It's hard to calculate how much you are drinking—but you should know

The festive season is a testing time, and those who wish to drink sensibly might use the “unit of alcohol”—a glass of wine or beer or a single measure of spirits—as a yardstick. But what is a unit, and how many is it safe to drink?

The discerning drinker could calculate the dose of ethanol in a drink knowing its volume and ethanol concentration. However, even the sober can find this difficult. Firstly, there are several ways of defining concentration. It is expressed as percentage ethanol by volume (% v/v) in Europe and as percentage proof in the United States, where 100% proof is 50% v/v (in England 100% proof was 57% v/v). The density of ethanol is 0.79 g/ml at room temperature, so, for example, 100 ml of ethanol 10% v/v contains almost 8 g of ethanol. Secondly, concentration can differ widely among apparently similar drinks. The strengths of beers range from about 3.4% to 9% v/v; white wine from 8% to 13% v/v; and …