Other sizes are generally half of a CMU, measured either vertically or horizontally. Modern concrete blocks are made of a combination of Portland cement, heavy- or light-weight aggregate, water and additives to improve curing. A standard concrete block is 7 5/8 in. wide by 7 5/8 in. tall by 15 5/8 in. long. With a 3/8-in. masonry joint, that equates to 8-by-8-by-16 inches when the block is installed in a structure. To reduce weight, concrete blocks usually have two or three hollow cores taking up about 50 percent of the block's volume.