Buy it by the box and use an empty Big Gulp cup to drink it. :-)
What is a unit of alcohol?
One unit of alcohol is 10 ml (1 cl) by volume, or 8 g by weight, of pure alcohol. For example:
A half-pint of average strength beer, cider, or lager (4-5% alcohol by volume) contains one unit. Note: many beers are now stronger than the average 3-4% alcohol by volume.
A small pub measure (25 ml) of spirits (40% alcohol by volume) contains one unit. A standard pub measure (35 ml) of spirits contains one and a half units.
A standard pub measure of fortified wine such as sherry or port (20% alcohol by volume) contains one unit.
A small glass (125 ml) of average strengh wine (12% alcohol by volume) contains one and a half units.
Note: Many wines are stronger - up to 14-15% alcohol by volume. Also, wines are often served in large 175 ml glasses.
A more accurate way of calculating units is as follows. The percentage alcohol by volume (%abv) of any drink equals the number of units in one litre of that drink.
For example: Beer at 6% abv has six units in one litre. If you drink half a litre (500 ml) - just under a pint - then you have had three units.
Wine at 12% abv has 12 units in one litre. If you drink a quarter of a litre (250 ml) - two small glasses, then you have had three units.
Some other examples
Three pints of beer, three times per week, is at least 18 units per week. That is nearly the upper weekly safe limit for a man. However, each drinking session of three pints is at least six units, which is more than the safe limit advised for any one day.
Another example: a 750 ml bottle of 12% wine contains nine units. If you drink two bottles of 12% wine over a week, that is 18 units. This is above the upper safe limit for a woman.