In the UK, one unit of alcohol is 8 grams of pure ethanol, which is 10mL of ethanol.  The % ABV (also referred to as % vol) measurement of a drink indicates how much of the volume of the drink is pure ethanol.  For example, a 40% ABV vodka is 40% pure ethanol by volume, therefore in 100mL of this drink there is 40mL of pure ethanol which is 4 units of alcohol or 32 grams of pure ethanol.  It is important to note different countries have different measurements for a unit of alcohol - for example some countries measure a unit of alcohol as 10 grams of pure ethanol, 20% more than in the UK.