When assuming the interest rate or discount rate, one must not forget the inflation rate. The rate of inflation may be assumed as a variable every year or a fixed value, and there are studies and economic research to calculate the rate of inflation. Its value differs from one country to another according to the nature of a country’s economy observed over time. The following equation calculates the highest rate of return and includes the rate of inflation:
The nominal interest rate of return ^ 5)
= (l+inflation rate) (1+interest rate)-l.
The previous example includes an interest rate of 10% and assumes an inflation rate of 4%. The real interest rate or discount rate is figured by the following equation:
Table 3.7 Net present value including inflammation
When the inflation rate is constant, the net present value is stable under the assumption that the rate of return is fixed. When the difference value of the interest rate changes every year, the net present value will be different.