Why use this calculator
The value of this tool is not the number it prints, it is the lesson the number teaches. Most people badly underestimate how long alcohol stays in the blood and how little it takes to cross a limit. Three ordinary beers in two hours puts an average-sized man at or over the 0.05 limit used in Australia and most of Europe, and leaves him over three hours from zero.
A smaller person, or anyone drinking wine in a modern 250 millilitre pour rather than a 150 millilitre serve, gets there faster still. Seeing the elimination curve makes the real point: alcohol leaves at a fixed slow rate of roughly one standard drink an hour, and nothing shortens it.
Coffee, a cold shower, food, water, exercise and sleep all change how you feel while changing nothing about the concentration in your blood. Use this to plan a night before it starts, never to audit yourself part way through one.
How to use it
- Choose pounds or kilograms, then enter your current body weight, which sets how far the alcohol is diluted through your body water.
- Choose your sex, which sets the Widmark distribution ratio.
- Pick the drink that most closely matches what you actually drank, because a craft beer, a restaurant wine pour and a home-poured spirit each carry very different amounts of ethanol despite all being called one drink.
- Enter how many of that serve you have had.
- Enter the hours since your first drink, not since your last, because elimination has been running the whole time.
- Finally choose the legal limit that applies to your licence, which for learner, provisional, professional and heavy vehicle drivers is frequently zero or 0.02 rather than the general limit.
What each result means
- Estimated BAC (g/100 mL)
- The estimated grams of alcohol per 100 millilitres of blood. It is a population average, not a measurement of you, and carries error of 20 per cent or more.
- Driving Decision
- The only answer this tool is qualified to give. No estimate can establish that you are under a limit or safe to drive a vehicle.
- Against Your Selected Limit
- How the estimate compares with the limit you chose. Anything within a few hundredths of the threshold should be read as over it.
- Peak BAC Before Elimination
- The starting concentration if every drink had fully absorbed and no time had passed. Real peaks arrive 30 to 90 minutes after drinking.
- Pure Alcohol Consumed
- Total grams of ethanol, converted into Australian standard drinks of 10 grams and UK units of 8 grams for comparison.
- Typical Impairment At This Level
- What effects are commonly reported around this concentration. Impairment of reaction time and judgement begins from the very first drink.
- Hours Until Estimated Zero
- How long the estimate takes to reach zero at the average elimination rate. Slower metabolisers need considerably longer than this.
- Hours Until Estimated Under Limit
- Estimated time to fall below the limit you chose. Treat it as a floor and add a wide margin, especially the morning after.
- Time Elapsed
- The hours you entered since your first drink, which is the only input that reduces the estimate.
The formula
The Widmark equation gives BAC as a percentage by weight per volume: BAC equals grams of pure ethanol consumed, divided by body weight in grams multiplied by the distribution ratio r, multiplied by 100, then minus 0.015 multiplied by the hours elapsed. The distribution ratio r is the fraction of body mass that behaves as body water, taken here as 0.68 for men and 0.55 for women, which are Widmark's original mean values. Grams of ethanol per serve are the serve volume in millilitres multiplied by the alcohol by volume fraction multiplied by 0.789, the density of ethanol. The elimination constant of 0.015 percentage points per hour is an average; measured rates range from about 0.010 to 0.020, and Widmark estimates typically sit within roughly 20 per cent of a measured blood sample but can be considerably further out for any one person.
How this number is used in practice
The most dangerous hour for a drink driving charge is the morning after, not the night before. Someone who finishes ten standard drinks at one in the morning is still carrying a substantial concentration at eight, because elimination only removes about one standard drink an hour and does not accelerate while you sleep.
Police across Australia, the United Kingdom and much of Europe run morning breath testing operations for exactly this reason. Limits also vary far more than people assume. Australia, France, Germany and Scotland use 0.05, while the United States, Canada, England and Wales use 0.08, and dozens of countries including the Czech Republic and Hungary have a true zero limit.
Learner and provisional licences, professional drivers, bus and truck drivers and anyone driving for work commonly face 0.00 or 0.02 regardless of the general limit.
The Widmark model is the equation courts and toxicologists have used since the 1930s, and it is genuinely useful in aggregate, but the assumptions behind it are strong. It treats alcohol as instantly and evenly distributed, ignores the absorption phase entirely, and uses a single average elimination rate.
In reality food in the stomach can delay a peak by an hour or more, body composition changes the distribution ratio, some medications and liver conditions change clearance substantially, and women often show higher concentrations than the equation predicts because of first-pass metabolism differences. Two people of identical weight and sex drinking identically can end up a third apart.
This is an educational estimate. It is not a breath test, it is not a blood test, it is not evidence, and it must never be used to decide whether to drive.
Worked examples
170 lb man, three 355 mL beers at 5 per cent, two hours after the first
Estimated 0.050, right on the 0.05 limit used across Australia and most of Europe, and about 3.3 hours from zero.
60 kg woman, two 150 mL glasses of wine at 12 per cent, one hour after the first
Estimated 0.071, over a 0.05 limit and under a 0.08 one, needing about 4.7 hours to reach zero.
