You don't really need a calculated field for this. On your workbook, click on the analysis tab > total > and turn on grand totals for columns. This will add a grand total row at the end of the table. Then you can go back to analysis click percent of table. This will give you the row contribution to the total value in percentages. Let me know if you have questions.
PS: I don't have access to tableau right now so the path I used above might not exactly match.
Once again Pooja,
Thank you for your response, it was helpful and met my need.
I am glad I could help. On a side note, please mark the answer correct by 'hitting the green star' on the response.
We can do Table Calculation, ( I am treating you are having only 2 values "Category" Field.)
I have added Dimension & Measure on the first report.
2) I have created another report as same as above in the first step. Then i have added table Calculation like below image.
3) Then hidden the header.
I hope it may help you,