To use the SUM Function to add a range of cells together:
Use the SUM function to add a range of cells together. First, click the Insert Function button, then select SUM from the list of functions, and click OK. In the Function Arguments dialog box, notice that by default, Excel has entered a range of contiguous cells for you. If these are the cells you want to add, check the result of your function at the bottom of the dialog box, and then click OK to insert the function into your worksheet.