What is a unit of alcohol?
A unit of alcohol is 10 ml of pure alcohol. This measurement is used in Britain to produce guidelines on how much it is healthy for adults to drink, in a day and over a week, and to measure what would constitute heavy drinking, binge drinking, etc. A unit is roughly the amount that an adult can digest in an hour.
Units of alcohol in a drink are calculated by multiplying the alcohol by volume (or ABV, normally shown as a percentage on the label) by the number of millilitres, divided by 1000, such that a half litre (500ml) can of beer at 4.5% ABV would be (4.5 x 500) / 1000 = 2.25 units.
Units of alcohol have different definitions around the world, but this site uses this measurement in its diary calculations.