i-Size and R129, in plain words
R129 is the height-based car seat regulation, and i-Size is part of it. GOV.UK says height-based seats stay rear-facing until a child is over 15 months.
By Pavel Ryzhikh, 3 November 2026.
This is general information from public sources, not medical advice. For anything about your child's health or development, speak to your doctor or health visitor.
R129 is a car seat regulation that goes by your child's height. i-Size is part of it, according to the NHS, so in everyday shopping the two names point at the same kind of seat. The older regulation, R44, goes by weight. Both are allowed in the UK.
What the labels look like
GOV.UK gives the simplest test. Only approved seats can be used in the UK, and each kind carries its own label:
| R129 (i-Size) | R44 |
|---|---|---|
Chosen by | Child's height | Child's weight |
Label | Capital E in a circle, and R129 | Capital E in a circle, and ECE R44 |
Rear-facing | Until over 15 months, says GOV.UK | Depends on the weight group |
How it's sold | A height range in cm | Groups such as 0+, 1, 2/3 |
If you're looking at a seat, that label settles the question quicker than any listing. It's usually on the seat itself.
What changed with R129
RoSPA's child car seat site lists three changes, and says R129 was introduced in July 2013.
Height replaced weight. You check that your child's height falls within the seat's range. RoSPA's reasoning is that parents tend to know their child's height better than their weight. It's also easier to judge when a seat is outgrown.
Longer rear-facing. GOV.UK says height-based seats have to be rear facing until a child is over 15 months old. RoSPA says rearward-facing seats give more protection to a child's head, neck and spine. The NHS suggests staying rear-facing for as long as your child fits.
A side impact test. RoSPA says i-Size includes one.
That's the plain version. We couldn't open the regulation text itself on the UN's site, so we can't describe what those tests involve. Makers sometimes print a version number after R129. We can't tell you what separates those versions either.
Does i-Size mean Isofix?
Often, but not always. The NHS says i-Size seats usually fit into Isofix points, while some fit with a seat belt. This trips people up.
A seat with an i-Size label may still need checking against your car. RoSPA says i-Size seats fit any i-Size approved vehicle, and elsewhere you should check whether the seat is approved for your car. Many makers publish a list.
Infant carriers such as those in choosing a first car seat often belt in, with an Isofix base as an extra. No Isofix in your car? Check how a seat fits before you buy it, which the NHS also advises.
Is my R44 seat still fine?
RoSPA says you don't need to swap a current seat for an i-Size one unless you wish to. That page says R129 and R44 run side by side. It also says R44 sales will probably end at some point, with no date set. The wording looks old, so we wouldn't rely on it for what shops can sell today.
The rule for using a seat is clearer. GOV.UK says weight-based seats with the E mark and ECE R44 label can be used in the UK, and it doesn't call for a swap. So you can relax about a seat you bought new and still fit. Moving to the next seat for any other reason is a separate question, covered below.
A hand-me-down R44 seat is a different matter. The NHS says "Do not buy a secondhand car seat", and the Lullaby Trust says not to buy or use one, as you can't be certain of its history.
What the 15 months means for you
Fifteen months is a floor for forward-facing, and nothing says to turn a child then. GOV.UK says a child can use a forward-facing seat once over 15 months. RoSPA says not to rush it, because rearward-facing is safer.
You may be near this point now. Infant carriers often run out of height or weight before or around this age, so the next seat tends to come first. Our guide to the next seat after an infant carrier compares three options, and the car seat list has the rest.
Do labels say anything about sleep?
Not in anything we could read. A seat of any kind is for the journey. The Lullaby Trust says car seats are not a main sleeping place, and the NHS says to take your baby out as soon as you arrive. For sleep at home the advice is on the back, on a firm, flat mattress, in a clear cot, in the same room as you for at least the first 6 months. Your doctor or health visitor can help with anything specific to your child.