An empirical formula has been developed for the criticality specialist who does not have readily available a computer that calculates the individual cylinder diameter for a critical uranyl nitrate solution slab-cylinder system. The formula is used to calculate the criticality condition for an accidental leak in an array of fissile-containing vessels which forms a solution slab under the array. The critical system consisted of a square array of 1, 4, 9, or 16 vertical, equal-diameter cylinders resting on and interacting with a horizontal slab. Both the array and the slab were filled with ∼495 g U/liter uranyl nitrate solution with the uranium enriched to 93.2 wt% 235U. The empirical formula, which predicts the critical unit cylinder diameter of the slab-array system, is where Da is the critical unit cylinder diameter of the array alone at 500 g U/ liter. The independent variables are the number of cylinders, N; the edge-to-edge spacing between nearest neighbored cylinders in cm, S; the array solution height in cm, H; the solution concentration in g U/liter, C; and the solution slab thickness in cm, T. The calculated unit cylinder diameter, Ds, in cm, is within ±11% of the experimentally measured diameter for 65 critical slab-array systems. This accuracy is sufficient for calculating the accident condition for nuclear safety purposes. Monte Carlo calculations were performed on some typical experimental configurations. The average keff ranges from 0.977 ± 0.017 to 0.996 ± 0.012. By increasing the slab thickness by the experimental error, the low keff was increased to 0.998 ± 0.012.