The Excel Sumproduct function returns the sum of the products of the corresponding values in two or more supplied arrays.
The format of the function is:
where the array arguments are arrays of numeric values, which may be supplied to the function directly, or as one or more cells or ranges of cells containing numeric values. All of the supplied arrays must be of equal length and nonnumeric values in the supplied arrays are treated as the value zero.
In Excel 2007 and later versions of Excel, you can provide up to 255 arrays to the function, but in Excel 2003, you can only provide up to 30 arrays.
Formulas:

Results:

The above spreadsheet on the right shows 3 examples of the Sumproduct function.
Note that:
Further information and examples of the Excel Sumproduct function can be found on the Microsoft Office website
If you get an error from the Excel Sumproduct Function, this is likely to be the #VALUE! error:
#VALUE!    Occurs if the supplied arrays have different lengths (i.e. contain different numbers of values) 