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Original poster

Does ISOFIX matter for a high-back booster for a nine-year-old?

18 replies

Anon501178 · 07/04/2026 22:00

DD has just turned 9- she has been using an isofix high back booster for several years now.She is quite short and slim for her age although don't know her exact height.Weight 25kg.It seems alot of kids her age are in backless boosters or even no car seat, but we would like to continue with the high back booster for awhile longer due to it offering much more protection than a backless one, which obviously basically just makes them sit higher.
We have another baby on the way soon and so looking at car options....it seems there are very few which can accommodate 3 isofix seats across the back.
I have always felt isofix is safer and better method of installation even for HBB as it moves around so much less when clamped in, but have been thinking that as she is using the cars normal seatbelt as her only restraint anyway and not a harness, does having isofix actually make much difference to the impact/safety to her in a crash, as however well the seat is clamped into the car, the restraint holding her to the seat is the same method of adult seatbelt, isofix or no isofix.
I am wondering if there are any car seat gurus on here who can help.Safety is a priority for us but if we can widen our car then that would be useful.

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Lavender2021 · 07/04/2026 22:06

One of the reason for isofix on a high back booster seat is so you don't have to remember to seat belt it in when empty or in the event of an accident it will fly around the car hitting anyone near it.

Workinggreen · 07/04/2026 22:09

Can’t isofix only go up to a certain weight anyway? Like 25kg I think?

isofix and seat belt are meant to be just as safe as each other when done correctly, the difference is only that isofix is basically impossible to do incorrectly.

even if there’s no isofix you need to check the car manual allows a car seat to go in that seat

Koalaslippers · 07/04/2026 22:11

The main advantage of isofix on a HBB is that it keeps it secure when the child isn't in it. Without it the seat needs to be seat belted in when the child isn't in it otherwise it can become a projectile in a crash. It's not essential on a HBB, more of a nice to have.

TheBoldZebra · 07/04/2026 22:20

I thought the maximum weight of child using isofix was 18kg? We have a rear facing axkid and had to swap to belted installation at 18kg.

OP posts:
Hotdoughnut · 07/04/2026 22:27

Isofix is actually not safe past a certain weight. Please look into this. Once a child is heavy enough, the high-backed booster is meant to move with them in the event of an accident, with seatbelt then catching them and the chair. Many people are not aware of this.

Workinggreen · 07/04/2026 22:34

As far as I know there is no upper limit on isofix, and it doesn't say so for this one.
a quick google says there is. And it says on this car seat instructions that after 100cm you should switch to using the seatbelt, though you can leave it attached to the isofix it is the seatbelt that’s securing the seat and child.

lochmaree · 07/04/2026 22:34

The Audi Q7 from 2015 has isofix points in 6/7 seats. It's what I've got my eye on for next car 😂

Original poster
Anon501178 · 07/04/2026 22:35

Have found afew sources after a quick Google search saying ISOFIX can be used upto a combined weight of 33kg (child and seat) interesting as hadn't heard that before, and it doesn't seem to be widespread information for some reason!

OP posts:
Original poster
Anon501178 · 07/04/2026 22:38

Workinggreen · 07/04/2026 22:34

As far as I know there is no upper limit on isofix, and it doesn't say so for this one.
a quick google says there is. And it says on this car seat instructions that after 100cm you should switch to using the seatbelt, though you can leave it attached to the isofix it is the seatbelt that’s securing the seat and child.

She has been using it with the adult seatbelt.The isofix secures the seat to the car, but this was my thinking...would it actually make any difference to the safety of the child in a crash if they are strapped in to the seat the same way regardless.
I might have to look at the weight of the carseat itself.

OP posts:
DragonsFurry · 07/04/2026 22:38

Hotdoughnut · 07/04/2026 22:27

Isofix is actually not safe past a certain weight. Please look into this. Once a child is heavy enough, the high-backed booster is meant to move with them in the event of an accident, with seatbelt then catching them and the chair. Many people are not aware of this.

