According to an embodiment, a calculation apparatus includes a first calculator, a second calculator and a third calculator. The first calculator performs a regression analysis to calculate internal state parameters including a quantity of active material and an initial charged capacity of at least the first electrode. The second calculator calculates, based on the internal state parameters, an upper limit charged capacity in a predetermined range based a charged capacity at which an open circuit voltage of the rechargeable battery reaches an upper limit voltage. The third calculator changes the quantity of active material of the first electrode to a fixed value, and recalculates the initial charged capacity of the first electrode used on the fixed value and the upper limit charged capacity.