The merit of a particular riprap design can be represented by numerical value of the riprap safety factor, defined as the ratio of moments resisting particle motion to the moments tending to rotate the particle out of the riprap blanket. A safety factor of unity denotes incipient motion. From theoretical considerations, a relation is established between the riprap safety factor and the magnitude and direction of the flow velocity in the vicinity of the riprap, the angle of the side slope, and the angle of repose for the riprap. The design procedures of the Corps of Engineers, the Bureau of Reclamation, the California Division of Highways, the Bureau of Public Roads and others are evaluated in terms of the riprap safety factor. Some of the designs have safety factors less than unity indicating that there could be a loss of riprap material when the design flows are obtained.