calculation in matrix

If I understood you correctly you want to perform calculation inside of Matrix field. Inside of this guide you can find how to perform form calculation in the Matrix field:

http://www.jotform.com/help/343-How-to-perform-form-calculation-in-the-Matrix-field 

In case that I did not understood your question correctly, please explain in more details what are you trying to achieve and also provide us ID of your form. 

Hope this will help. Let us know if you need further assistance.