I measured my carpet as directed and chose the low-pile based on that. This mat is too soft for my carpet and the wheels on my desk chair sink in creating divots that it's hard to wheel out of. I stopped using it after a week and it's now sitting on my cellar's cement floor.
The 36" x 48" clear chair mat is the perfect size for my rolling office chair at my desk. I recommend buying one to save your carpet and making it easier to roll with a chair or have a stationary chair.