Britax Marathon Strap Adjustment Instructions

Correctly using a car seat might seem overwhelming, as it has many straps and adjustments. If you have purchased the Britax Marathon, you can help it work correctly by making sure the straps are properly adjusted 1. The Marathon’s straps adjust to fit children from five to 65 lbs., and the Marathon 70 fits children from five to 70 lbs., so adjusting the straps is an important step to ensure that your children are safe in their seats, whatever their weight may be.

Loosen the harness straps. Pull on the straps while lifting the harness adjuster lever, which you can find at the bottom front of the Marathon.

Unhook the shoulder straps from the harness yoke. You will find this on the back of the car seat.

Pull the straps through the front of the seat. Pull them completely through the shell, foam and cloth.

Place your child in the car seat to determine which shoulder slot fits best. When your child is rear-facing, the seat’s harness should go through the slot that is at or directly below your child’s shoulders. If your child is rear-facing and too small to fit in the lowest strap slot, choose another seat for your child, as the Britax Marathon user guide recommends 1. If your child is forward-facing, the shoulder straps should be at the shoulders or in the slot directly above. The top of the child’s ears must be below the top of the seat. If they are not, then your child is too big for the Britax Marathon 1.

Take the child out of the seat.

Thread the harness straps through the correct slots to the back of the Marathon.

Reattach the harness straps to the harness yoke.

Pull the harness adjuster lever to tighten the straps as needed to fit your child.


The Britax Marathon can go to a reclined position when it is forward-facing. If your child is in the recline position, you might need to adjust the straps to have them fit correctly.

Loosen the harness adjuster lever when you take your child out of the seat every time. That way, you can tighten it each time your child gets back in the car. Some clothes are bulkier than others, and tightening the straps with each use will help keep your baby safe.


Read your Britax Marathon user’s guide carefully to avoid incorrectly adjusting your straps.

After you install the straps, check that they are not twisted anywhere, as this could result in the straps not working properly in an accident.

Both sides should be threaded through the same-height shoulder strap slot.

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