Leave aside for a moment whether the International Monetary Fund's proposal to tax banks is fair (where is the levy on automakers to repay the gross $81bn American taxpayers gifted Detroit?). If the G20 group of nations is determined to extract a “substantial” contribution, how big should it be? Details are still scarce. But politicians should bear a few numbers in mind before they start hurling percentages around.

The first proposed tax is the financial stability contribution, intended to “pay for the fiscal cost of any future government support to the sector”. The IMF recommends making all financial institutions pay the levy based on the size of their liabilities, minus tier one capital and insured deposits. For US banks, for example, that equals about $2,200bn, according to Federal Deposit Insurance Corporation data for the fourth quarter of last year. Taxing that at a politically nice-sounding rate of 1 per cent nets Washington $22bn-odd a year.

But the IMF puts the net fiscal cost of the last financial crisis to American taxpayers at 3.6 per cent of gross domestic product – some $513bn – as of end-2009. That means it would take a whopping 23 years to fill the hole using the new levy. No wonder the IMF proposes hitting the banks for yet more money, this time a financial activities tax on profits and remuneration. Roughly how hefty would a Fat tax have to be to pay the cost of the bank bail-out within, say, a decade?