Many online mortgage calculators are designed to calculate the effect of refinancing your mortgage. These calculators usually require information about your current mortgage (such as the remaining principal, interest rate, and years remaining on your mortgage), the new loan that you are considering (such as principal, interest rate, and term), and the upfront or closing costs that you will pay for the loan. Some may ask for your tax rate and the rate of interest you can get on investments (assuming you will invest your savings).

Refinance calculators will show the amount you will save compared with the costs you will pay, so that you can determine whether the refinancing offer is right for you.

Source: A Consumer's Guide to Mortgage Refinancings