Standard cement block sizes are 4 inches, 6 inches, 8 inches, 10 inches and 12 inches. Cement blocks are identified by their depth, which is the thickness of the wall they create.
A concrete block, also called a concrete masonry unit, has a size based on its nominal dimension of depth times height times length. The nominal dimension is the actual dimension of the block plus the width of the mortar joint.
The actual size of a standard 4-inch block in inches is 3 5/8 x 7 5/8 x 15 5/8, and its nominal size is 4 x 8 x 16 The actual size of a half block is 3 5/8 x 7 5/8 x 7 5/8. Its nominal size is 4 x 8 x 8.