A key part of our evaluations of booster seats is assessing the ability of each booster to provide a good belt fit on our 6-year-old-child size dummy in a variety of test vehicles and seating locations. But the dummy is far from being representative of all of the ages, sizes and shapes of children that will ride in booster seats.

To illustrate the point, we invited my own 6-year-old son Griffin to stop by our auto test center so we could compare him to the 6-year-old-size dummy. Interestingly, my “average-Joe” 6-year-old turned out to be nearly exactly the same height (approximately 48”) and only 1 pound heavier than his 52-pound inanimate counterpart. But, as you can see in the photo, while they are close in height and weight, there are differences in their stature. My son’s head is bigger, his torso is longer (which means his legs are shorter), and his shoulders are broader. All of these factors would potentially make the lap and shoulder belt fit him differently than they fit the test dummy.

Our Ratings will tell you which Booster seats work well. But because every child is different, that’s just a starting point in finding the best booster. You need to know how to evaluate how well a booster works when it comes to belt-fit for your particular child in your particular vehicle. Here are six simple questions to help you determine if a booster fits your child properly and when he is ready to ride with a vehicle seat belt alone. If you can’t answer yes to all of them then the booster may not be the best fit.

1. Does the child sit all the way back against the auto seat or booster seatback without slouching?
2. Do the child's knees bend comfortably at the edge of the booster seat so that he’s not tempted to slouch?
3. Does the shoulder belt cross the shoulder between the neck and arm and across the collarbone?
4. Is the lap belt as low as possible, touching the tops of the thighs so that it will engage the hips during a crash?
5. Can the child stay comfortably seated like this for the whole trip?
6. Do the belts stay in place and without slack once your child starts to move around?

Once you know how the belts should fit you may find some differences between your results and ours. We expect you will.

See our buying advice and newly updated Ratings (available to subscribers) for booster seats as well as buying advice and Ratings for a wide range of products for babies and kids.