In this paper, a simplified model for predicting the penetration rate of Abrasive Water Jets (AWJs) in a piercing operation is presented. It proposes the use of a lumped mass approach for the jet combined with Bitter's theory of erosion for interpreting the material removal associated with the piercing process. The lumped mass approach considers the jet as a series of cylindrical slugs penetrating into the material and Bitter's theory of erosion predicts the cutting mechanism associated with jet penetration. By balancing the momentum of any cylindrical slug with drag forces exerted on it by the outgoing slurry, the geometry of the jet profile is predicted. The jet profile is then used to interpret the geometry of the pierced hole. The rate of material removal is used to predict the variation of depth of the pierced hole with time. Finally, the results predicted with the theoretical model are validated with experimental data (Hashish, 1996). Copyright © 2005 Inderscience Enterprises Ltd.