BudgetBlog reader and national debt aficionado Brooke A. fired up Excel (or maybe even OpenOffice.org Calc?) and calculated what a 30-year mortgage would look like on a $9.5 trillion loan -- a loan principle equal to today's national debt. So, if the government stopped borrowing today and began making monthly installments on the national debt over 30 years, the monthly payment would be $51 billion -- that's $612 billion annually. In the first year of repayment, interest costs would equal $471.8 billion. And the cost to service the debt over the life of the mortgage would be some $8.9 trillion, bringing the total amount to pay off the national debt to $18.3 trillion. Oy. Get the national debt amortizationn spreadsheet here.