Before using any field for calculation you need to configure that field for calculation. Configuration of calculation field is described step by step below:
1. Go to the Application in which you want to use the calculation field.
2. Select the Calculation tab from the top and click on "New Calculation" button.
3. Select the Field on which you want to use calculations from the popped up drop down and click on "Create". (A field can be selected only once for calculation).
4. To configure the selected field for calculation, click on "Edit" of the selected field from the Action dropdown.
5. Select field for calculation, write your code, save, test code for validation and publish it. Now when calculation is setted up on a field it will execute every time you create or update an item.
Supported fields for calculation :
2. Rich Text (HTML)