On the Table Tools Layout tab in the Data group click Formula. In the Formula dialog box. The Table Tools tabs become available.
Then click the tables Layout contextual tab in the Ribbon.
In the Formula box check the text between the parentheses to make sure Word includes the cells you want to sum and click OK. When you click inside a table in Word a new tab label Table Tools appears above the Design tab in the tool bar. Click the blank cell you want to show the calculated result then click layout Formula. In Word you can do other calculations in table such as average production and so on.