I've used this over the years and found it excellent http://www.drcalculator.com/mortgage/ however recently I have experienced problems with the PREPAYMENT aspect.
If you put in say 200k over 20 years @ say 5% it calculates €1320 per month which is fine, however if you drop to the lower part of the page and drop on €200 per month in the Prepayment Data field then nothing happens. When it was functioning correctly it used to show that the term would decrease by x yrs and x months and that the total interest saved would be €xx.
Any suggestions in case I am now doing something wrong or if there are any other sites with this info I would appreciate it.
I don't know if the calculator is based on Excel, but there is a general bug in Excel. It means that sometimes it doesn't take the E/sum of option correctly.