Summarize this article:

Double-click the Excel document that you want to edit. Doing so opens it in Excel. In order to include all of the cells in the longest column, you'll need to know to which row the column extends. For example, if you have three columns and the longest one has values from row 1 through row 20, your formula will need to include rows 1 through 20 for each column you want to add even if this includes blank cells. If you're adding the A column and the B column, for example, your beginning column is the A column and your ending column is the B column. Click the cell in which you want to display the sum of your columns. Type =SUM( ) into the cell. In the parentheses, type in the beginning column's top cell number, type a colon, and type in the ending column's letter along with the longest column's ending row. For example, if you're adding columns A, B, and C, and your longest column stretches to row 20, you would enter the following: =SUM(A1:C20) Doing so will display the sum of all of the columns in your selected cell.

Summary:
Open your Excel document. Determine which of your columns is the longest. Determine your beginning and ending columns. Select a blank cell. Enter the "SUM" command. Enter the cell range. Press ↵ Enter.