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The alcohol equation - do you know what you're drinking?

When calculating the size, strength and units of an alcoholic drink, you’ve got a fair few numbers to consider to stay within the recommended guidelines.

These numbers can make it all too confusing, and before you know it, it’s like a mathematical equation. To help you keep track, here’s a breakdown of what they all mean.


  • Size and measures Size and measures

    Taking beer or lager as an example, glass size and measurements are fairly straightforward compared to some other drinks, such as spirits or wine. Lager, beer or ale is usually offered as either a pint or half pint.

    And although the strength of wine may vary, if you drink in a pub you’ll usually be offered a measurement – 175 ml or 250ml.

    So far, so good. But then it gets trickier – how do you know how much alcohol by volume (ABV) is in your drink and does that affect how many units you’re drinking?

  • Volume and units Volume and units

    ABV is expressed as a percentage (usually on the packaging or menu) of how much total alcohol is in your drink.

    A unit is a measurement of 10ml or 8g of pure alcohol. It takes about one hour for your body to process one unit.

    A pint of strong lager, for example, can be as much as 6% ABV, whereas some ales can be as low as 3.5%. Depending on the strength of your drink, the number of units it contains will vary.

    To calculate how many units are in your drink yourself, you can do this sum:

    Strength of your drink (ABV %) multiplied by the amount (ml) divided by 1000 = the number of units

    So, if you have a pint (568ml) of strong lager (6% ABV), then you’re looking at over three units in your drink. When you apply this to sensible drinking guidelines, you can see why it’s so easy to go over.

    Proposed new guidelines recommend that you should not regularly drink more than 14 units over the course of a week. If you do drink as much as 14 units, you should spread it over three days or more, rather than 'saving up' units.

    An easy way to cut back on your intake is to have several drink-free days each week.

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  • Curved or straight? Curved or straight?

    And to throw another formula into the mix, could the shape of your glass affect how quickly you drink your beverage? Some research suggests it could influence your drinking speed and also your perception of how much drink your glass holds.

    The results of this small study showed that people drank alcoholic drinks faster from a curved glass than those who drank from a straight glass. People also had more trouble guessing the midpoint on the curved glass than the full glass.

    How much we can read into this is debatable due to the small study size and potential for other factors affecting the results. However, it’s still an interesting observation about our drinking habits.

  • Putting it into practice Putting it into practice

    The truth is, when you’re out with friends enjoying a couple of drinks, these numbers and dimensions are the last thing on your mind. Research shows that, in general, people don’t know enough about what they are drinking. This is partly down to the labelling on the packaging or bottle. But it’s also a sign that people either don’t understand what these mean or aren’t keeping track of their drinking.

    It’s no wonder sticking within the guidelines can seem tricky. So, what can you do to make things easier?

    1. Know your numbers. Take some time to learn how many units are in your favourite drink and how much alcohol they have in them.
    2. Keep track. Use an alcohol calculator to help you keep track of how much you’ve had to drink.
    3. Cut down. If you regularly drink more than the recommended limits, then try cutting back. Alternate alcoholic drinks with soft ones, choose a smaller measure or glass size, and don’t feel compelled to keep up with others.

    The numbers are there for a reason – to protect your health and keep you and others safe. Taking a bit of time to understand them could help you keep your drinking in check and your health on track.

  • Resources Resources

    Further information


    • What is an alcohol unit? Drinkaware., published October 2013
    • Alcohol guidelines. House of Commons, Science and Technology Committee. January 2012.
    • How much is too much? Alcohol Concern., accessed 20 November 2013
    • Attwood AS, Scott-Samuel NE, Stothart G, et al. Glass shape influences consumption rate of alcoholic beverages. PLoS ONE 2012; 7(8):e43007. doi:10.1371/journal.pone.0043007
    • Message in a bottle: does the public have enough information about what they are drinking? Alcohol Concern., published July 2009
    • Are you ready to cut down? Drinkaware., published October 2013
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  • Author information Author information

    Produced by Natalie Heaton, Bupa Health Information Team, November 2013. 

    This information was updated in January 2016 following revisions to the Department of Health’s guidelines for alcohol consumption.

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