Possible in more than one way.
Is this a filtered or unfiltered portal?
Sum ( PortalTable::NumberField )
in a calculation field defined in the layout's table will sum the contents of an unfiltered portal.
Likewise, you can define a summary field in the portal table and put it on your layotu to get the same total.
For a filtered portal, you can put the summary field I just described inside a one row portal to the same table with the same portal filter and you will see the desired total.
If you are using FileMaker 12, you can also use ExecuteSQL in a calculation field to compute the total of the portal field.
This is great. Sorry if this question is so basic by I looked for a simple answer for more than two hours...