What Is Blood Alcohol Content?
Blood Alcohol Content (BAC) โ also called Blood Alcohol Concentration โ is a measurement of how much alcohol is present in your bloodstream, expressed as a percentage. A BAC of 0.05% means there are 0.05 grams of alcohol per 100 millilitres of blood.
BAC is the standard used by Australian law enforcement to determine whether a driver is over the legal limit. You can estimate your current BAC using our Blood Alcohol Calculator, but understanding how BAC works gives you a much clearer picture of what's actually happening in your body.
Australia's Legal Limits
Australia has tiered BAC limits based on licence type:
- 0.00 BAC: Learner and P1/P2 provisional licence holders, professional drivers (bus, truck, taxi), and anyone driving a vehicle that requires a special licence
- 0.05 BAC: Full licence holders โ this is the limit most people associate with drink driving
It's important to note that impairment begins well before 0.05. Research consistently shows measurable effects on reaction time, divided attention, and decision-making beginning around 0.02โ0.03 BAC. The legal limit is not a safe limit โ it's a legal threshold.
How Alcohol Gets Into Your Blood
When you drink, alcohol is absorbed through your stomach lining and small intestine directly into the bloodstream. This process begins within minutes. A few factors affect absorption speed:
- Stomach contents: Food โ particularly fatty or high-protein food โ slows gastric emptying and significantly delays absorption. Drinking on an empty stomach accelerates intoxication
- Drink type: Carbonated drinks (champagne, spirits with soda) are absorbed faster than still drinks at the same alcohol content
- Drinking rate: Your liver can only process roughly one standard drink per hour. Drinking faster accumulates alcohol in your bloodstream faster than it can be cleared
What Affects Your BAC Level
The same number of drinks produces different BAC levels in different people. Key variables:
- Body weight: A heavier person has more total blood and body water, so the same amount of alcohol is more diluted. A 50kg person will reach a significantly higher BAC than an 80kg person drinking the same amount
- Sex: Women typically reach higher BAC levels than men of the same weight from the same amount of alcohol. This is due to differences in body composition (women generally have a higher proportion of body fat, which doesn't absorb alcohol) and lower levels of the enzyme alcohol dehydrogenase in the stomach
- Time: Your liver processes approximately 0.015โ0.017 BAC per hour regardless of what you do (coffee, water, and food do not speed this up)
- Genetics: Enzyme activity varies between individuals and across ethnic groups, affecting how quickly alcohol is broken down
A Standard Drink in Australia
One Australian standard drink contains 10 grams of pure alcohol. This is equivalent to:
- A mid-strength beer (375ml at 3.5%)
- A small glass of wine (100ml at 13%)
- A 30ml shot of spirits
Note that many pours โ particularly restaurant wine glasses (150โ180ml) and pre-mixed cans (5โ7%) โ contain 1.5โ2 standard drinks. It's easy to undercount significantly without checking labels.
How Long Until You're Sober?
The only thing that reduces BAC is time. At 0.015 BAC cleared per hour, here's an approximate clearance schedule:
- BAC 0.05: ~3.5 hours to zero
- BAC 0.08: ~5.5 hours to zero
- BAC 0.10: ~7 hours to zero
These are rough averages โ individual clearance rates vary. Our Blood Alcohol Calculator gives a personalised estimate based on your weight, sex, number of drinks, and time elapsed. Many Australians are still over the limit the morning after a heavy night โ more common than most people realise.
Practical Tips for Staying Under the Limit
- Count standard drinks, not glasses โ containers vary enormously
- Eat a substantial meal before and during drinking
- Alternate alcoholic drinks with water
- Use a personal breathalyser before driving โ a good one costs less than a taxi for a year. Browse personal breathalysers on Amazon AU to find a reliable option
- When in doubt, don't drive โ rideshare is always cheaper than a fine or worse
Important: The Blood Alcohol Calculator provides estimates only and should not be used to determine whether it is safe or legal to drive. BAC varies between individuals. If in doubt, don't drive.