By the capacity of Kirchhoff’s rectangular plate differentials, buckling and deflection are solved together and since the deflection-factor is readily recognizable, buckling results are partially confirmed. In this way, unsafe buckling results are identified for correction. Deflection factors are needed to eliminate many un-useful harmonic buckling-solutions; a plate with a larger surface area will have a larger factor. The all-round simply-supported plate at an aspect ratio of “s*=3.5” is found to be only 30-percent as stiff as that of “s*=1.0”; the two plates cannot have the same critical load. This new capacity-analysis is able to eliminate the discontinuities prevalent in strength versus aspect-ratio correlations, emanating from joining isolated harmonic waves as final. A large number of plates in mixed boundary conditions have been successfully treated by the new method.