SUMPRODUCT function in Excel

The SUMPRODUCT function in Excel multiplies corresponding components in the given arrays, and returns the sum of those products.

Syntax:  SUMPRODUCT(array1,array2,array3, …)  Array1, array2, array3, … are 2 to 30 arrays whose components you want to multiply and then add. Remarks  The array arguments must have the same dimensions. If they do not, SUMPRODUCT returns the #VALUE! error value.  SUMPRODUCT treats array entries that are not numeric as if they were zeros. Example  Our training video example of sumproduct in Excel demonstrates the calculation of total inventory. Remark  The preceding example returns the same result as the formula SUM(A4:A6*B4:B6) entered as an array. Using arrays provides a more general solution for doing operations similar to SUMPRODUCT. For example, you can calculate the sum of the squares of the elements in A2:B4 by using the formula =SUM(A2:B4^2) and pressing CTRL+SHIFT+ENTER.

Further reading
Use SUMPRODUCT in Excel 97-2003 to Summarize Worksheet Data
Performing calculations based on multiple criteria, using SUMPRODUCT function