The Pinch Test
1. Put your child in their winter coat and sit them in their car seat. Secure the harness and tighten it until you can no longer pinch any webbing between your thumb and forefinger.
2. Without loosening the harness, unbuckle and remove your child from the seat.
3. Remove the coat, put your child back in the seat and buckle the harness. The straps should still be set the way they were when the child was wearing their coat. If you can pinch any webbing between your thumb and forefinger, then the coat is too bulky to be worn under the harness.