The amount of alcohol in the blood is known as the blood alcohol concentration or BAC. Your BAC depends on how much you've drunk and how quickly you drank it. Other important factors affecting BAC are:

Your size and weight
If you're small, your blood alcohol volume is obviously less than that of someone who is larger. So the same amount of alcohol will probably affect you more.

Your sex
Women can't drink as much as men. That's not a male conspiracy but a biological fact! Women are generally smaller. They also have proportionately less body water and more body fat - and alcohol doesn't dissolve easily in fat. That's why, drink for drink, women end up with more alcohol in their blood than men.

Your water level
If you're dehydrated, alcohol will have a greater effect than if your body's water concentration is normal. That's why drinking alcohol in summer or after exercise affects you more.

The amount you've eaten
If you drink a unit of alcohol on an empty stomach, almost all of it will be absorbed in an hour. But if there's food in your stomach, the process will be slower and the alcohol reaches your brain and the rest of your body more slowly.