The alcohol content in the blood relates to the quantity of alcohol consumed, to the time needed for consumption (how fast) and to other factors. The most important are:

Height and weight:
The smaller someone is the more he/she will be affected by the same quantity of alcohol, in comparison to a bigger person

Sex
Women can consume less alcohol than men, firstly because their body is generally smaller than men's and secondly, because their body contains less water and more fatty tissue in comparison to men. Alcohol is more easily dissolved in water than in fat.

The quantity of water in the body
If we are dehydrated, the effects of alcohol will be more intense. This is quite obvious in the summer, or after exercise.

The quantity of food
If we consume one alcohol unit with an empty stomach, most of it will be absorbed by the body in one hour. On the contrary, if the stomach is full, the process will be delayed and alcohol will be transferred to the brain and the other body organs at a slower pace.