This is what I thought too. According to nurse that I knew, the main purpose of a high-backed booster is to move with the child and support them, acting like a stretcher so the emergency services can move the child out of the car more easily.

lochmaree · 07/04/2026 22:41

I thought for HBBs, the isofix is to secure it to the cars seat, while the seatbelt secures the child onto the HBB. @bertiebotts maybe be able to answer the isofix question.

BertieBotts · 08/04/2026 00:31

The 33kg combined weight only applies to seats which have an integrated harness, ie where the child is attached to the seat and the isofix attaches the seat including strapped-in-child to the car. You will never need to work this out as it's always counted into the weight limit on the harness for the seat, that's why it's not widely known.

There is no maximum for the isofix on a high back booster, all it does is stabilise the seat. The child is restrained by the adult seatbelt, which has no upper weight limit, so an isofix weight limit wouldn't make sense in that context.

In theory, you don't need to use the isofix fittings for a pure high back booster seat, as the seatbelt restrains both seat and child. But for some combination seats the manual specifies that it does need to be used. The manual for this seat which I've just got from the Graco website is a bit confusing with lots of back and forth from text to images and text in unclear order, but it says:

"The installation with ISOFIX connectors in booster mode is optional. If ISOFIX connectors are used, make sure the indicator colors on both connectors are completely green when attached to the anchor points"

The top tether is the part which is not really clear - usually in these combination type seats, you would only use the top tether when the harness is being used, and must always use it when the harness is being used. In booster mode, the tether should be detached. But the manual just opts to not mention the tether at all in the section for installation for 100-150cm. IMO this is unclear because it doesn't explicitly tell you to remove the tether, and in the general instructions for "child restraint installation" it does show how to fix the tether and states only that when the tether is not in use, it should be stowed away. It doesn't give any guidance about when it should be used or not.

Based on more explicit guidance in other seats, you don't need the tether for booster mode, and it's possible that it can impact on the performance of the seat. If you want it clearer though I would suggest writing to Graco to ask. I know some of the Joie seats do allow the top tether to be used in booster mode, which is unusual.

By the way, there is a button on the underneath of the seat near the front, which can be squeezed a bit like a recline lever and this should allow the isofix connectors to slide in and out. If it doesn't completely go in, try extending it to its maximum and then see if the isofix will rotate and slot back in the other way around, that's how our Joie booster seat works. The button should also be used to reduce any gap behind the back of the seat caused by the length of the isofix connectors.

I don't think it's true that the isofix prevents the seat moving with the child. You don't really want their pelvis moving around very much at all. Top tether might prevent the seat from moving which is my understanding of why it's usually not recommended, and why you should be careful not to "wedge" a booster seat under the car headrest. IIRC there is a very slight theoretical benefit from isofix in terms of side impact protection, but it doesn't make a huge amount of difference, and if she will be in the middle then she is not at high risk from side impacts anyway.

BertieBotts · 08/04/2026 00:32

Lavender2021 · 07/04/2026 22:06

One of the reason for isofix on a high back booster seat is so you don't have to remember to seat belt it in when empty or in the event of an accident it will fly around the car hitting anyone near it.

This is true though. Most people don't do this but you are supposed to.

Original poster
Anon501178 · 08/04/2026 16:46

BertieBotts · 08/04/2026 00:31

The 33kg combined weight only applies to seats which have an integrated harness, ie where the child is attached to the seat and the isofix attaches the seat including strapped-in-child to the car. You will never need to work this out as it's always counted into the weight limit on the harness for the seat, that's why it's not widely known.

There is no maximum for the isofix on a high back booster, all it does is stabilise the seat. The child is restrained by the adult seatbelt, which has no upper weight limit, so an isofix weight limit wouldn't make sense in that context.

