Among the countries in the 100% club: Japan, Greece, Italy, Portugal, Ireland and the U.S. (U.S policymakers and credit-rating firms tend to put the U.S. debt-to-GDP ratio at 73%, based on marketable securities held by the public. Gross debt — the $16 trillion figure we see most often — includes money the government owes itself, like for Social Security. But the IMF uses the gross debt figure here.)