First principles calculations have been carried out to investigate the structural, magnetic and electronic properties of the multiferroic manganite TbMnO3 with and without Ca doping. For pure TbMnO3, results show that spiral-spin antiferromagnetic ordering is energetically found favorable. The presence of Ca doping at the manganite Tb1-xCaxMnO3 with x = 0.25, 0.5 and 0.75 directly affects the Mn ordering. For x ≤ 0.5, the ferromagnetic ordering is most favored. While for x = 0.75, the most stable magnetic configuration is antiferromagnetic arrangement. It is found that the decrease in the intensity of the ferromagnetic double-exchange interaction has been correlated with the increase in the Ca content and the Mn-O-Mn bond angle.