AbstractCalculations of the critical sizes of cubic arrays of interacting fissionable units are compared with critical experiments. The units are of two types: vessels containing 5 liters of an aqueous solution of highly enriched uranium, and cylinders of highly enriched uranium metal. The arrays are surrounded by various thicknesses of hydrogenous reflectors. Agreement between calculation and experiment is reasonably good. The similarity of the results obtained with the widely differing types of units invites confidence in general applications of the method of calculation. Tables are presented for computing critical and safe sizes of cubic arrays of 8, 27, 64, or 125 identical units as a function of the albedo of the reflector surrounding the array and of the reactivity of an individual unit.