The optical absorption of nanowire arrays is calculated with the rigorous coupled wave analysis. The effect of different parameters such as periodicity, filling ratio and thickness are studied. There are potential applications of these arrays for photovoltaic applications and there are experiments on Si nanowire arrays. For that reason the main objective of this study is to find the maximum absorption of nanowire arrays over a frequency area that covers the visible spectrum. There is a random location and orientation of the nanowires in those experiments. For that reason the effect of disorder in the absorption results is also examined. Although, the main focus of this study is Si nanowires, other materials are also calculated.