Do I use height or weight for an R129 vs R44 car seat?

Which number applies to this shell?

The regulation on this orange label, not the group you remember from the last seat. GOV.UK splits the two families: height-based seats show a capital E in a circle and R129; weight-based seats show E and ECE R44. You do not “pick height or weight” as a household rule. You pick the rule that seat was approved under.

RoSPA’s using the right child restraint page says the same in one line: R44 seats are designed for specific weight ranges; R129 (i-Size) seats for specific height ranges. Age is how shops talk. It is not the approval number.

If you have just moved from a Group 0+/1 box to a 40–105 cm listing, centimetres apply because the new shell is R129 — not because height “wins” over weight for every seat in the house.

If the seat is R129, do I ignore weight?

No. Height is the selection rule. Weight can still be a stop rule. nidirect says Regulation 129 seats are based on length/height and show a maximum weight, so you need both figures. Joie’s i-Size explainer says most seats still have an upper occupant weight you must observe.

If the seat is R44, do centimetres count?

Not as the approval group. GOV.UK’s R44 table is kilograms: Group 0 (to 10 kg), Group 0+ (to 13 kg), Group 1 (9–18 kg), Group 2 (15–25 kg), Group 3 (22–36 kg). The NHS choosing a baby car seat page repeats those weight bands and treats age as “about”.

A head sticking above the shell, or harness slots that no longer sit in the right place, can still mean the seat is outgrown before the kilo limit. Those are this-manual checks. They do not turn an R44 seat into a height-based R129 seat. You do not “convert” 13 kg into 85 cm on an R44 label.

How do I check height and weight on this seat today?

1. 1
   Photograph the orange label Write down R129 or ECE R44, the height band or the weight group, and any extra max-kg line. If you cannot read the amendment or the range, you cannot apply the right number.
2. 2
   Measure the child the way that label expects R129: standing height (Which? says for a baby, crown to heel with legs as straight as you can). R44: weight — Which? describes weighing yourself, then yourself plus the child, and subtracting.
3. 3
   Apply only that seat’s numbers If the listing is 40–105 cm and 19 kg, both apply to that R129 shell. Do not keep a child in it because they are “still Group 1 weight” if this seat is not R44 Group 1.
4. 4
   Treat age as a shop filter only MKUH’s NHS leaflet: seats are approved on height (R129) or weight (R44), so use size, not age. Birthday is for the 12-years-or-135 cm adult-belt rule on GOV.UK, not for picking a group.

What if I have an R44 and an R129 in the same car?

Each child is checked against their seat. The toddler in a 40–105 cm R129 is a height (plus that seat’s max kg) problem. The younger baby in a Group 0+ R44 is a 13 kg problem. There is no household average.

When you retire the R44 and buy R129, you switch measurement system at the same time. That is why the 0+/1 to 40–105 cm jump feels like two numbers fighting. They are not fighting. The old number retired with the old label.

In shortIn short: R129 = height, plus any max weight on that label. R44 = weight group. Age is only a rough guide. When both a centimetre band and a kilo limit are printed on one shell, both apply to that shell — the first limit you hit is the one that ends this seat.