This analysis calculated the mass moment of inertia of a nonviscous fluid in a slowly rotating rectangular tank. Given the dimensions of the tank in the x, y, and z coordinates, the axis of rotation, the percentage of the tank occupied by the fluid, and angle of rotation, an algorithm was written that could calculate the mass moment of inertia of the fluid. While not included in this paper, the change in the mass moment of inertia of the fluid could then be used to calculate the force exerted by the fluid on the container wall.