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

Updated December 2019.

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When my oldest was meeting the so-called 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. Yikes – I only just turned her forward facing a few months earlier! She wouldn’t be boostered 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 fit matters so much.

The transition from a forward-facing harnessed seat to use of the adult seat belt 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.

What is the legal minimum?

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 they 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.

How do booster seats work with the adult seat belt?

First – note the emphasis on adult here. Seat belts are designed to fit adult bodies, and children are not just mini adults. 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, making contact with the chest.

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).


What matters for booster fit?

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.

Won’t a booster fit more easily in my car?

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.

How do I choose a booster seat?

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 combination seat or 3-in-1 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.

Too many choices. Where do I start?

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! 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.