I measured the carpet thickness as instructed on the Staples website before ordering this mat. I've only had it 5 days and have a chair with five rollers on it. The mat already has permanent dents from the rollers. Pretty much a waste of money.
It could have been more firm so chair doesn't sink in so much. I bought this one because it doesn't have the nubs on the bottom and was a bit firmer than the cheaper mats.