Hi,
For achieving your task.
1. Create a variable.
2. On Add/Modify variable screen, select calculation type as sum, Variable Expression should be the field for which you want to do total. Variable class type should be double.
3. Drag and drop the variable to the summary band of report.
For calculations you should type cast your data in report to double.
Hope this helps.
Regards,
Shahnavaz
2.