Debt-to-GDP ratio is useful for comparing the debt loads of two countries, but not terribly useful in assessing the serviceability of debt for a single country.

That is, suppose two countries both have $100B in debt. One of them is a small island nation; the other is a global superpower. Obviously the global superpower will be better able to handle that - dividing by GDP helps make that clear.

However, this simple division doesn't tell you some important things. How much of the debt comes due very soon? It's worse if the answer is "most of it." How was it incurred? "Winning a war" is much better than "losing a war."

The United States has lots of debt, and personally I'm worried about the long term serviceability of it. But the ratio to GDP isn't why!