Return period is often used by investors as an index of investment effectiveness taking into account the time value of money. In order to determine this parameter the equation NPV=0 needs to be solved as a function of 't'. As it is shown it is an exponential equation for which no solution in an analytical form can be obtained and only iterative methods can be used: secant, tangent or bisection methods. The field literature recommends determining consecutive cash flows and comparing them with incurred expenditures (to an accuracy of a unit used for consecutive payments). The authors proposed formulas which are easy to use and burdened with small error, and which determine investment return period.