Most people who drink at home end up pouring more than what they would get at a pub in a single measure, a new survey has found.
The government survey, Know Your Limits Campaign, found that among 600 people tested, the average amount poured was 38ml, compared with a standard 25ml.
Those aged 31 to 50, who are the most generous pourers, gave an average of 57ml.
For a person thinking they were drinking 7.5 units a week, the larger measures would equate to 17 units.
It could also mean that people wrongly think they are drinking within the NHS recommended limits of two to three units a day for women and three to four units a day for men.
The alcohol industry has been offering free measuring cups with certain mixer drinks this Christmas.
"Many of us enjoy a drink, especially at new year," the BBC quoted public health minister Gillian Merron as saying.
"But it's easy to get carried away and it's worrying to see just how much more people might be unwittingly pouring for themselves and their friends at home on a regular basis.
"If you want to minimise your risk of diseases like cancer, heart disease and stroke, it's worth paying attention to the size of your measures," he said.
Tests with vodka, gin and whiskey were carried out on 600 people in London, Liverpool and North Shields at the beginning of December.
Adults between 18 and 65, who were regular home drinkers, were asked to pour what spirit measures they would normally pour at home, with or without ice cubes.
When asked to pour how much they thought a single 25ml shot would be, the average amount poured was 38ml, with the highest amount measuring 182ml.
Men poured considerably more than women, 43ml compared with 32ml.
The experiments also suggest that the size of your wine glass really does matter.
When asked to pour the equivalent of one unit into a large (250ml) wine glass, the average amount poured was 157ml - almost exactly twice the correct amount of 76.25ml.
In a smaller wine glass (175ml), it was 131ml, which is still 55ml more than the correct standard measure.
The drinks company Diageo gave away more than 60,000 free metal unit measures in supermarkets earlier this year to help consumers get their measures right.
And have given a further 300,000 measures out in Scotland over the last two years.
Some Schweppes mixer drinks have also had bottle caps adapted to be measures this Christmas.
"It is important that people understand how much they are drinking and the easiest way is to ensure that you have a wine and spirit measure at home," Jeremy Beadles, chief executive of the Wine and Spirit Trade Association, said.
"These are readily available and indeed during the last year both producers and retailers have run promotions to offer them free to shoppers," he stated.
Professor Ian Gilmore, President of the Royal College of Physicians and Chair of the Alcohol Health Alliance spoke of being aware of how much one is consuming.
"It is worrying that people have little appreciation of how much they are drinking when serving themselves, especially given the rise in home drinking fuelled by cheap alcohol from supermarkets," he said.
"Part of this lack of awareness comes from ever larger glass sizes and drinks such as wine and beer increasing in strength.
"This advice comes at a welcome time as recent data shows that three quarters of people intend to see in the New Year at home.
It is also timely as people think about how they can improve their health in 2010," he added.