i-Size car seat (R129): how to choose by height in Portugal

i-Size car seat (R129): how to choose by height in Portugal

The biggest change in the R129/i-Size standard is simple: the car seat is no longer chosen "by kg" but "by cm ". This reduces confusion and helps you buy better.

1) First step: measure the height (cm) and don't "guess"

  • Measure the child barefoot, leaning against the wall.

  • Keep track of the measurements (e.g., 74 cm, 92 cm, 118 cm).

  • Purchase based on the product's width range (e.g., 40–75 cm, 76–105 cm, 100–150 cm).

Why? i-Size approves chairs by height, not by weight.

2) The most common “phases” (in practice)

Stage 1 — newborn and toddler (e.g., ~40–75 cm)

Normally:

  • "Egg" with internal harness.

  • It can have an ISOFIX base (very practical).

Search:

  • Newborn reducer.

  • Good lateral protection.

  • Easy to attach/detach.

Stage 2 — baby/toddler (e.g., ~61–105 cm)

Here are the most commonly used models:

  • i-Size chairs with 360º rotation

  • ISOFIX installation + support leg/top tether (depending on the model)

Important:

  • In the R129/i-Size system, the child should face backward until 15 months of age .

  • Many parents keep their children rear-facing for longer periods (for comfort and protection).

Stage 3 — older child (e.g., ~100–150 cm)

Normally:

  • Booster seat with backrest (recommended) or without backrest (later).

  • The child begins to use the car's seatbelt , guided by the car seat.

Objective:

  • The belt should be correctly positioned over the shoulder and pelvis (not the neck/stomach).

3) i-Size ≠ “fits any car”: compatibility is mandatory

Even though it's "standard," you have to confirm it:

A) Does the car have ISOFIX?

ISOFIX is a seatbelt-free fastening system that reduces the risk of incorrect installation.

B) Is your place i-Size?

Some cars have seats marked as "i-Size". If yours doesn't, many car seats are still compatible — but always check the manufacturer's list.

C) Do you have real space?

Rear-facing seats take up more depth. Confirm that the front passenger is in a safe position.

4) How to choose in 60 seconds (rule of thumb)

  1. Child's height (cm) → choose the correct range

  2. ISOFIX in your car? → If yes, prioritize i-Size/ISOFIX.

  3. Small baby? → baby car seat + base (if possible)

  4. Daily use? → 360º helps a lot.

  5. Older child? → booster seat with backrest, if possible.

5) Common mistakes when buying (and how to avoid them)

  • Buying "thinking about the future" and ending up with a chair that's too big now.

  • Ignore car compatibility

  • Choosing a backless booster seat too early

  • Buy online without checking the label UN R129 / i-Size

FAQ

What is the advantage of choosing by height?
It reduces errors and makes the choice more straightforward in the i-Size standard.

Until when is it mandatory to face backwards?
Up to 15 months (R129/i-Size applicable).

Is a booster seat without a backrest okay?
It's better to postpone and opt for a backrest (side protection + seatbelt guide).