Britax DUALFIX i-Size — EU Car Seat

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Britax DUALFIX i-Size

Britax DUALFIX i-Size

Britax’s DUALFIX convertible car seat has long been one of the more popular swivelling car seats on the market here in Europe. Now, Britax has updated the old favourite to make it compliant with the new R129 (commonly referred to as i-Size) regulations. Suitable from birth to 105 cm or 18 kg, the new DUALFIX i-Size looks pretty awesome, but can it live up to the hype? Let’s see how it stacks up!

CSFTL Quick Stats


  • Lowest harness position: 19 cm (7.5 inches)
  • Highest harness position: 37 cm (14.5 inches)
  • Length (from anti-rebound bar to front of base): 63 cm (25 inches)
  • Width at widest point: 46 cm (18 inches)
  • Weight: 15 kg (33 pounds) It’s heavy!

Unique Features

It Swivels!

Britax DUALFIX swivelled for loading

Britax DUALFIX i-Size swivelled for loading

I have to admit that I had never used a swivelling seat before I got the DUALFIX i-Size to try out. I didn’t think a swivel was really necessary — loading my older son into his rear facing seat in our Ford Focus was never much of a problem. The swivel feature seemed like something that I just didn’t really need.

Then we got a rather small BMW i3 as a lease car, and my whole perspective on the swivel changed. Now I was loading my (much bigger) younger son into his seat through a teeny tiny gap because the rear doors on the i3 open backward instead of forward. It was awful. So when Britax offered to let me review the DUALFIX i-Size I absolutely jumped at the chance!

And oh my goodness, what a difference it makes. No more bumped heads and screaming babies. The swivel is smooth and easy to do — just push the button on the side of the seat and swivel the seat to the side. Then swivel it back around after baby is safely harnessed and off you go. It’s like magic!

I was worried that the slight lean of the seat when it’s facing the door would make it hard to centre my son and get him properly adjusted before rotating him back around, but I haven’t found that to be the case. He doesn’t slouch to one side, and I haven’t had any trouble with the harness being uneven.

Infant Insert

Britax DUALFIX insert

Britax DUALFIX i-Size insert

The DUALFIX i-Size features a very chunky infant insert to help position smaller babies.  The insert must be removed when the baby reaches 60 cm long — since 76 cm is the minimum height to use this seat forward facing, this insert can only be used when the seat is installed rear facing  This height limit could mean that young babies without good head control are too upright once the insert is removed. The insert is intended to ensure that smaller babies are well supported and properly positioned to keep their airways open and the harness correctly positioned.

The sides and back have polystyrene blocks with padding over top for added side impact protection, and the head portion has thick, cushy padding.  The insert is washable, but all of the foam must be removed before washing. A handy zipper on the back makes it a doddle to remove and replace the foam in the body portion of the insert.

Fun Cover Options

We got a preview of Britax’s new patterns and colours at Kind und Jugend last year. We were lucky enough to get the Letter Design pattern on the DUALFIX i-Size, and it’s fabulous! I love patterned covers not only because they’re fun to look at, but also because they tend to hide stains a bit better than some of the monotone colours. I’m not ashamed to admit that I have spent an inordinate amount of time trying to find hidden words in the pattern, but so far I’ve only come up with “moi”.

The cover is removable and washable on delicate at 30 degrees, followed by drip drying.

No-Rethread Harness

Britax DUALFIX i-Size headrest adjustment handle

Britax DUALFIX i-Size headrest adjustment handle

The harness on the DUALFIX i-Size automatically adjusts when the headrest is raised or lowered. The manual says to ensure the the straps come out at or just slightly above the child’s shoulder (never as high as the ear) both rear and forward facing.

This is a good illustration of how European car seats do not follow the rule of “at or below for rear facing” that North American seats follow. There are enough positions on the headrest that it’s pretty easy to find a setting that is just at or a tiny bit above the shoulders.

To adjust the headrest and harness, make sure the harness is loose, then squeeze the handle at the back of the headrest and push it down or pull it up to the correct setting.

Anti-Rebound Bar

Britax DUALFIX i-Size anti-rebound bar adjustment handle

Britax DUALFIX i-Size anti-rebound bar adjustment handle

The DUALFIX i-Size features a two-position anti-rebound bar. This allows for more legroom once baby is older. To adjust the anti-rebound bar, squeeze the handle on the base with one hand while pulling up on the bar with the other hand. Put the bar in the desired position, then push it down to lock it in place.

Installation: ISOFIX Only

Britax DUALFIX i-Size in a BMW X2 -- passenger seat is very far forward

Britax DUALFIX i-Size in a BMW X2 — passenger seat is very far forward

The DUALFIX i-Size is an ISOFIX only seat, so it can’t be installed with the vehicle seat belt. The seat is bulky enough that it can be awkward to install, especially in small vehicles. While it fit pretty well in a BMW i3, it was very difficult to install in a BMW X2. Surprisingly, I was able to fit it in our Ford Focus behind my somewhat tall fiance without moving his seat, though it definitely involved some fairly impressive acrobatics on my part!

Before installing the seat, decide whether the anti-rebound bar should be in the outer or inner position. If the inner position leaves a gap between the vehicle seat back and the bar, go ahead and put the bar into the outer position.

Britax DUALFIX ISOFIX release strap

Britax DUALFIX ISOFIX release strap

Next, pull the strap under the rebound bar to extend the ISOFIX arms. I found this was often very difficult to do — I pulled the strap, but nothing happened. Finally I realised that the strap was meant to pull on a mechanism to “unlock” the ISOFIX.  Then, I was able to easily squeeze this mechanism with my hand instead of pulling the strap, which made everything much easier.

