A battery calculating method, apparatus, and program are provided. The battery capacity calculating method and apparatus can very accurately calculate a residual capacity of a secondary battery especially in the last stage of discharging independently of environmental conditions such as a temperature or a deteriorated state. the battery capacity calculating apparatus is equipped with a voltage measuring circuit which measures the terminal voltage (V.sub.mea) of a battery at the time of discharging, a current measuring circuit which measures the current value (I) of the battery at the time of discharging, and a control circuit which calculates a residual capacity and/or residual power. The operation unit in the control circuit calculates a discharged capacity (Q.sub.mea) and an apparent discharged capacity (Q.sub.ocv) based on a terminal voltage (V.sub.mea) and a current value (I) measured by the voltage measuring circuit and the current measuring circuit, respectively, estimates a discharge curve (C.sub.pre) in the future including the last stage of discharging based on a capacity shift (.DELTA.Q) being the difference between the discharged capacity (Q.sub.mea) and the apparent discharged capacity (Q.sub.ocv) during discharging, and calculates the residual capacity and/or the residual power of the battery based on the estimated discharge curve (C.sub.pre).