A seasonal index indicates how a periodic amount -- typically a month -- compares to the average of all periods in an extended period, such as a year. Because seasonal indexes measure price fluctuations, they're commonly used in sales forecasting, but seasonal indexes can be used to analyze any activity that is influenced by the season or specific time of year. Microsoft Excel is an excellent tool for calculating seasonal indexes.

## 1. Open the Excel Workbook

Open the Excel workbook that contains your data. Your data should be arranged in adjacent columns or rows to simplify the functions and their calculations.

## 2. Totals and Averages

In the cell below the last entry of the period amounts, type the function =SUM(...) , replacing the ellipses with the cell references of the cells you want to total up for all of the period amounts. Underneath the total, type in an =AVERAGE(...) function, using the same cell references, to calculate the average period amount. In the example shown, the two entries are =SUM(B2:B13) and =AVERAGE(B2:B13).