You can’t use a online mortgage calculator when offline.. The online calculator is ‘served’ by the webserver you are accessing to get the online mortgage calculator. That said, if you are good at ‘hacking’ around and can read html, it is possible to capture the javascript and html used by the mortgage calculator and put it in a file based web page on your computer.. where you would not need to be online to use it.
There are also spread sheets that do the calculation and the formula is known so you could make a spread sheet… or you can go buy a financial calculator..
