To Booster or Not To Booster: Why Fit and Maturity Matter So Much

Updated January 2019.

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When my oldest was meeting the ‘milestone’ of age four, and reaching the ‘magical’ weight of 40 lbs, I started getting surprised looks and inquiring remarks about when she would be out of her ‘baby seat’ and into a booster.  Not for a while for our family (a year to a year and a half I thought) but with boostering on the horizon, I started to pay more attention to what boosters are out there and what the fit is like — and why it matters so much.

The transition from a forward-facing harnessed seat to use of the adult seat belt only is a period of surprisingly high misuse.  Many 40 lb kids are in poorly fitting backless boosters without adequate head support — putting them at tremendous risk for spinal injuries, head injuries, and internal injuries. I consider age five a minimum for boostering, with many kids not ready until age six or seven.  Four year olds — and certainly not three or two year olds — are simply not mature enough for a booster, and their little bodies aren’t ready for the added strain of only three contact points of a seat belt versus the five in a five-point harness.

I prefer to see kids start in a high-back booster and then move to a backless after a few years once the novelty of boostering has worn off. Read more about harnessed seats that convert to good boosters, and the different types of dedicated boosters.

In British Columbia (as in many other provinces and territories) a child may not ride in a booster until they are 40 lbs, the belt fits them properly while doing so, AND they wear it properly at all times.  A boostered child must use a lap/shoulder belt and never a lap belt only.  BC, just like every other region in Canada, is a ‘proper use’ province, meaning that not only does a child need to meet the minimum height and weight limits, they must USE the restraint properly at all times.  That means no leaning over, no belt behind the back or under the arm, no unbuckling, no slumping.  Ever.  We like to call it developmentally ready – some kids have it at five, many by six, most by seven.  If they’re not mature enough to remain in position at all times, even when sleeping, they should remain harnessed in a seat suitable for their height and weight.

It’s fairly easy to get a child’s proportions and know that s/he will fit in a harnessed seat, and for how long, both rear- and forward-facing.  How well a car seat might fit in the car is another matter entirely though, and it surprises many to learn that booster fit is even less predictable. It varies greatly from child to child, vehicle to vehicle, and even in different seating positions in the same vehicle.

Boosters work by positioning the child so the adult seat belt fits properly over the strongest parts of the body.  The lap belt must be low across the pelvis/hips and the shoulder belt must lie across the collarbone.  The seat depth of a booster is shorter than a vehicle seat so the child’s knees will bend comfortably rather than slouching, which contributes to proper belt fit.  Slouching causes the shoulder belt to rub against the neck or lie over the face, and causes the lap belt to ride up onto the soft abdomen.

Some boosters do a poor job of positioning one or both of the lap and shoulder belts, and knowing what to look for when booster shopping is a very important part of injury prevention.  If you don’t want to have to re-buckle the booster seat back in when it’s not occupied (so it’s not a projectile for you), pick a booster that can be UAS-ed (latched) into the vehicle.

Are you a visual learner? This short movie shows the difference between a child in a booster versus just the seat belt (it’s informative, but not gory).


Booster fit is further complicated by quirks of vehicles – awkward buckle stalks, belt geometry that doesn’t work with a particular booster, head restraints that don’t adjust or angle forward, or no head restraint at all.

Booster riders MUST have head support up to at least the tops of their ears.  This is complicated by the fact that some BOOSTERS require in-vehicle head support up that far too, so if you have an older vehicle with a bench seat and no head restraints, or head restraints that interfere with the top of the booster, you’ll need to shop carefully for a booster that doesn’t need a head restraint behind it, AND fits your child well.

The reverse of having an underage 40 lb child in a booster is having an underweight seven year old in a booster. Legally that seven year old must still be harnessed, and despite the potential issues with peers, that child is not safe or legal at 38 or 39 lbs in a booster. There is an increased risk of submarining (sliding under the lap belt) and ejection; both result in poor outcomes.

Parents with three kids in the back seat are often anxious to move one child to a booster to make the three-across situation more pleasant.  The reverse is usually true, as it’s extremely hard to buckle a booster seat in a tight situation.  In that scenario, keeping a child harnessed as long as possible usually results in far fewer scraped knuckles and frustrated kids.

When booster shopping, think carefully.  Is your child ready for the responsibility of a booster?  Then take them shopping with you and try them in your vehicle, in each position.  Make sure there’s a lap/shoulder belt where the child will be, and that there is a vehicle head restraint if the booster requires it.  If your child is on the younger side of booster readiness (five-six) aim for a high back.  If your vehicle doesn’t have a head restraint, get a high back that doesn’t need support.  If your child is older, and there’s a head restraint in the vehicle, a backless is probably fine but make sure the belt fits well either way.

When is a booster seat outgrown?  When a child reaches the stated height limit or weight limit, when the belt guide is no longer at or slightly above the shoulder in a high back booster, or when the tips of the ears reach the top of the shell in a child/booster seat converted to booster mode.  Read your manual carefully to know what steps need to be taken to convert a harnessed seat to booster mode, as all seats are different.

Don’t rush a child out of a booster either – in BC (other provinces vary) a child must remain in a booster until they are at least age nine or 4’9″ tall, but more importantly, make sure the adult seat belt fits them once they reach that stage.  Continuing to booster past that is preferred if the belt fits them better with the booster than without, so long as the child remains within the weight and height restrictions of their particular booster.  If the belt doesn’t fit properly, the child is not protected.

And now, the shopping part.  To make that a bit easier, we’ve narrowed down the options, but it’s only a starting point.  Conversely, a booster not on this list might work just fine for your child in your car, and if the belt fits well then go for it!

Best bets for good belt fit:
We can’t guarantee that these boosters will fit your child and your car well, but they’re a great place to start. Read about our favourite dedicated booster seats here.