200 lb man, six 44 mL spirit serves at 40 per cent, four hours after the first
Estimated 0.076, still under a 0.08 limit on paper but about 5.1 hours from zero and clearly impaired.
When people use it
- Understanding how many hours after a night out you may still be over a limit
- Seeing how much alcohol is actually in a craft beer, a large wine pour or a home-poured spirit
- Planning transport and accommodation before an event rather than deciding afterwards
- Explaining to someone why coffee, food and a shower do not lower blood alcohol
- Comparing how the same drinks affect people of different body weights
Tips
- Never drive after drinking. This calculator is an educational estimate and can be wrong by enough to cost you a licence, a life, or both.
- Count the pour, not the glass. A restaurant serves wine in 200 to 250 mL pours, which can be nearly two standard drinks in one glass.
- Plan the ride home before the first drink. Decisions made at 0.05 are demonstrably worse than decisions made at zero.
- Allow for the morning after. Ten drinks finishing at 1 am can leave you over a 0.05 limit well past 8 am.
- Check your own licence conditions. Learner, provisional, professional, taxi, bus and heavy vehicle drivers usually face a zero or 0.02 limit.
- Alternate alcoholic drinks with water to slow the pace, while knowing water does not reduce blood alcohol at all.
- If someone is unrousable, breathing slowly or cannot be woken, call emergency services. Alcohol poisoning kills people who are left to sleep it off.
What it assumes, and where it stops being accurate
- The Widmark model assumes all alcohol is absorbed instantly and distributed evenly, so it overstates concentration in the first hour and understates the delay caused by food.
- Elimination is treated as a constant 0.015 percentage points an hour, but measured rates run from about 0.010 to 0.020 and vary with drinking history and liver health.
- Distribution ratios of 0.68 for men and 0.55 for women are population averages. Body composition, age and pregnancy all shift them.
- It is not accurate for people with liver disease, on medication that affects alcohol metabolism, who have had bariatric surgery, or who are very lean or very obese.
- Serve sizes are nominal. Home pours and bar measures routinely differ from the volumes listed here by 50 per cent or more.
- The result has no legal standing. Only an evidential breath or blood test administered by police measures blood alcohol.
Terms used on this page
- Blood alcohol concentration
- Grams of alcohol per 100 millilitres of blood, written as a decimal such as 0.05.
- Widmark formula
- The 1930s equation estimating blood alcohol from dose, body mass, a distribution ratio and elapsed time.
- Widmark ratio (r)
- The fraction of body mass that behaves as body water, about 0.68 for men and 0.55 for women.
- Standard drink
- A defined quantity of pure alcohol: 10 g in Australia, 8 g in a UK unit, about 14 g in the United States.
- Elimination rate
- How fast alcohol leaves the blood, averaging 0.015 percentage points an hour with a range of 0.010 to 0.020.
Questions people ask
- Can I use this to decide whether to drive?
- No, and this is the most important thing on the page. The estimate carries error of 20 per cent or more, ignores absorption, food, medication and liver health, and has no legal standing whatsoever. If you have had any alcohol, the only safe assumption is that you cannot drive. Arrange another way home before you start drinking.
- How accurate is the Widmark formula?
- In aggregate it is reasonable, and forensic toxicologists still use it. For any one person it typically sits within about 20 per cent of a measured blood sample, and can be considerably further out. Food in the stomach, drinking pace, body composition, medication and liver function all shift the real figure in ways the equation cannot see.
- What is the legal limit where I live?
- It varies widely. Australia, France, Germany and Scotland use 0.05, while the United States, Canada, England and Wales use 0.08, and many countries including the Czech Republic and Hungary use zero. Learner, provisional, professional, taxi, bus and heavy vehicle drivers commonly face 0.00 or 0.02 regardless of the general limit that applies to other drivers.
- How long does it take to sober up?
- Blood alcohol falls at roughly 0.015 percentage points an hour, which is about one standard drink an hour, and nothing speeds it up. Coffee, cold showers, food, water, exercise and vomiting can change how alert you feel while leaving the concentration in your blood untouched, which is exactly why feeling fine is such an unreliable guide.
- What counts as a standard drink?
- It depends on the country. Australia defines a standard drink as 10 grams of pure alcohol, the United Kingdom uses an 8 gram unit, and the United States uses about 14 grams. That is why this calculator asks for the actual serve: the same glass of wine can be one drink, one and a half units or two standard drinks depending on where you are.
- Why does sex change the result?
- The Widmark ratio reflects the proportion of body mass that behaves as body water, and women on average carry a lower proportion than men at the same weight, so the same alcohol is diluted less. Differences in first-pass metabolism in the stomach contribute as well. These are averages, and individual variation within each group is large.
Use this with
BAC Calculator answers one part of the question. These pick up where it stops: Alcohol Dilution Calculator, Water Intake Calculator, BMI Calculator and Calorie Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- World Health Organization: alcohol. Global health effects and drink driving evidence
- National Institute on Alcohol Abuse and Alcoholism. Standard drink definitions and alcohol metabolism
- NHTSA: drunk driving. Impairment at given concentrations and US legal limits
- Australian Government Department of Health: alcohol. Australian standard drink and national guidelines
- NHS: alcohol advice. UK units and low-risk drinking guidance
BAC Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.