Here's one way to calculate it (htp: Burning Our Money): GDP less debt owed to foreigners, per capita.
But there's also total debts (foreign and domestic) compared to GDP, in which case the US owes 4 times what it earns, and goodness knows exactly what the equivalent is here in the UK.
And then there's future liabilities - pensions and healthcare costs that have effectively been promised, and will have to be paid for decades to come. The US Comptroller General, David Walker, has been touring the US for two years like Cassandra of Troy, trying to alert the public about this. His latest reported estimate is that these promises are equivalent to a lump sum debt of $53 trillion, and rising annually by $2 - $3 trillion.
It's an unholy mess, on a scale so large that even approximate calculations seem almost irrelevant. However, I strongly suspect that it is a misrepresentation to say that the problem is principally American. If only the truth were made plain - but those in the best position to say, have the greatest incentive to remain silent, or to mislead us.