Because the math can be fuzzy, economists have tended to be wary of such measurements. Italy holds a place in the annals of creative accounting for “il sorpasso,” or “the overtake,” of 1987, when the government revalued G.D.P. to include the black-market economy. So large were the figures, which included estimates of tax evasion and illegal workers, that almost overnight Italy leapfrogged over Britain to become the world’s fifth-largest economy.

Now, as Italy struggles to pull out of a recession and labors under one of the developed world’s biggest debt burdens, it will start counting smuggled alcohol and tobacco. Mr. Oneto declined to make a forecast, but he noted that Italy’s overall black market already accounted for 15 percent of the country’s €1.5 trillion economy.

Still, Mr. Oneto said his agency would not go so far as to include one of the biggest possible economic enhancers — business conducted by the Italian mob, which is thought to generate around €180 billion in annual revenue, equal to about 7 percent of Italy’s G.D.P. “The mafia is too difficult to pin down,” he said.

For most governments, illicit activity may count for little more than a rounding error. In the Netherlands, known for its red-light districts and marijuana coffee shops, such activity counts for 0.4 percent of G.D.P.

The biggest boost to G.D.P. figures will come from something with no titillation value: a Eurostat revision allowing governments to count research and development as investments rather than costs. Finland and Sweden, hotbeds of high-tech capitalism, could increase the size of their economies by as much as 5 percent, according to Eurostat.

In Italy, Ireland, Portugal and Spain, which are struggling to overcome recessions, G.D.P. could increase by as much as 2 percent, Eurostat estimates, while Germany and France could see expansions of as much as 3 percent. Britain might show a gain of 3 to 4 percent, Eurostat said.

Britain’s Office for National Statistics recently announced it would start counting vice for the first time in the country’s economy, which is rebounding from a recession. It has begun by going back five years in its recounting. In 2009, the only year for which it has thus far done a tally, prostitution and drugs including cocaine, heroin, cannabis, Ecstasy and amphetamines added nearly 10 billion pounds, or $17 billion, to British G.D.P., equivalent to 0.7 percent.