A concrete masonry unit is the hollow block of the masonry trades, and its hollowness is the design feature: the vertical cells of conventional two-core units accept reinforcing steel and core-fill grout, turning a stack of blocks into a structural wall. The standard nominal widths for grouted work are 6, 8, 10, and 12 inches — 4-inch units are deliberately excluded from the NCMA grout-volume tables, and NCMA recommends against grouting them at all, because their small cells cannot be grouted and consolidated reliably. In running bond, the cores of two-core block land on an 8-inch on-center module, which is what "all cells" grouting means in practice.
The two recurring CMU quantity questions have table answers. Grout: an 8-inch hollow wall takes about 36.1 ft³ of grout per 100 ft² fully grouted, dropping roughly in proportion to spacing when only reinforced cores are filled — 18.1 ft³ at 16 in o.c., 6.1 ft³ at 48 in o.c. — per the NCMA estimating tables, with the grout itself specified to ASTM C476. Strength: on the design side, a wall’s capacity comes from the chosen block-and-mortar combination, checked for characteristic strength, slenderness, and eccentricity under codes like TMS 402 and Eurocode 6.