There is a common misconception that your rug underlayment has the same dimensions as your rug. However, the rule of thumb for any rug is that your rug underlayment should be about one inch smaller than your rug on all sides. That way the edges of the pad will not be visible from the edges of your rug.. Most importantly, a pad that is smaller than your rug will hold the edges and corners in place and prevent them from folding over.