Glasses of wine, meanwhile, typically packed more alcohol per volume -- 14 percent instead of 12 percent -- than those used to define a standard drink.
Factors like the type of establishment, the region of Northern California and the gender of the bartender didn't seem to affect the sizes of the drinks, Kerr said.
The findings were expected to be published in the September issue of Alcoholism: Clinical and Experimental Research and are available online.
Some people use the definition of a standard drink to figure out how much they can drink before becoming drunk.
"If the chart says you can have five standard drinks (before you get drunk), you can only have three to four of these actual drinks," Kerr said.
Dwight Heath, a professor of anthropology at Brown University who studies alcohol consumption, said the study "points out a dirty little secret of alcohol research: The definition of 'standard drink' is inaccurate and out-of-date."
Researchers have failed to recognize and adjust to "changes in culture," Heath said.
Learn more about alcohol and potential health risks at Pace University.
SOURCES: William C. Kerr, Ph.D., senior scientist, Public Health Institute's Alcohol Research Group, Emeryville, Calif.; Dwight Heath, Ph.D., professor of anthropology, Brown University, Providence, R.I., September 2008, Alcoholism: Clinical and Experimental Research
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