I guess you are referring to EditForm as calculated columns are available in DispForm by default. To do this you must add this to your Custom JS:
var res = spjs.utility.getItemByID({
"listName": _spPageContextInfo.pageListId,
"id": spjs.dffs.data.thisItemID,
"viewFields": ["ID", "YourCalculatedColumn"]
});
var yourCalcFieldValue = res.YourCalculatedColumn.split(";#")[1];
Then add a table row to your tab and use this in the Row body textarea:
{{var:yourCalcFieldValue}}
Replace YourCalculatedColumn with the internal name of our calculated field.
Alexander