Car seat groups exist to match the seat to the child's size. But the labelling is old, the rules changed, and the shop floor is full of seats that use both systems simultaneously. Here is what you actually need to know.

Step 1. Understand the Two Labelling Systems

Most car seats on sale in the UK are labelled under one of two standards — and some carry both.

ECE R44 — the older group system

This is the system most parents recognise: Group 0+, 1, 2, 3. It categorises seats by weight range. It is still legal and widely sold, but it has been superseded for new approvals since 2023.

ECE R129 (i-Size) — the newer standard

i-Size seats are categorised by the child's height rather than weight, and they require ISOFIX installation as standard. The key practical difference: i-Size mandates rear-facing up to 15 months, which the older standard did not enforce.

The shortcut: if a seat is labelled i-Size, it meets the current safety standard. If it is labelled by group only (0+, 1, 2, 3), it is approved under the older standard — still legal, still safe if correctly fitted, but worth understanding the difference.

Step 2. Group 0+ — Birth to Approximately 15 Months

Weight range: 0–13 kg. This covers newborns up to roughly 12–15 months, though some children outgrow this group earlier if they are on the larger side.

Group 0+ seats are always rear-facing. This is non-negotiable for this age group — rear-facing distributes crash forces across the whole back and head, which matters most when the neck muscles are still developing.

What the spec sheet should confirm

  • Weight limit: 13 kg is standard. Some extended seats go to 13 kg with a higher weight limit — check the label carefully.
  • Installation: ISOFIX with a support leg or base is significantly more secure than belt-only installation. Check whether the base is included or sold separately — on some models it adds £80–£150.
  • Compatibility: not all bases work with all cars. The manufacturer's website lists compatible vehicles by make and model. Check this before purchasing.
  • Travel system compatibility: if you want to click the seat onto a pushchair chassis, confirm the car seat and pushchair are from compatible brands. This is the primary reason parents buy travel systems — the Doona+ range takes this furthest, converting directly between car seat and stroller with no separate chassis needed at all.

When to move on

When the child reaches the seat's weight limit, or when the top of their head is less than 1 cm from the top of the shell — whichever comes first. Do not wait until both conditions are met.

Step 3. Group 1 — 9 Months to Approximately 4 Years

Weight range: 9–18 kg. The overlap with Group 0+ (from 9 kg) means you have a choice: you can move at 9 months, or stay in the infant seat until it is outgrown.

The guidance from road safety organisations is clear: keep children rear-facing as long as possible. A rear-facing Group 1 seat typically takes children to 18 kg. A forward-facing Group 1 seat is also legal from this stage — but rear-facing is safer for as long as the child fits.

Rear-facing Group 1 seats

Look for seats described as "extended rear-facing" or ERF. The child faces the rear until 18 kg (approximately age 4), then the seat converts to forward-facing for Group 2 use.

  • Check: maximum rear-facing weight — this varies between 15 kg and 25 kg depending on the model.
  • Check: whether ISOFIX is included or optional. ERF seats are large; a secure base matters more, not less.

Forward-facing Group 1 seats

Legal from 9 kg. The harness holds the child in the seat rather than the adult seatbelt. Check that the harness height is adjustable without rethreading — some cheaper models require disassembly to reposition the harness as the child grows.

Step 4. Group 2 — Approximately 4 to 6 Years

Weight range: 15–25 kg. At this stage, the child is old enough for the car's own seatbelt, but still needs a seat to position it correctly across the chest and hips rather than the neck and stomach.

Group 2 seats are high-back boosters with a guide for the seatbelt, not an internal harness. The back supports the head and spine and provides side-impact protection.

What the spec sheet should confirm

  • Side-impact protection: look for seats with deep side wings that extend to head height. Descriptions like "side impact tested" or "extended side-impact protection" indicate this has been tested beyond the minimum standard.
  • Head rest adjustment: the head rest must reach the top of the child's ears — check the adjustment range in the spec, not just the photo.
  • Seatbelt routing: the belt guide should direct the lap belt across the hips, not the stomach. Check product descriptions for "low belt guide" or similar.
  • ISOFIX: optional at this stage but adds stability when the child is getting in and out.

Step 5. Group 3 — Approximately 6 to 12 Years

Weight range: 22–36 kg. Group 3 seats are backless booster cushions — they raise the child so the adult seatbelt sits correctly without a back or headrest.

These are only appropriate once the child is tall enough that a seatbelt fits correctly without a high back. The general rule: if the shoulder belt crosses the neck rather than the chest, the child still needs a high-back Group 2 seat.

In practice, many parents skip backless boosters entirely and keep children in a high-back Group 2/3 combination seat until they no longer need one. A combination seat that covers Groups 2 and 3 with a removable back is the more versatile option.

When a child no longer needs a car seat

UK law requires a car seat until the child reaches 135 cm in height or their 12th birthday, whichever comes first. Height is the more reliable measure — some children reach 135 cm well before age 12.

Step 6. What to Check in the Spec Sheet Before Buying

Regardless of group, these are the details that matter when comparing models:

  • Weight and height limits: confirm both upper limits, not just the lower entry point.
  • ISOFIX: included, optional, or absent — and whether your car has ISOFIX anchor points (most cars built after 2004 do; check your vehicle handbook).
  • Installation method: ISOFIX with top tether, ISOFIX with support leg, or seatbelt only. Each has implications for which cars it fits.
  • Recline positions: important for sleeping children in Group 1 and 2 seats. More positions means more flexibility on longer journeys.
  • Cover washability: the cover will need washing. Check whether it is removable without tools.

Group 0+ Options Mentioned in This Guide

Budget: Doona+ Car Seat & Stroller — £269, converts between car seat and stroller directly.

Mid-range: Doona+ Car Seat & ISOFIX — £399, adds an ISOFIX base for a more secure in-car fit.

Premium: Doona X Car Seat & Stroller — £499, the newer generation of the same convertible design.

FAQ

Can I skip Group 1 and go straight from 0+ to Group 2?
Not safely. Group 2 seats use the adult seatbelt as the restraint, which is not appropriate until the child weighs at least 15 kg and can sit correctly without slumping. Group 1 fills that gap.

Is ISOFIX mandatory?
No — seatbelt installation is still legal and safe if done correctly. ISOFIX removes the installation error variable, which is why it is recommended. i-Size seats require ISOFIX by standard.

Can I use a second-hand car seat?
Only if you know its full history. A seat that has been in any collision — even a minor one — should not be reused. The damage is not always visible. If you cannot confirm the history, do not use it.

What does "Group 2/3" mean on the label?
It means the seat covers both groups — typically a high-back booster that works from 15 kg to 36 kg. These are combination seats, and they are practical if you want a single purchase that lasts several years.

Does a more expensive seat mean a safer seat?
Not directly. All approved seats meet minimum legal safety standards. The difference in price typically reflects installation ease, comfort features, longevity across multiple groups, and test performance above the minimum. A well-fitted basic seat is safer than a poorly fitted premium one.