In theory, you don't need to use the isofix fittings for a pure high back booster seat, as the seatbelt restrains both seat and child. But for some combination seats the manual specifies that it does need to be used. The manual for this seat which I've just got from the Graco website is a bit confusing with lots of back and forth from text to images and text in unclear order, but it says:

"The installation with ISOFIX connectors in booster mode is optional. If ISOFIX connectors are used, make sure the indicator colors on both connectors are completely green when attached to the anchor points"

The top tether is the part which is not really clear - usually in these combination type seats, you would only use the top tether when the harness is being used, and must always use it when the harness is being used. In booster mode, the tether should be detached. But the manual just opts to not mention the tether at all in the section for installation for 100-150cm. IMO this is unclear because it doesn't explicitly tell you to remove the tether, and in the general instructions for "child restraint installation" it does show how to fix the tether and states only that when the tether is not in use, it should be stowed away. It doesn't give any guidance about when it should be used or not.

Based on more explicit guidance in other seats, you don't need the tether for booster mode, and it's possible that it can impact on the performance of the seat. If you want it clearer though I would suggest writing to Graco to ask. I know some of the Joie seats do allow the top tether to be used in booster mode, which is unusual.

By the way, there is a button on the underneath of the seat near the front, which can be squeezed a bit like a recline lever and this should allow the isofix connectors to slide in and out. If it doesn't completely go in, try extending it to its maximum and then see if the isofix will rotate and slot back in the other way around, that's how our Joie booster seat works. The button should also be used to reduce any gap behind the back of the seat caused by the length of the isofix connectors.

I don't think it's true that the isofix prevents the seat moving with the child. You don't really want their pelvis moving around very much at all. Top tether might prevent the seat from moving which is my understanding of why it's usually not recommended, and why you should be careful not to "wedge" a booster seat under the car headrest. IIRC there is a very slight theoretical benefit from isofix in terms of side impact protection, but it doesn't make a huge amount of difference, and if she will be in the middle then she is not at high risk from side impacts anyway.

Thanks so much for all this useful advice! I was hoping you might comment as have heard from other posts that you know your stuff.
I guess i was just thinking in an accident that the isofix would mean the seat does not move around and stays secure, whereas with the highback boosters only secured by the seatbelt they just seem so wobbly and move around all over the place even when DD is sitting in it, so feel like in a crash they would move around all over the place, dislodge and potentially cause further injury.

OP posts:
pepperminticecream · 08/04/2026 16:52

Really grateful for this thread. I have been trying to figure this same thing out and the posts on here clarified it all for me.

WhatNextImScared · 08/04/2026 16:53

Lavender2021 · 07/04/2026 22:06

One of the reason for isofix on a high back booster seat is so you don't have to remember to seat belt it in when empty or in the event of an accident it will fly around the car hitting anyone near it.

No most seatbelted ones have got some other form of fix to prevent that eg a headrest tag

BertieBotts · 08/04/2026 23:03

Anon501178 · 08/04/2026 16:46

Thanks so much for all this useful advice! I was hoping you might comment as have heard from other posts that you know your stuff.
I guess i was just thinking in an accident that the isofix would mean the seat does not move around and stays secure, whereas with the highback boosters only secured by the seatbelt they just seem so wobbly and move around all over the place even when DD is sitting in it, so feel like in a crash they would move around all over the place, dislodge and potentially cause further injury.

Show quote history

They might move in the rebound phase. This is less important - it's the initial phase of the crash where it matters. The high back booster itself is doing absolutely nothing at all in this phase, unless you have a side impact, then it might be a barrier between your child's head and the side of the car. But in a frontal crash, all the booster does is ensure that the seatbelt is in the right position to do its job. There is no difference in this case between an isofixed vs unfixed booster. It's hard to find direct comparisons but if you watch older crash test videos from when you'd have an isofix vs non isofix version of the same booster, the movement is normally about the same. The seatbelt is the thing which is going to stop your child from flying around the car.

The only way it could dislodge is if you don't have the lap belt running properly under the guides on both sides. The part of the diagonal belt close to the buckle is usually also under the guide.

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