Using a spreadsheet is the best way for this. Just add a column called "Additional Payment" and input the extra amount you are paying that month. It will automatically calculate that for you. If you are doing this with a calculator, simply reduce the principal by the amount of your additional payment and complete the calculation. Make sure you have informed your lender that any additional payment amount is to be applied to your principal balance. Otherwise it may be applied to your next payment, and this will extend the life of your loan rather than decrease it.