Vertex42 has a great Debt Snowball Calculator. Their website has a short video tutorial. This is how they described the Excel spreadsheet:
How fast could I get out of debt? How much can you save in interest payments? That is what our Debt Reduction Calculator can help you figure out. Getting out of debt is not easy, but with a good plan and firm determination, it is entirely possible. The Debt Reduction Calculator is a simple spreadsheet available for Microsoft Excel®, OpenOffice, and Google Sheets, that helps you come up with a plan. It creates a debt snowball payment schedule that can help you manage your payments to most effectively pay off your debts.