The = (Formula) field code calculates a number by using a mathematical formula. You can use the Formula command (in the Data group on the Layout tab under the Table Tools tab, which appears when you select the table) or press Ctrl+F9 to insert a field in a table or in regular text.

If you have a spreadsheet application, such as Microsoft
Excel, embedding all or part of a worksheet in a document is often
easier than using the = (Formula) field in a table.

Syntax

When you view = (Formula) field code in your document, the syntax looks like this:

{ = Formula [Bookmark ] [\#Numeric Picture ] }

Note A field code tells the field
what to show. Field results are what is shown in the document after
having evaluated the field code. To toggle between viewing the field
code and the field code results, press Alt+F9.