About 70 pounds per cubic foot, depending on its size and the amount of entrained air. Concrete cinder blocks are well-known for their weight. It's easy to believe that they would be very heavy when submerged in water, but this is not the case. Hollow concrete masonry unit blocks actually float in water, and one cubic foot of concrete will displace about 130 pounds of water. Since each block has a basic shape of a rectangular prism, it creates gaps of air inside. As water flows in the voids, the weight of the block decreases. The actual weight of a cinder block in water varies depending on the dimensions and entrained air bubbles, but for a standard 8" x 8" x 16" size, it would displace about 70 pounds per cubic foot. As blocks become larger and/or more porous, the displacement and weight will be less. Solid concrete block will submerge, by the way.