A simple equation predicting the dimensions of sediment load casts is obtained from the fluid-mechanics literature. The equation relates the density and viscosity of the upper and lower sedimentary materials to the size of the expected load cast. These results are extended to predicting load cast size as a function of median grain size and sediment concentration, which are of more utility in geological studies. The results are based on the assumption of Newtonian fluid behavior, and predict load cast dimensions of less than 10 cm for a wide range of sediment conditions. It is hypothesized that although the results are of the correct order of magnitude, non-Newtonian rheologic properties are important to load cast formation given the common occurrence of load casts larger than those predicted.