This is an easy to find but very special type of concrete that will say 'Self-Leveling' right on the bag. It is designed to flow like water, hence level itself, but set hard like concrete. Although great for levelling floors, its thin consistency means that it is not as strong as regular concrete and you cannot use this outdoors or where it will not be covered by something else. But this is the product of choice for levelling out sub-floors. For a very large floor you may want to check the price before proceeding because building a level sub-floor could be less expensive when you need a lot of it.