i-Size vs R129: are they the same thing?

Short answer: R129 is the regulation; i-Size is one of its categories. Every i-Size seat is approved to R129, but many R129 seats are not i-Size.

What “i-Size” means in the regulation

R129 defines i-Size as an Integral Universal ISOFIX Enhanced Child Restraint System: a seat with its own harness or shield, attached by two ISOFIX connectors plus a top tether or support leg, designed for use in every i-Size seating position of a vehicle (§2.3.1). Vehicles declare i-Size positions under UN R14/R145 and R16.

There is also the i-Size booster seat — a non-integral high-back booster that fits the ISO/B2 envelope and is primarily for i-Size positions (§2.3.3).

R129 seats that are not i-Size

• Universal Belted seats, installed with the adult belt only (§2.3.2).
• Universal booster cushions (§2.3.4).
• Specific vehicle ISOFIX seats, booster seats, belted seats and cushions — approved for listed vehicles or ISO size classes (§2.7).

How to tell from the seat

An i-Size seat carries the i-Size logo (at least 25 × 25 mm) on the part with the ISOFIX attachments, and its approval mark says “i-Size universal ISOFIX” or “i-Size booster seat” (§4.7.1, §4.8.1, §5.4.2.1). A specific vehicle seat says so, and lists or references the vehicles it fits. See how to read an R129 label.

Why it matters

For parents, i-Size means “fits any i-Size position without checking a list”. For brands, choosing i-Size means meeting the ISO/F2x or ISO/R2 envelope, the support-leg geometry and the 33 kg combined-mass cap — design constraints that shape the product from day one.