Spreading of oil on calm water is investigated for the stage where gravity and inertia forces act as the major promoting and resisting forces. The treatment of Hoult (1972. Annual Review of Fluid Mechanics, 4, 341-368) is generalized to include the case where the volume of an oil strip is not necessarily constant and varies as a power-law function of time. A similarity solution is developed, and the spreading laws and the velocity and oil thickness profiles are determined, providing a theoretical basis which can be used to test different leading-edge boundary conditions proposed in the literature. In addition, the constant-volume-based approximate spreading-law formula is compared with the results obtained theoretically in this study.