, and how they must be restrained, depends on their weight.

GOV.UK also sets out some fitting rules:

• You must only use a child car seat if your car's seat belt has a diagonal strap, unless the seat is specifically designed for use with a lap belt or is fitted using ISOFIX anchor points.
• You must deactivate any front airbags before fitting a rear-facing baby seat in a front seat.
• You must not fit a child car seat in a side-facing seat.

Children with disabilities or medical conditions follow the same rules, with some allowances. See the GOV.UK page for details.

i-Size (R129) and R44: what the labels mean

You'll see two sets of approval labels on seats in the UK. They run side by side.

R129 (i-Size)

The NHS explains that i-Size is a European safety standard for baby and child car seats and is part of regulation ECE R129. According to the Child Car Seats website (run with RoSPA), the main changes it brought in are:

• Height-based, not weight-based. You check your child's height is within the range of the seat. Parents tend to know their child's height better than their weight.
• Rear-facing until at least 15 months.
• A side-impact test to make sure seats give better protection in side collisions.

The NHS says i-Size seats usually fit into ISOFIX points in your car, but some can be fitted with a seat belt, so check how a seat fits before you buy if your car doesn't have ISOFIX.

R44 (weight groups)

R44 seats are grouped by weight. GOV.UK lists them like this:

GOV.UK also notes that manufacturers can now only make booster cushions approved as group 3. It doesn't affect existing group 2 booster cushions, which you can still use.

The Child Car Seats website says you don't need to swap a seat you already have for an i-Size one unless you wish to. Whichever type you choose, GOV.UK is clear that only approved seats can be used, so look for the E-mark label and the R129 or R44 reference before you buy, especially if you're buying online or second-hand.

Whichever you pick, the NHS says to always choose a seat that's right for your child's current height and weight, and to check the seat's limits as they grow.

Why rear-facing matters

The NHS and RoSPA both say rear-facing seats give better protection. RoSPA says they provide greater protection for a baby's head, neck and spine than forward-facing seats, and notes that many parents assume children prefer facing forwards and turn them round at the first opportunity. The NHS says to use a rear-facing seat for as long as your child fits in it. The Lullaby Trust calls a rear-facing seat for a baby "essential for safety".

The legal minimum for height-based seats is over 15 months. That's a minimum, not a target. When your child outgrows a baby seat (they're over the maximum weight or their head is above the top of the seat), RoSPA says there are two choices: move to a forward-facing seat, or use an extended rear-facing seat, which lets a child travel rear-facing for longer, typically up to around four years old, and in some cases up to around six.

ISOFIX or seat belt?

ISOFIX is a set of fitting points built into both the car and the seat. RoSPA says an ISOFIX seat is plugged into the matching points in the car, so you don't need to use the seat belt to secure it, and that an extra top tether or support leg stops the seat tilting or rotating in a crash. The NHS says ISOFIX connectors are designed to make fitting simpler, and that most modern family cars have them.

How to find out if your car has ISOFIX:

• Look for ISOFIX labels between the base and back of the rear seats. The fitting points themselves may be visible, or hidden in the cracks between the cushions.
• Check your vehicle's handbook, or ask the manufacturer or dealer.

If your car doesn't have ISOFIX, RoSPA says you'll need a seat that can be fitted using the seat belt. Neither approach is "better" in itself: what matters is that the seat is approved, suits your child, fits your car and is fitted correctly.

Will it fit your car?

RoSPA is blunt: not all car seats fit all vehicles, so check compatibility before you buy. Its suggestions, and the NHS's, boil down to:

• Try seats in your car. The NHS says it's best to try a few in your car before deciding, and to find a retailer willing to help you do this. Ask whether staff have been trained in fitting car seats. RoSPA says the best way to check fit is to get a qualified fitter to try a range of seats in your car.
• Check the compatibility lists. The NHS says some manufacturers have online guides showing which cars their seats fit.
• Think about every car it'll be used in, such as a grandparent's car.
• Know the answers a retailer will ask. RoSPA lists: your child's weight, height and age; which vehicle(s) the seat will be used in; whether they have ISOFIX points; and whether they have underfloor storage. RoSPA also has a buyer's checklist on its Child Car Seats website.
• Think about daily use. The NHS says if you'll be lifting your baby in and out of the car a lot, you may prefer a lightweight seat with a base that stays in the car.

