A standard drink or (in the UK) unit of alcohol is a measure of alcohol consumption representing a fixed amount of pure alcohol. The notion is used in relation to recommendations about alcohol consumption and its relative risks to health. It helps to inform alcohol users. A hypothetical alcoholic beverage sized to one standard drink varies in volume depending on the alcohol concentration of the beverage (for example, a standard drink of spirits takes up much less space than a standard drink of beer), but it always contains the same amount of alcohol and therefore produces the same amount of intoxication. Many government health guidelines specify low to high risk amounts in units of grams of pure alcohol per day, week, or single occasion. These government guidelines often illustrate these amounts as standard drinks of various beverages, with their serving sizes indicated. Although used for the same purpose, the definition of a standard drink varies very widely from country to country. Labeling beverages with the equivalent number of standard drinks is common in some countries.
Definitions in various countries The definition of what constitutes a standard drink varies very widely between countries, with what each country defines as the amount of pure alcohol in a standard drink ranging from 8 to 20 grams (10 to 25 ml). The sample questionnaire form for the World Health Organization's Alcohol Use Disorders Identification Test (AUDIT) uses 10 g (0.35 oz), and this definition has been adopted by more countries than any other amount. Some countries choose to base the definition on mass of alcohol (in grams) while others base the unit on the volume (in ml or other volume units). For comparison, both measurements are shown here, as well as the number of standard drinks contained in 500 millilitres (16.9 US fluid ounces) of 5% ABV beer (a typical large size of beer in Europe, slightly larger than a US pint of 473 mL). The terminology for the unit also varies, as shown in the Notes column.
Calculation of pure alcohol by mass It is possible to calculate the pure alcohol mass in a serving from the concentration, density of alcohol, and volume:
volume × alcohol by volume × density of alcohol = Pure alcohol mass {\displaystyle {\text{volume}}\times {\text{alcohol by volume}}\times {\text{density of alcohol}}={\text{Pure alcohol mass}}}
For example, a 355 ml (12.0 US fl oz) glass of beer with an ABV of 5.5% contains 19.525 ml of pure alcohol, which has a density of 0.78945 g/mL (at 20 °C), and therefore a mass of 15.41 grams (0.544 oz).
355 m L × 0.055 × 0.78945 g / m L ≈ 15.41 g {\displaystyle 355\,\mathrm {mL} \times 0.055\times 0.78945\,\mathrm {g} /\mathrm {mL} \approx 15.41\,\mathrm {g} }
or
0.355 L × 0.055 × 789.45 g / L ≈ 15.41 g {\displaystyle 0.355\,\mathrm {L} \times 0.055\times 789.45\,\mathrm {g} /\mathrm {L} \approx 15.41\,\mathrm {g} }
When the drink size is in fluid ounces (which differ between the UK and the US), the following conversions can be used:
One should bear in mind that a pint in the UK is 20 imperial fluid ounces, whereas a pint in the US is 16 US fluid ounces, and that 1 imperial fl. oz. ≈ 0.96076 US fl. oz.: this means that 1 imperial pint ≈ 1.20095 US pints.
Calculation of units of alcohol by volume Calculating alcoholic content in countries that use units of alcohol is straightforward, as the volume multiplied by the Alcohol by volume (ABV) gives the alcohol content. In the UK, one unit of alcohol equals one percentage point per litre, of any alcoholic beverage.
volume (in litres) × alcohol by volume (ABV) = alcohol content (units of alcohol) {\displaystyle {\text{volume (in litres)}}\times {\text{alcohol by volume (ABV)}}={\text{alcohol content (units of alcohol)}}}
The standard UK units of alcohol in a drink can be determined by multiplying the volume of the drink (expressed in litres) by its percentage ABV. For example, a standard 750 millilitres (25 US fl oz) bottle of wine at 12% ABV contains:
0.75 L × 12 % = 9 units {\displaystyle {\begin{aligned}0.75{\mbox{ L}}\times 12\%&=9{\mbox{ units}}\end{aligned}}}
One UK standard measure of spirits (25ml, 0.025 L) at 40% ABV contains:
0.025 L × 40 % = 1 unit {\displaystyle {\begin{aligned}0.025{\mbox{ L}}\times 40\%&=1{\mbox{ unit}}\end{aligned}}}
One imperial pint (568ml, 0.568 L) of beer at 4% ABV contains:
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