People are more likely to pour extra alcohol into short, wide glasses than tall, narrow glasses, a study says.
Whisky was one of the drinks people were asked to pour
The US researchers from Cornell University asked 198 students and 86 bartenders to pour a shot of alcohol.
They found students poured 30% more into the short glasses, while bar workers faired only slightly better at 20%, the British Medical Journal said.
The groups poured more than a standard shot measure into both types of glasses.
Students also said they thought the tall glasses held more, suggesting they were trying to compensate for size when pouring into the short, wide glasses.
The people participating in the study were asked to pour four different types of drinks - vodka tonic, rum and cola, whisky and gin and tonic.
Practice led students to pour less into tall glasses, but did not affect the amount put in the short ones, while bar staff poured less in each.
Lead researcher Brian Wansink said the findings suggested people should think more carefully about the measures they pour and it might be useful to mark on glasses to signify what equates to a shot.
And he added: "If short tumblers lead even bartenders to pour more alcohol than tall highball glasses the way to better control alcohol consumption is to use tall glasses or to use glasses with the alcohol level marked on them - and to realise that, when alcoholic drinks are served in a short wide glass, two drinks are actually equal to two and a half."
Moira Plant, professor of alcohol studies at the University of the West of England, said: "The real problem is that people just do not known what constitutes a unit of alcohol.
"Evidence suggests that at home, people will pour themselves larger measures than those served at pubs.
"Meanwhile, pubs have started serving drinks in larger glasses and offering cheap promotions to encourage people to drink more.
"People need to become more aware."
One UK unit contains eight grams of pure alcohol.
One unit of alcohol is about equal to half a pint of ordinary strength beer or
a small pub measure of spirits.