=SUM() Function

=SUM() is used to add up [sum] a group of cells.

Example
A budget spreadsheet list of Expense items, as shown below.
We need a total of all expenses at the bottom of the list.
=sum() is used to add up all those items.

Steps
1. Select the cell you want the answer to display
    Using the example below select B14

2. Type =SUM(

3. Identify the first cell in the list of cells * [see below]
    Type: B6

4. Next type ":"  [a colon]

5. Identify the last cell in the list of cells 
    Type: B13

6. Press ENTER.

* Identifying the cells

There are different ways to identify the cells in the =SUM() function

1.    Select the cells with the mouse which should be included. 
       Select the first or last cell and drag your mouse through the whole list.   Most Popular

2.    Type in the cells with a colon between them.

3.    Type in the cells use 2 periods .. between them. This is an old method which Excel accepts  and then converts to the colon.