China's National Bureau of Statistics said in a survey that households with an annual income ranging from 60,000 yuan (US$7,250) to 500,000 yuan should be categorized as middle class, as a growing number of people enjoying higher living standards.

The bureau defined the standard on a survey with 300,000 questionnaires released and conversions of international standards, Cheng Xuebing, an NBS official, said.

"We set the number according to the World Bank's criteria of the medium level of international GDP per capita, ranging from US$3,470 to US$8,000, and transferred it after currency exchange, purchasing power conversion and GDP per capital to average income conversion," Cheng said.

"The proportion of middle class in China will expand to 45 percent in 2020 from 5 percent today."

The survey, which cost at least 6 million yuan, reflected only the living standards in the urban areas, because people who earned that much money mainly lived in cities, Cheng said.

Lu Dale, a Chinese University of Hong Kong professor, said it was too easy to define middle class solely by income.

"It also depends on what kind of house one lives in, consumption habits, living environment, education level, and social influence," Lu said.

Under these terms, Hong Kong's middle class accounted for at most 20 to 25 percent of its total population, Lu said.