This is not a mortgage, but a company loan. $200,000 over 25 years, paid weekly at 5% annual rate. But it compounds weekly. I’ve only been able to find spreadsheets and calculators that compound monthly, which changes the payment amount! I need one to track the balance left if and when I make extra payments [...]

go to your browser and type in "mortgage calculator" I like these sites because it allows you to add extra money each month to see how quickly you could pay off your loan.. you DO need to know the interest rate though or at least have a pretty good idea of what it would be. [...]