There are two car seat regulations in Europe, ECE R44 which has been around since the early 1980s (the current version is R44/04), and ECE R129 or i-Size which was introduced in 2013.
ECE R44/04 weight groups
Group 0, from 0-10kg, birth to 9 months Group 0+ from 0-13kg, birth to 18 months Group 1 from 9-18kg, 9 months to 4 years Group 2 from 15-25kg, 4 to 7 years Group 3 from 22-36kg, 6 to 12 years
These groups all overlap slightly. The minimum weights given are the minimum weights for which these seats have passed a crash test. But that doesn't necessarily mean that it is safe to use them that early. It is always safer to use a lower group until the maximum weight has been reached before moving to the next one. This is especially important when making the switch from rear facing to forward facing, and from five-point harness to booster seat. Group 2's 15kg is actually from three years, but we have deliberately put four, because a booster seat is not safe for a three year-old.
ECE R129 or i-Size
There are a few differences between R44/04 and i-Size car seats. In an i-Size car seat all children must rear face until they are at least 15 months old, regardless of their weight. I-Size car seats have height limits, they don't have set weight groups like R44/04 ones do. Infant seats generally go up to around 83-87cm (the limits vary, so please check your instruction manual to check up to which height your seat can be used). I-Size car seats for older children up to about four all go up to 105cm. They do still have a seat-specific weight limit which can be found on the orange sticker on the seat shell, and this limit must not be exceeded, but most children will reach the seat's height limit before they're too heavy. Advanced technology 'Q' crash test dummies simulate a child’s fragile body more accurately than the dummies used for R44/04. I-Size car seats have all passed a side-impact test which is not compulsory in R44/04 testing. All i-Size seats are fitted into the car with ISOfix and all new cars from 2013 onwards have at least two i-Size compatible seats.
The Orange Sticker
All car seats have an orange approval sticker, usually on the back. This sticker tells you whether the seat meets R44/04 or i-Size, it gives the weight and height limits, and the seat's approval number. The circle with the E in it indicates that the seat is approved for use in Europe and the number is the country it was tested in. For example 1 is Germany, 4 is the Netherlands, 5 is Sweden, 11 is the UK, etc. Each country has its own number, but seats from any European country may be used in any other. It will say Universal meaning the seat will fit in most cars, or Semi-Universal if they have a support leg which you may not be able to use if your car has storage compartments in the footwell. The Y indicates that the seat has a harness with a crotch strap.
Seats bought outside Europe which do not have this sticker are not legal in Europe and must not be used here.