Child car seats: choosing the right size and fitting ISOFIX

A child seat must fit the child, the vehicle and the seat manufacturer’s instructions. ISOFIX does not mean every seat fits every car. Check the approval label, height or weight range and approved seating positions before buying.

Choose by the child and approval

GOV.UK says children normally use a child car seat until they are 12 or 135cm tall, whichever comes first. Read the current child seat rules, then follow the specific seat instructions. Height-based R129 seats and older weight-based approvals have different labels and limits. Do not move a child up a seat size just because of age if they remain within the current seat’s limits.

For a used seat, ask about its age, crash history, parts and manual. Check recalls and damage. A seat without a legible approval label or complete harness is difficult to assess.

Test the fit in the car

The vehicle handbook identifies ISOFIX points, top tether anchors and permitted seating positions. Seat belt and ISOFIX are different installation methods; do not combine them unless both manuals allow it. Try the actual seat before buying: seat shape, buckle position, support leg and space for the front passenger can all matter.

Follow every fitting step. Harness straps should be flat and snug. Bulky coats can hide slack; follow the seat maker’s advice about clothing. Never use unapproved inserts or accessories. If unsure, ask a trained child-seat fitting service.

After a collision, check with the manufacturer whether the seat should be replaced, even when damage is not visible. Bring the seat on a family-car test drive and check everyday access. For a used vehicle, combine this with a history check and dealer protection information.