excel - calcuating proportion of sales in powerpivot -

i have powerpivot table looks this:

product, units_sold, year         10         1990         5          1990 b         12         1900 c         50         1900 

i want create calculated field contribution overall sales of each sale, e.g. in case divide every number 77.

how can in powerpivot?

use formula calculated column:

=[units_sold]/sum([units_sold]) 

rename column , choose formatting home menu.