Blood alcohol content is a measure of the amount of alcohol present in a quantity of blood. It is usually described as the amount of alcohol in mg per 100ml of blood. The set legal drink driving limit in the UK is 80mg/100ml blood or 0.08%.
How much alcohol you drink and can increase your BAC and also the quicker you drink the quicker your BAC will rise. Females will usually have a higher BAC even if the drink the same amount of alcohol in the same amount of time as a man.