The chair mat that I purchased was for 1/4 inch carpet. It is simply too thin and the chair wheels create a divot that you can only get out of by rocking the chair until it moves out of the divot. I am not a heavy person at 175 pounds.
I purchased this item as a replacement for an older worn out model. Unfortunately it's not holding up. It's thickness does not hold the weight of the chair with a person in it without dimpling. I think it needs to be thicker to do it's job.
This mat like another larger one that I purchased at the same time does not have the gripping power to stay in place on my medium-pile carpet. The mat moves on the carpet constantly and has to be repositioned at least once a day. Very annoying.