Great post by Paul Krugman.

Amid all the debt hysteria, it’s worth taking a look at the actual arithmetic here — because what this arithmetic says is that the size of the deficit in the next year or two hardly matters for the US fiscal position — and in fact the size over the next decade is barely significant.

Start with interest rates. What matters for debt sustainability is the real interest rate, since what matters is keeping real debt, not nominal debt, from growing. (World War II debt never got paid off, it just eroded in real terms to the point where it was trivial). As of yesterday, the US government could lock in 30-year bonds at a real interest rate of 1.25%. That means that a trillion dollars in extra debt would mean $12.5 billion a year in additional real interest payments.

Meanwhile, the CBO estimates potential real GDP in 2021 at about $18 trillion in 2005 dollars, or around $19 trillion in 2011 dollars.

Put these together, and they say that an extra trillion in borrowing adds something like 0.07% of GDP in future debt service costs. Yes, that zero belongs there. The $4 trillion S&P said it needed to see clocks in at less than 0.3% of GDP.

These are not, to say the least, make or break numbers.

Yep. But what’s going on the US Congress has nothing to do with reality. or rationality, come to that.