Follow these steps to add a formula field in a table:
To add a formula field in a table, go to Fields in the table's settings and select New fields.
Select a field type for the formula.
Add a label and select Add fields.
Enter the formula
Once you add the field, you'll see the list of fields for the table. If you don't see the fields, go to the table's Settings menu and select Fields.
From the list of fields for the table, select the formula field.
A formula box displays, which is a working space for you to write your formula.Learn more about different field types in Formula field types.
Select the formula box to start typing or select Choose fields & functions to add functions to your formula.
Finish setting up the field, and then select Save.