Sum can be simply calculated as:
1) First, Check that the field(whose sum is to be calculated) data type should be in "integer" or "float" (as per needed).
2) Right Click on the field(whose sum is to be calculated) in Crystal Report.
3) Select Insert --> summary.
4) Check "Choose the field to summarize" dropdown and "calculate this summary Dropdown".
5) Click OK (It will come in Crystal Report footer part).