The number is pretty hard to calculate because it should include any obligation, not just official debts. If the government promised to, say, cover certain expenses in retirement like health care, the true debt should include a realistic estimate of just how much that health care would cost. Not that anyone can predict which new treatments will appear or what they would cost.
It really is amazing how the country decided to do two things simultaneously:<p>1) Create a world order heavily dependent on globalization wherein the international economy has very little redundancy and relies heavily on one country's dominance in security and economic matters to keep everyone more-or-less happy<p>2) Make the revenues needed to maintain such a world order impossible to get because of a flat-out refusal to pay taxes on profits accrued from such a world order