Syntax: = SUM( number1, [number2], [number3], …)
The SUM formula adds together all of the numbers specified in the arguments. The arguments in the formula that are in brackets (i.e., [number2], [number3], etc., are optional.
The inputs can be numbers, formulas, a cell reference, or a range. If a range or cell reference is specified, only numbers, or text that Excel can convert into a number will be included—text, empty cells and logical values, such as the greater than or less than operators, will be ignored).
Note that if any of the arguments or cell values are errors (i.e., #N/A, #VALUE, etc.) the SUM formula will result in an error.
=SUM( 1 , 2, 3, 4, 5, -1) This formula will add together each number specified and in this case the result will be 14.
=SUM(A1, B3, 2) This formula will add A1 and B3 and then add 2.
=SUM(A1:A5) This formula will add together each of the cells in the range
The SUM formula can also be used to add numbers together across multiple worksheets or files.