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Units of alcohol are a measure of the volume of pure alcohol in an alcoholic beverage. They are used in some countries as a guideline for alcohol consumption. In some other countries a "standard drink", different from country to country, is defined for the same purpose.
One unit of alcohol is defined as 10 millilitres (7.9 grams) in the United Kingdom; typical drinks provide 1–3 units. In Australia "a 'standard drink' is the amount of a beverage that contains ten grams of alcohol at 20 degrees Celsius (12.7 ml)". In the US a "standard" drink is one that contains about 0.6 US fluid ounces or 14 grams of alcohol, about 77% more than a UK unit.
Labelling is usually required to give an indication of alcoholic content of a serving. In the United Kingdom the number of units in a typical serving is printed; Australia requires that "the label on a package of an alcoholic beverage must include a statement of the number of standard drinks in the package".
A typical healthy adult can metabolise about one unit per hour, depending on many factors.
The number of UK units of alcohol in a drink can be determined by multiplying the volume of the drink (in millilitres) by its percentage ABV, and dividing by 1000.
The formula uses ml ÷ 1000. This results in exactly one unit per percentage point per litre, of any alcoholic beverage.
Since 4% can be expressed as .04, .04 × 568 ml gives the amount of alcohol in terms of ml—which, when divided by 10, shows the number of units.
When the volume of an alcoholic drink is shown in centilitres, determining the number of units in a drink is as simple as volume × percentage (converted into a fraction of 1).
Thus, 750 millilitres of wine (the contents of a standard wine bottle) at 12% ABV contain:
It is often stated that a unit of alcohol is supplied by a small glass of wine, half a pint of beer, or a single measure of spirits. Such statements may be misleading because they do not reflect differences in strength of the various kinds of wines, beers, and spirits.
Most spirits sold in the United Kingdom have 40% ABV or slightly less. In England a single pub measure (25 ml) of a spirit contains one unit. However, a larger 35ml measure is increasingly used (and in particular is standard in Northern Ireland), which contains 1.4 units of alcohol at 40% ABV. Sellers of spirits by the glass must state the capacity of their standard measure in ml.
On average, it takes about one hour for the body to metabolise (break down) one unit of alcohol. However, this can vary with body weight, sex, age, personal metabolic rate, recent food intake, the type and strength of the alcohol, and medications taken. Alcohol may be metabolised more slowly if liver function is impaired.
From 1992 to 1995 the UK government advised that men should drink no more than 21 units per week, and women no more than 14. (The difference between the sexes was due to the typically lower weight and water-to-body-mass ratio of women.) The Times reported in October 2007 that these limits had been "plucked out of the air" and had no scientific basis.
This was changed after a government study showed that many people were in effect "saving up" their units and using them at the end of the week, a phenomenon referred to as binge drinking. Since 1995 the advice was that regular consumption of 3–4 units a day for men, or 2–3 units a day for women, would not pose significant health risks, but that consistently drinking four or more units a day (men), or three or more units a day (women), is not advisable.
An international study of about 6,000 men and 11,000 women for a total of 75,000 person-years found that people who reported that they drank more than a threshold value of 2 units of alcohol a day had a higher risk of fractures than non-drinkers. For example, those who drank over 3 units a day had nearly twice the risk of a hip fracture.