I got a little tired of relying on "black box" online mortgage calculators when trying to figure out my monthly payment and amortization schedule, so I created this simple Excel spreadsheet. [screenshot, download]
As you can tell from the screenshot, you provide as input the term of the loan in months (up to 360, which is 30 years), the annual interest rate, the principal (i.e. the total amount borrowed), and any extra principal you might want to pay (same amount each month and/or on a month-by-month basis). As output, you will get the monthly payment, total interest over the loan's lifetime, and the full amortization schedule.
Instructions: Download the spreadsheet by right-clicking here and selecting "Save Target As..." or "Save Link to Disk..." depending on your browser. Open the spreadsheet in Excel (or a free Excel-compatible spreadsheet program). You may edit any cell that is bordered in bright red, but don't modify the rest of the spreadsheet.
Download: In case you somehow missed it the first two times, here it is again.
Credits: This spreadsheet was creating using the info posted at Bret Whissel's amortization calculator.
Also, if you find a mistake in my calculations or make some other improvement, please e-mail me at