Where in the car?

• Rear seats are safer than the front. RoSPA says it's safer for your child to travel in the back, and the NHS agrees.
• Airbags. RoSPA says never to put a rear-facing baby seat in the front if there's an active passenger airbag, because the airbag could hit the seat and fling it forward. GOV.UK says to deactivate any front airbags before fitting a rear-facing seat in a front seat.
• The middle rear seat. RoSPA says that if it has a three-point (lap and diagonal) belt, it's the safest place for a child seat, unless the manufacturer's instructions say otherwise, because it's furthest from the sides of the car. If you're using ISOFIX or i-Size, RoSPA says you can only use the middle seat if it has ISOFIX points.
• Behind the passenger. If you can't use the middle, RoSPA says to use the seat behind the front passenger rather than behind the driver, as you can normally get the child in and out on the pavement side. The NHS says to always put your baby into their seat from the pavement side.

New, second-hand and how long a seat lasts

The NHS says it's ideal to buy a new seat. It advises against buying a second-hand one because it could have been in an accident, may be missing parts including the instructions, and may not fit your car. If you do accept one from friends or family, the NHS says only to do so if you know its history, it's not too old and it comes with instructions. The Lullaby Trust makes a similar point: damage may not be visible, and even wear and tear over time can affect how well a seat protects a baby. The NHS specifically says not to buy one from a second-hand shop or through classified ads.

On expiry, RoSPA says UK car seats aren't marked with an expiry date. Many manufacturers recommend replacing or upgrading after five years, though some seats suitable from birth up to 36kg are designed to last longer. RoSPA also recommends replacing a seat that has been involved in a collision. Check what your seat's maker says in the instructions.

Car seats and sleeping babies

This is a safety point many new parents don't hear until later. The Lullaby Trust says car seats are designed to keep babies safe while travelling, but are not designed to be a main sleeping place, so use them for transport only. It's fine for a baby to fall asleep in a car seat while you're driving, but take them out as soon as you reach your destination and put them on a firm, flat surface to sleep. The NHS gives the same advice.

The Lullaby Trust also suggests taking regular breaks on long journeys, and removing hats and outdoor clothes such as snowsuits and coats once your baby is in the car, to avoid overheating.

If you're planning a travel system, which clips a car seat onto a pushchair frame, see our pram and pushchair guide.

Questions to take to the shop

• Is it approved? E-mark label with R129 or ECE R44
• Is it right for my child's current height and weight?
• How long can it stay rear-facing?
• Does my car have ISOFIX? Does this seat fit it?
• Can I try it in my car, and will staff show me how to fit it?
• Does it fit every car it'll be used in?
• Not on the recalls list

If you're in any doubt, RoSPA suggests popping into a reputable shop to ask for advice. The NHS adds that your local council's road safety officer can give detailed advice.

Where this information comes from

We haven't tested any products. Everything above is based on these official and expert sources, which we read and checked for this guide. Guidance can change, so always check the latest version, and follow the instructions that come with your product.

• Child car seats: the law (using a child car seat or booster seat) — GOV.UK
• Choosing a baby car seat — NHS
• Our guide to choosing the right car seat — RoSPA
• i-Size seats — Child Car Seats (RoSPA)
• Car seats and safer sleep — The Lullaby Trust
• Sudden infant death syndrome (SIDS) — NHS
• Product Safety Alerts, Reports and Recalls — GOV.UK (OPSS)

This guide is general information, not medical, legal or individual safety advice. If you're unsure which product suits your baby, ask a trained retailer, your midwife or health visitor.