calculate the Net Present Value (NPV) of an investment calculate gross return, Internal Rate of Return IRR and net cash flow Start by entering the initial investment and the period of the investment, then enter the discount rate, which is usually the weighted average cost of capital (WACC), after tax, While it's often used in the world of corporate finance, it can also be used for everyday purposes. Generally, NPV can be calculated with the formula NPV = ⨊(P/ (1+i)t) – C, where P = Net Period Cash Flow, i = Discount Rate (or rate of return), t = Number of time periods and C = Initial Investment. Expected rate of return is the ideal rate for discounting cash flows to find NPV. Expected rate of return would be your cost of capital. Cost of capital depends on how you are going to fund your investment. If it is only by Equity, then cost of equity would be relevant. If it is only debt, then after tax cost of debt.