Concrete blocks come in several standard sizes, the most common of which is called a concrete masonry unit (CMU). 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.
The Concrete Masonry Unit
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. These cores make the blocks easier to reinforce during masonry construction.
Partition or Floor Blocks
Partition and floor blocks are 3 5/8 inches wide by 7 5/8 inches tall by 15 5/8 inches long, which equates to 4 by 8 by 16 inches when installed with a 3/8-inch masonry joint. Two partition or floor blocks stacked on top of each other with a 3/8-inch masonry joint between them are the same size as a standard concrete masonry unit. Partition and floor blocks may be solid or have two or three hollow cores.
Half blocks are 7 5/8 by 7 5/8 by 7 5/8 inches, or 8 by 8 by 8 inches with a 3/8-inch masonry joint. Half blocks are used for beams, lintels and as end blocks in walls where the layers of standard blocks are offset by 50 percent. Half blocks for different applications may have one or two hollow cores or be U-shaped.
Concrete blocks come in many other more specialized sizes and shapes, most of which are based in some way on the standard masonry unit. Blocks come in shapes designed to accommodate doors and windows, soffits, pipes and rafters, or two make curved corners or 45-degree angles. There are also solid concrete bricks made in the same 2 1/4 by 4 by 8 inch dimensions as the more familiar red clay bricks.