Calcium is an important metal ion in biological systems. The coordination structure of hydrated Ca²⁺ has been studied by a variety of methods. However, there is relatively little work done on the effect of hydroxide coordinating to Ca²⁺. This work uses density functional theory electronic structure methods to examine the coordination of Ca²⁺ with both water and hydroxide, in order to determine the most favorable coordination state of the metal ion with the two ligands. Minimum energy structures of [Ca(H₂O)_n]²⁺, where n is one through eight, and [Ca(H₂O)₆]²⁺•nH₂O, where n is one and two, were obtained through geometry optimizations. In the gas phase, the hydrated Ca²⁺ ion favors the six, seven, and eight coordination states nearly equally. Including a PCM solvent model appears to result in the seven coordinate structure being most stable. When one water on each complex was deprotonated to form [Ca(H₂O)_nOH]⁺, where n is one through seven, the six coordinate structure is favored appears most stable in the gas phase. In the seven and eight coordinate structures, water ligands leave the first coordination sphere, and they form hydrogen bonds to the hydroxide.