convert a 15 character id to a 18 character id in salesforce

1. Navigate to the  Custom Fields and Relationships of the Object:

Setup> Customize> Object> Fields> Custom Fields and Relationships> New

2. Select Formula as the Field Type

3. Name the Field and select Text as the Return Type

4. Input the following Formula into the Formula Editor:

CASESAFEID(Id)

5. Set Field Visibility, add/ remove from Pagelayout(s), and Save