To find the units you multiply the serving size in litres by the number for the ABV. So, a 250 ml glass of wine at 12% ABV is 0.25 * 12 = 3 units. In the past

To find the units you multiply the serving size in litres by the number for the ABV. So, a 250 ml glass of wine at 12% ABV is 0.25 * 12 = 3 units. In the past when units were created wine was about 8% ABV, and a serving size was about 125 ml, which gives 0.125 * 8 = 1 unit per glass. When you see advice saying "one small glass of wine is about 1 unit" it's a bit misleading, because currently in the UK one small glass is going to be about 175 ml, and the ABV is going to be about 12%, which gives about 2 units per glass.