A 'unit' in Scotland/UK is 1cl of pure alcohol. So to get the number of 'units' of alcohol you multiply the the 'strength', in % ethanol by volum, by the volume of the container it is sold in. So a 1 liter bottle of vodka is 1l x 40% = 40 units and a bottle of wine in 0.75 x 13% = 9.75 units.