Define a Calculated Field

On the Field Information page, enter a Field Title and Database Field name.
From the Data Type drop-down list, select Calculated.
Click the Expression Builder button. The Expression Builder dialog box opens.
From the From Table drop-down list, select the alias of the database file containing the fields you want to use in the expression. The Fields list shows all the fields contained in that database file.
From the Fields list, scroll down and double-click the first field for the expression.
Note: Expression Builder enters the field name using the table alias (such as "Tkpe.")
In the Expression box, place the insertion point next to the field name and enter the logical operator or the mathematical operator that you want to use. You can also make a selection from either the Logical list or the Math list.

The Logical list and logical operators are used to show a relationship (<) and to build a compound expression (AND).
The Math list and mathematical operators are used to perform a calculation (+).

Continue to add fields, values and operators as needed.
When done, click Verify. The Expression Builder presents a message indicating if you’ve used the correct structure for your expression. For example, if you have an equal sign, it will confirm that you have a value after the equal sign.
When you are satisfied with the expression, click OK to close the Expression Builder. Your completed expression appears on the Field Information page.

Note: If you have just added a custom field that you want to use in an expression, first click Apply on the custom page to save the new field, then create the calculated field using the preceding steps. If you do not save the new field first, you may not see it in the Fields list when you are building your expression.

Click here to see an example.