How does the bill propose to calculate this misalignment? It relies not on Big Macs, but on the less digestible methods favoured by the IMF. The size of those tariffs would reflect the scale of the undervaluation. A bill passed by the House of Representatives last month would treat undervalued currencies as an illegal export subsidy and allow American firms to request countervailing tariffs.

If a currency war is in the offing, America's congressmen seem increasingly determined to arm themselves. But according to burgernomics, the yen is only 5% overvalued, not much of a casus belli.

The Japanese are so far the only rich country to intervene directly in the markets to weaken their currency. Their franc is the most expensive currency on our list. And the Swiss, who avoid most wars, are in the thick of this one. The index also suggests that the euro is overvalued by about 29%. In Brazil a Big Mac costs the equivalent of $5.26, implying that the real is now overvalued by 42%. The tensions caused by such misalignments prompted Brazil's finance minister, Guido Mantega, to complain last month that his country was a potential casualty of a “currency war”.

Big mac index example with pesos