If you do pull the strap to extend the ISOFIX arms, be prepared for them to fly out all of a sudden. You do NOT want them, for instance, pointing down towards your leather vehicle seats when they suddenly come out. Trust me on this.

Britax DUALFIX ISOFIX indicators

Britax DUALFIX ISOFIX indicators

Make the sure the load leg is rotated out from under the seat. Then line up the ISOFIX arms with the ISOFIX anchors in the vehicle and push them into place. The indicators on the arms will turn green when they’re correctly installed. If you have trouble reaching the ISOFIX anchors, you can attach the included black plastic anchor guides to them to make them easier to access.

Britax DUALFIX i-Size load leg indicator

Britax DUALFIX i-Size load leg indicator

Push the car seat towards the vehicle seat back until it makes firm contact. Finally, extend the load leg by pushing the two buttons on the middle and pulling out the bottom of the leg. Push the foot firmly onto the vehicle floor — the indicator will show green when it is properly installed.

The DUALFIX weighs a whopping 15 kg (33 pounds), so this install is not something you’re going to want to redo very often!

Fit to Child

Newborn Doll

Britax DUALFIX newborn fit

Britax DUALFIX i-Size newborn fit

Our Ikea doll is our newborn stand-in. He has a torso that is roughly equivalent to a newborn’s torso (and you can see that he fits perfectly in one of my son’s preemie sized outfits!). Ikea baby’s fit in the DUALFIX i-Size is beautiful. The harness tightened easily and snugly on him.

Newborns require an angle no more upright than 45 degrees, and sometimes this can be difficult to achieve with ISOFIX seats. The DUALFIX i-Size had an ok newborn angle in our BMW i3, though it teetered on being too upright. We’d always recommend trying the seat and checking the angle before you put a newborn in it.

8 Months Old

Britax DUALFIX i-Size 8 month old fit

Britax DUALFIX i-Size 8 month old fit

This model is 8 months old, about 70 cm and about 8.5 kg. He loves the DUALFIX i-Size! He spent his first several rides staring adoringly at his brother or looking out the window and giggling. The harness is very smooth to tighten and loosen, and the swivel feature makes it incredibly easy to get him in and out of his seat.

I did find that his head slumped a bit when he slept. However, as he’s a healthy 8 month old, I know that head slump is not a concern. When he got uncomfortable, he moved!

He’s currently on the most reclined position, and he seems very comfy. He has quite a bit of room to grow in the seat, though at his current growth curve he may still need another harnessed seat once he outgrows the DUALFIX i-Size.

1 Year Old

Britax DUALFIX i-Size 1 year old fit

Britax DUALFIX i-Size 1 year old fit

Here’s our model again on his first birthday! He’s now about 74 cm (29 inches) and 9.8 kg (21 pounds 10 ounces). He still gets a great fit in the DUALFIX i-Size. He now usually rides one or two settings up from the most reclined, but I still put it fully reclined if I think he’s going to fall asleep — the ability to recline the seat on the go makes this very easy to do!

Unfortunately, we live in a country that is made entirely out of sand. Now that my son is more mobile, he has brought some of this sand into his seat. Sand is never a great thing for car seats, but for the DUALFIX i-Size it’s an even bigger problem. If sand gets into the track that the seat swivels on it becomes very difficult to turn. I’ve had to make extra sure to give him a good cleaning before he gets in the seat and have needed to vacuum out sand from the DUALFIX i-Size’s little crevices more than once.

He also has a tendency to want to bring Duplo bricks into the car. This practice has now been completely banned because I value the skin on my hands and have lost significant chunks trying to dig those bricks out from inside the swivel mechanism.

6 Years Old — Outgrown

Britax DUALFIX i-Size 6 year old fit

Britax DUALFIX i-Size 6 year old fit

My petite 6 year old model normally rides in a high back booster, but I was curious how or if he would fit in the DUALFIX i-Size. At 111 cm tall he’s well over the standing height limit of 105 cm, but at 17.7 kg he’s still a hair under the weight limit. So while he definitely can’t ride in the seat, I was curious if he still fit.

And, much to my surprise, he does still fit! The harness is right at his shoulders; one big sneeze and he’ll be too big, but right now he still fits by harness height. This means that he definitely could have stayed in the DUALFIX i-Size until he reached the standing height limit when he was 5.

Britax DUALFIX i-Size 6 year old rear facing fit

Britax DUALFIX i-Size 6 year old rear facing fit

He fits both rear facing and forward facing, though he did complain that his legs were cramped while rear facing. Again, he’s well over the standing height limit, so he couldn’t ride in the seat, but if he were smaller I could reinstall with the rebound bar in the outer position to give him a bit more leg room.

I was worried that it would be difficult to swivel the seat with a child so close to the weight limit of the seat. However, while it was certainly more difficult to swivel than with my younger model, it was far from impossible.

Important Information: Where to Find

Expiration: Britax doesn’t specify a maximum period of use.

Britax DUALFIX i-Size manual storage

Britax DUALFIX i-Size manual storage

Manual storage: A hinged door at the back of the car seat hides a little compartment for the manual.

Airline approval: The DUALFIX i-Size is NOT approved for use on aircraft.

Overall Thoughts

The DUALFIX i-Size takes an old favourite and improves on it in so many ways. From an improved anti-rebound bar to the freedom to choose which recline you’d like at any time and a longer lasting harness, the DUALFIX i-Size ticks a lot of boxes. All of this comes in a hefty package (look elsewhere if you need to move the car seat between vehicles regularly!) and with a higher price tag, but if you’re looking for an i-Size seat it’s a great option to consider.

Note: Britax also markets the SWINGFIX i-Size. The SWINGFIX is identical to the DUALFIX except it can’t be used forward facing. As a seat that can only rear face, it has passed the Swedish Plus test.