The rule
A child must normally use a child car seat until they are 12 years old or 135 cm tall, whichever comes first. After that, an adult seat belt. The height threshold matters because a great many children reach 135 cm well before their twelfth birthday, and a good many do not reach it until after it.
Height rather than age is the better thing to think about throughout. A seat belt is designed to cross the collarbone and sit low across the hips; on a child who is too short for it, the diagonal strap crosses the neck and the lap strap rides up over the abdomen, which is precisely the arrangement that causes injury in a crash.
Two approval standards, both legal
Seats sold in the UK are approved under one of two standards, and both are legal to use. The difference is how the seat is matched to the child.
R129, usually sold as i-Size. The label carries a capital E in a circle and the text R129. These seats are matched to the child's height. This is the newer standard and the one new seats are generally approved under.
ECE R44. The label carries a capital E in a circle and the text ECE R44. These are matched to the child's weight, in groups:
- Group 0, up to 10 kg — lie-flat or lateral baby carrier, or a rear-facing baby seat with a harness
- Group 0+, up to 13 kg — rear-facing baby carrier or baby seat with a harness
- Group 1, 9 to 18 kg — rear- or forward-facing seat with a harness or safety shield
- Group 2, 15 to 25 kg — rear- or forward-facing seat, including high-backed boosters and booster cushions, used with a seat belt, harness or shield
- Group 3, 22 to 36 kg — the same types as Group 2
The groups overlap, and more than one type of seat may be suitable for a child of a given weight. The seat's own label and the manufacturer's instructions are the authority on what it is approved for, not a general chart.
Rear-facing, and for how long
Under R129 a child must travel rear-facing until they are over 15 months old. That is the legal minimum, not a target. A forward-facing seat becomes an option after 15 months; it does not become the better option.
The reason to go on longer is mechanical. A toddler's head is large and heavy relative to the body and the neck is comparatively weak, and in a frontal impact a rear-facing seat spreads the load across the back and head instead of throwing the head forward against the restraint. Extended rear-facing seats exist for exactly this and are worth considering.
The equivalent under R44 is weight-based, which is why a baby can legally be turned forward-facing at 9 kg under that standard — a point at which many babies are nowhere near 15 months old. This is the clearest practical argument for choosing an R129 seat.
Backless booster cushions: what changed
This is the detail that catches people out, usually when handing a seat on to a younger sibling or a grandparent.
Manufacturers can now only make booster cushions approved as Group 3 — that is, for children of about 22 kg and upwards, and the change is commonly quoted as applying to children over 125 cm. Booster cushions approved earlier as Group 2, for children from 15 kg, remain legal to use.
Legal and advisable are not the same thing here. A backless cushion raises a child so the lap belt sits correctly but offers nothing at all for the upper body, no side protection and no guide for the diagonal strap. A high-backed booster does all three. Where there is a choice, the high-backed seat is the better one, and for a smaller child it is not really a choice.
Published figures on this differ in whether both the height and the weight condition must be met or only one. The seat's label settles it for that seat, and that is the thing to read.
Fitting
- A child car seat may only be used with a diagonal seat belt, unless the seat is specifically designed for a lap-only belt or is fitted using ISOFIX anchor points.
- A front airbag must be deactivated before a rear-facing baby seat is fitted in a front seat. An inflating airbag strikes the back of a rear-facing seat with enormous force, and this has killed children.
- Child car seats must not be fitted in side-facing seats.
- The back seat is the safer place, and the middle rear seat the safest of those where a seat can be properly fitted there.
- Where the back seats cannot accommodate another child seat, a child may travel in the front — in the correct seat, with the airbag rule above observed.
Fitting is the step most often got wrong, and it is worth having checked rather than assumed. Many retailers and some fire services and local road safety teams offer free checks.
Second-hand seats and seats after a crash
A second-hand seat is not recommended, and the reasons are practical rather than fussy: a seat may have been in a collision without any visible damage, may be missing parts or instructions, may have been recalled, and may simply not fit the car it is going into. A seat from a known history — an older sibling's, kept in the house the whole time — is a different proposition from one bought from a stranger.
Manufacturers advise replacing a seat that was in the car during a crash, whether or not the child was in it and whether or not damage can be seen. Structural damage to the shell and webbing is not reliably visible.
When a child may legally travel without one
The exceptions are narrow and specific, and knowing them is more useful than guessing in the moment.
- No seat belts fitted. A child aged three or over may travel in a back seat without a child seat or belt. A child under three may not travel in a vehicle with no seat belts, except in a taxi.
- Taxis and minicabs. If the driver does not provide a correct child seat, a child may travel without one. They must sit in the rear; a child aged three or over wears the adult belt, and a child under three travels unrestrained.
- Coaches and minibuses. A child may travel without a car seat and should sit in a rear seat. Where seats or suitable belts are absent, a child aged three or over uses a seat belt.
- An unexpected but necessary short journey. A child aged three or over may use an adult seat belt. "Short" is not defined, and the exception is for genuine necessity, not for convenience or a forgotten seat.
- Disability or a medical condition. A disabled person's belt or a restraint designed for the child's needs may be used, and a doctor can issue an exemption certificate where no restraint can be used at all.
Two things follow from that list. The taxi exception is real but it is an exception, not a recommendation — a child in a taxi on an adult belt is less well protected, and bringing a seat is better where it is possible. And the "unexpected necessity" exception does not stretch to a planned trip.
The penalty, and the insurance point
Using an unsuitable or incorrectly fitted car seat risks a fine of up to £500.
The quieter consequence is the insurance one: where a child passenger was not in the correct seat and is injured or killed, cover may not apply, and that includes temporary cover taken out to drive someone else's car. It is worth knowing before lending a car or borrowing one.
Where American advice goes wrong here
A large amount of parenting material in circulation was written to United States rules, and on this subject the differences are not cosmetic.
- "Rear-facing until one year." The UK minimum is over 15 months under R129, and longer is better. One year is not the rule here.
- Booster thresholds in pounds. American advice gives weights — 40 lb to move into a booster, 60 to 80 lb to leave one. The UK threshold for leaving a seat altogether is 135 cm or 12 years, measured by height and age.
- References to state law. There is one national rule.
- Airbag on/off switches. Common in the United States, uncommon on UK cars. Where there is no switch, a rear-facing seat does not go in the front at all.
- Seat standards. American seats are approved to a different standard and are not approved for use here; a seat bought in the United States is not legal in a UK car.
The journey itself — keeping a small child reasonably content for three hours in the back — is a separate matter, covered under travelling with toddlers.