You can get some idea by simply seeing the change in your refund before and after you enter the mortgage interest. You don't really get money directly for any one entry though. The mortgage interest deduction may be the amount that allows you to itemize rather than use the standard deduction because itemized deductions in total have to exceed the standard before you switch over to using itemized deductions. On the other hand, if you haven't entered any other itemized deductions yet you may see no change in your refund because the interest alone may not be enough to exceed your standard deductions. Tax returns just aren't that simple.
