How to calculate a real return
To figure out the real return, you need to know two things. First, you need to figure your after-tax nominal return, which you can get by dividing the value of your investments at the end of the period in question by their value at the beginning of the period. Second, you need to determine the inflation rate over that same period, which you can get by looking at historical data on the CPI.
Once you have those two percentage figures, the calculation is relatively simple. Add 1 to both figures, and then divide the return-based number by the inflation-based number.