Very few methods are available for predicting the secondary fragment velocity and secondary fragment production of grouted and unreinforced concrete masonry unit (FGUR CMU) walls when subjected to blast loading. A novel energy-based methodology is developed that calculates the secondary fragment velocity of an unreinforced fully grouted concrete masonry unit (CMU) wall under blast loading. The computationally efficient, engineering-level methodology was originally derived for fragment velocity calculation of unreinforced ungrouted CMU walls. Existing approaches often require large computational resources or fail to predict secondary fragment velocity accurately; this approach calculates fragment velocities with minimal computational effort. The applicability of this analytical model to unreinforced fully grouted CMU walls is discussed. The selection of a resistance function and assumptions about fragment behavior are examined. Velocity calculations throughout wall fragmentation are compared to velocities from quarter-scale and full-scale experiments and reasonable agreement is observed.