http://www.usdebtclock.org/world-debt-clock.html
Interesting that in the US we're always told doomsday stories about our massive national debt. Yet you look at the numbers and actually there are 5-6 countries with worse debt-GDP ratios than the US. In fact Japan is way, way, out front of any other country in terms of debt at almost 2.5x GDP (and worse off even than Greece). Portugal, Belguim, Canada, France, UK, and Brazil all have higher debt-GDP ratios than the US. So maybe the US debt is not the end of the world.
Like other's said, population matters too. China's GDP is #2, but their population is also 4.5x the US. So their GDP would need to be over $90 trillion to be similar to the US per capita.