Blood alcohol levels : how accurately can they be guessed?

Volunteers were asked to guess their blood alcohol level immediately before having their venous level measured by gas chromatography. Thirty-one percent were able to guess to within 20mg per 100ml of the measured amount. There was a tendency for people to over-estimate if the level was low and to under-estimate the higher values. Blood alcohol concentrations ranged from nil to 195mg per 100ml with both high and low being recorded throughout the evening.