Know the Facts

Helping you make informed decisions about your drinking

Blood Alcohol Concentration

Blood alcohol concentration (BAC) is a measure of alcohol in your body, expressed as grams of alcohol per 100mls of blood. For a BAC of 0.05, every 100mls of your blood contains 0.05 grams of alcohol. Your BAC can be measured through your breath, blood, and urine.

Factors that influence BAC

As a general rule of thumb, it takes an average healthy person about one hour to process one standard drink. But remember, this is a guideline only! Two people who drink the same amount of alcohol can have very different BACs. This is because everyone processes alcohol at a different rate. Your BAC is affected by the amount of alcohol that you drink and the period of time over which you drink it. If you consume a lot of drinks in a short amount of time, your BAC will be higher than if you drank them over a longer period of time. Your BAC will also depend on a number of other factors including your:


Due to differences in body composition, the same amount of alcohol will generally lead to a higher BAC in a woman than in a man.


Younger people will reach a higher BAC faster than older people due to their faster metabolism.

Body size

A larger person has more blood to dilute the alcohol, and so will take longer to reach the same BAC as a smaller person. Also, muscle absorbs alcohol but fat does not, so the more body fat a person has, the faster their BAC will rise.

Liver function

Damaged or unhealthy livers cannot process alcohol as efficiently as healthy ones, resulting in higher BACs.

Stomach content

Food in your stomach slows absorption of alcohol, making your BAC rise slower than if you drank on an empty stomach.


The liver enzymes used to break down alcohol differ from person to person, depending on genetic make-up. These enzymes work at different rates, meaning some people will naturally process alcohol faster than others.

BAC and driving

Drinking one standard drink will increase your BAC by about 0.02 (remember this can depend on the factors mentioned above). As your BAC rises, so does your risk of being involved in a crash. This is why drivers with an open licence have to keep under a BAC of 0.05. Once your BAC reaches 0.05 you are twice as likely to crash as someone with a BAC of 0.00. For this reason, if you plan on drinking, plan on not driving. Try these alternatives instead:

  • Stay over at a friend’s place
  • Use public transport
  • Take a taxi
  • Assign a designated driver
  • Arrange for a friend or family member to pick you up
  • Ask for a lift from someone going in your direction

The only way to lower your BAC is with time. The more drinks you had, the more time you will need. If you have a lot of drinks and drink late into the morning, your BAC will likely still be over 0.05 when you wake up, so avoid driving early the next day after a big night out.


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