Computed Field lets you add a custom "computed fields" to your content types. These computed fields are populated with values that you define via PHP code. You may draw on anything available to Drupal, including other fields, the current user, database tables, ... .
Add a field type: 'Computed' to a content type.
Choose the settings you want and save.
Fill in the computed code (php), for example
This example adds two field values...