Three terms are used in referring to the overall dimensions (width, height, and length) of concrete masonry units (cmu): specified, actual, and nominal.
Specified dimensions are those specified for the manufacture of masonry units or the construction of masonry. Specified dimensions compensate for a nominal 3/8 (0.375) inch joint for precision and split face cmu, and a 1/2-inch (0.5) joint for SlumpstoneTM. (See "nominal dimensions" below.) Thus, a precision or split face unit with a nominal size of 8x8x16 has specified dimensions of 7.625 x 7.625 x 15.625; SlumpstoneTM as 8x6x16 has specified dimensions of 7.5 x 5.5 x 15.5
Actual dimensions are the measured dimensions of the unit. ASTM Standards allow the actual dimensions a permissible variation from the specified dimension. For example, the outer dimensions of a 10x8x16 unit (9.625x7.625x15.625 specified) are allowed a variation of + or - 1/8 inch. In this case the width could range from 9.5 to 9.75 inches.
NOTE: The Permissible Variations in Dimensions section of ASTM C 90 makes a distinction for split face and slump units since the textured faces vary: only the non-split, overall dimensions of split units, and only the overall height dimension of slump units are considered.
Nominal dimensions are those used in stating the unit size. They are equal to the specified dimensions plus the thickness of the mortar joint.
Dimensions must be stated in proper order - width, height, and length (WHL). For example, an 8-inch wide, 16-inch long, 4-inch high unit is termed an 8x4x16. Reversing any two dimensions will describe a completely different unit. Using that same unit, but referring to it as 4x8x16 would inadvertently indicate a unit 4 inches wide and 8 inches high (not 8 inches wide and 4 inches high).