An equation that is useful for rapid machine computation of the directivity characteristics of a planar array with discrete identical elements and uniform phase progression is derived. Computer time and cost are reduced by an order of magnitude or more when this equation is used rather than the computational techniques that involve integration of the array power pattern. Parameters of the new equation are the number of elements and the interelement spacings along two principal axes, the beam-pointing angles, the array excitation in amplitude, and an element voltage pattern of the form (cos θ)ν+1/2, where ν > −1. An example of the usefulness of the equation is given by means of its application to a uniformly excited rectangular array. Curves of directivity versus interelement spacing and scan angle for square arrays with a (cos θ) element power pattern are presented.