Car seat groups and i-Size explained

The two systems: R44 weight groups and R129 i-Size

In the UK two safety standards run side by side, and a seat will be approved under one of them. The older standard, R44, sizes seats by your child's weight and divides them into groups: Group 0+ for infants, Group 1 for toddlers, Group 2 and Group 3 for older children. You will still see seats described as, say, Group 1/2/3. The newer standard, R129, marketed as i-Size, sizes seats by your child's height in centimetres instead.

The change to height-based sizing was made because it is far easier to measure a child's height than to know their exact weight, so parents are more likely to use the right seat at the right time. R129 also brought two safety improvements: it requires a child to stay rear-facing until at least 15 months, and it adds a side-impact test that the older R44 standard never included. Both standards are legal, but i-Size is the more modern brief, and it is the one we recommend for any new purchase.

The stages from birth to around 12 years

Whichever system a seat uses, a child passes through broadly the same life stages, and the seat formats map onto them:

• Infant carrier (birth to ~15 months) — a rear-facing carrier that lifts out of the car and often clips to a pushchair, such as the Maxi-Cosi Pebble 360. Under i-Size this is roughly 40 to 85 cm.
• Toddler or all-in-one seat (~birth or 15 months to ~4 years) — a larger seat, often rotating, that keeps a child rear-facing for longer, such as the Joie i-Spin 360. Under i-Size this typically runs to 105 cm.
• High-back booster (~4 to 12 years) — raises the child so the adult seatbelt sits correctly and adds side protection. Some all-in-one seats, such as the Silver Cross Motion, build this stage in, taking a child to around 145 cm.

You do not have to buy a separate seat for each stage. An all-in-one combines two or three of them, which is why we have included several rotating all-in-one seats in our comparison. The trade-off is that a do-everything seat is never quite as tailored to one stage as a specialist seat, and it is bulkier.

Which should you buy?

For a new purchase we would choose an R129/i-Size seat fitted with ISOFIX. It is easier to size correctly, harder to fit wrongly, and built to the more modern safety standard. The next decision is how many stages you want one seat to cover: a dedicated infant carrier gives you the best newborn experience but a shorter life, while an all-in-one gives you longevity at the cost of out-of-car flexibility. Our buying guide walks through that choice, and our guide to ISOFIX versus belt-fitted seats covers how to fit whichever you choose.

If you already own a good R44 seat that fits your car and is within its expiry date, there is no urgent need to replace it. R44 seats remain legal to use. The case for i-Size is strongest when you are buying new, where the extra side-impact protection and longer rear-facing are worth having.