14 comments to To Booster or Not To Booster: Why Fit and Maturity Matter So Much

  • Jennifer

    Great article. I am feeling pressured from my kindergartener to advance to a booster seat as she sees all the other kids in her class in one. My daughter is just about to turn 6 but is only 34 pounds. She is of average height for her age and her 5 point harness carseat is definitly becoming too small, as the straps are now sitting below her shoulder level at their highest setting. She’s most likely going to need to be in the carseat another 2 years or so at the rate she’s gaining. Also, it has become increasingly difficult to buckle her up now with her wearing her winter coat as the straps are not long enough. I was wondering if you could suggest some car seats that are designed for taller children

  • jshapka

    Check your email Jennifer – I sent you a reply, it might be in your spam folder.

  • Ely

    My 5 years old is severe underweight; average height for her age; she has been asking for a grown up seat, but still hasn’t reach te 30 pounds mark; looking for information online. Any suggestions

  • jshapka

    Ely — I sent you a response to your email. For anyone else reading a child must remain harnessed until 40lbs, which is tough for the older kids who don’t reach that for a long time. Her peers may very well be riding in backless boosters (which is not ideal) and her being stuck in a perceived ‘baby seat’ is challenging. Kids under 40lbs are not legal in a booster, and are at an increased risk of internal injuries, as well as submarining right under the belt. Maybe she’s mature enough to see crash test videos of this

    • Madeleine

      Hello, I would be interested in receiving some advice also. My 5 year old will start school this September and currently weighs about 26 pounds (11.8 kg to be exact). She is “outgrowing” her current seat hightwise with her head close to the top…even though she is not really that tall (about 1meter last checked at Dr.’s office)

      • jshapka

        Your child needs to be harnessed until consistently 40lbs. Would you like to get in touch on our Facebook page We’re much quicker to reply there as comments are moderated and we don’t remember to look at them other than when we’re running a contest (to be very honest!).

  • Makes me glad I homeschool – my kids have NEVER said anything about wanting to be in a “big kid seat” because of their peers being in them. My oldest only hit forty pounds after she turned six and stayed in a five point harness until that time. My oldest boy was six and a half when he finally hit forty pounds. He wanted to move to a booster and talked about it occasionally because his older sister was in one but it was never an issue of peer pressure. They spend time with other kids outside our family (I have five kids) but peers don’t seem to have any influence at this point in how they feel about things.

    This is a good time to start teaching our kids that peer pressure has to be ignored in the name of safety – that pressure leads so many to do stupid things later on in life anyway.

    My one year old will stay rear facing longer than his older siblings did and my three and five year olds will stay in five point probably as long as my oldest two did. I like knowing that I’ve done everything I can to keep them safe in the car.

  • Tara

    Hello, my son and I were recently in a car accident. And while I am suffering from a possible fractured clavicle and a very sore neck and back, ny eight year old son is totally fine. He was in his Britax Parkgate. We are now looking to replace his booster seat as it has now been in an accident, and I am struggling whith what to buy. He is a fairly big kid at 72lbs. I was looking for info on the safety stats of the cleck oobr as it uses the latch system, or the Diono. Or if I should just buy another Britaxb Parkgate. Do you know where I can find the safety stats on these booster seats to help me make my decision Thank you very much for for your help!


  • Elizabeth

    Do you have any information about the Graco Affix I ordered one and noticed that one side of the LATCH comes loose if you pull on the seat. I saw some other people comment on this online and someone said it isn’t safe because it could make the child twist to the side in a collision. I am going to return it for the new 2013 Britax Parkway SGL or The First Years Compass B570. I am trying to decide (this is for the car we don’t use as often and to have as an extra if we need to transfer the seat to a friend’s car). Do you know how much each of these seats weigh on their own Also, do you know if the B570 has the LATCH system Some reviews say that it does, but none of the descriptions, even on THe First Years website say anything about it.

    • jshapka

      Elizabeth ? I have heard good things about the Affix but haven?t used it myself. It?s important to note that the lower anchors of a booster do not restrain the child at all; they prevent the booster from becoming a projectile when the seat is unoccupied, and the seat belt restrains the child when it is occupied. My first step would be to contact Graco to ask about the loosening as there?s maybe an easy way to avoid that.

      The new Parkway SGL isn?t out yet in Canada so I don?t have much to say about that one yet. The B570 has lower anchors but Amazon is the only place I know of that carries it ( The belt fit can be very good with it but not as reliable as some of the others. The Parkway and the Compass seats are both high back whereas the Affix is backless. How old is the child the seat is for, and what are the head rests like in the vehicle (adjustable, removable, fixed, leaning forward). The Parkway SGL (current version) is about 9.7lbs and I don?t have a number on the Compass seat although Amazon lists the shipping weight at 7kg (17.8lbs) but that sounds high to me. Jen

  • […] and for all but the tallest or long torsoed kids it’s reasonable to expect Foonf to last to booster readiness.? It’s important to remember that each child is different and due to their shape might fit a […]

  • Sarah

    I love this article but wish it had a but more info about the dangers of bolstering too soon. Perhaps some stories or videos.

  • Shauna

    I have a seven year old. He is a solid 85lbs. 138cm, nt tall enough for leaving a booster seat.
    However, in our car the booster seat does not fit him well. He has better support without the seat.
    The booster seat does not support his femur, as it is too short.
    The seat belt fits without the booster
    But yet I am considered illegal if he isn’t in a booster

    • jshapka

      Have you considered using a different booster that does provide good leg support Or using something lightweight (such as a rolled sleeping bag or styrofoam cooler — things that won’t be a dangerous projectile in a crash) under his feet for support Are you using the 5-step test to evaluate seat belt fit