FamilyBuys
Guide · researched by our editor

i-Size car seats by child height: a UK guide

By Frank Jones · Published

In short

Choose an i-Size seat by your child’s height, not their age. By law, height-based seats must be rear-facing until your child is over 15 months old, and children must normally use a car seat until they are 12 or 135cm tall. Check the seat maker’s vehicle fit list for your car, and buy new rather than second-hand.

The rules that matter

Source: GOV.UK: Child car seats, the law.

The height ladder

  1. 40cm (birth) to about 75–87cm: Infant carrier, rear-facing. Carry-in seats that click into a base or fit with the belt. Most babies outgrow them at roughly 12 to 15 months.
  2. 40cm to 105cm: Birth-to-about-4 seat, rear-facing then forward-facing. Usually ISOFIX and often rotating. Rear-faces to at least 15 months by law, and many seats allow longer.
  3. About 61–76cm to 105cm: Toddler seat. For when your baby outgrows the carrier. Many fit on the same base as the carrier.
  4. 100cm to 150cm: High-back booster (R129). Uses the car’s adult seat belt, guided by the booster. Some also clip to ISOFIX for stability.
  5. Over 135cm, or 12 years old: Adult seat belt. GOV.UK says children must normally use a car seat until they are 12 or 135cm tall, whichever comes first.

Before you buy: three checks

  1. Vehicle fit list. Look up your exact car (model and year) on the seat maker’s vehicle fit list. ISOFIX bars, support legs and seat shape all vary.
  2. Your child’s height and the seat’s range. Buy for the stage your child is at now, with room to grow. Check the cm range on the seat’s label.
  3. Buy new. We advise against second-hand seats. A seat that has been in a crash may have damage you can’t see (Child Car Seats, RoSPA). Many retailers will help you check the fit.

Still planning for a new baby? The car seat is on our new-parent checklist, and a travel system from our best prams list may take an infant carrier.

Car seat questions

What does i-Size mean?

i-Size seats are approved to the UN R129 regulation and are chosen by your child’s height in centimetres rather than weight. GOV.UK says approved height-based seats have a label with a capital E in a circle and R129.

How long must my child rear-face?

GOV.UK says height-based seats must be rear-facing until your child is over 15 months old. Many i-Size seats let children rear-face for longer, to 105cm, so check the seat’s manual.

Will an i-Size seat fit my car?

Not always. Check the seat maker’s vehicle fit list for your exact car model and year before you buy, and try it in your car if you can. ISOFIX points and a support leg both have to suit your car.

Can I buy a second-hand car seat?

We advise against it. The Child Car Seats website, run by RoSPA, says a seat may have been in a crash and have damage you can’t see, and may come without its instructions. Buy new from a retailer who can help you check the fit.

When can my child stop using a car seat?

GOV.UK says children must normally use a child car seat until they’re 12 years old or 135cm tall, whichever comes first.

Sources

  1. GOV.UK – Child car seats: the law
  2. Child Car Seats (RoSPA) – Choosing child car seats
  3. Child Car Seats (RoSPA) – Second-hand child seats

General information, not legal or safety-fitting advice. Always follow the seat maker’s instructions.