BAC Calculator

About the BAC Calculator

Blood alcohol concentration, or BAC, is the share of your blood that is alcohol. A BAC of 0.08% means 0.08 grams of alcohol in every 100 millilitres of blood, and it is the measure that drink-drive laws, and most research on the effects of alcohol, are written in.

It can be estimated from what you drank, your body weight and sex, and the time since you started, using a formula devised by the Swedish chemist Erik Widmark in the 1930s. That formula is still the basis of the calculations forensic experts use today.

This BAC calculator does that estimate, and then does something many do not: it shows how wide the realistic range is around it. The range is wide enough that no calculator can tell you whether you are under a legal limit, and this one never claims to.

How to Use the BAC Calculator

Choose your units and sex, and enter your body weight.

Pick the drink you had, or enter the size and strength of something else, and how many of them.

Enter the hours since your first drink. Alcohol is being cleared from the moment you start, so the clock starts then.

How the Widmark Formula Works

  alcohol (g)  =  drinks × volume (ml) × ABV × 0.789
  peak BAC (%) =  alcohol (g) ÷ (r × body weight (g)) × 100
  BAC now (%)  =  peak − 0.015 × hours

r is the Widmark factor, the share of the body that alcohol spreads through. Alcohol dissolves in water, not fat, so r is essentially the body's water fraction. Widmark's averages are 0.68 for men and 0.55 for women.

0.015% an hour is the average rate at which the liver clears alcohol.

Step-by-Step Example

Man, 80 kg. Four pints of 4% beer. Two hours since the first.

  Alcohol:    4 × 568 ml × 0.04 × 0.789       =  71.7 g
  Peak:       71.7 ÷ (0.68 × 80,000) × 100     =  0.132%
  Now:        0.132 − 0.015 × 2                =  0.102%
  To zero:    0.102 ÷ 0.015                    ≈  7 hours

That estimate is above both the 0.08% and 0.05% limits. But the realistic range around it runs from about 0.06% to 0.15%, depending on the individual.

Why the Estimate Is So Uncertain

Every input to the formula is an average, and real people differ.

Body water ranges from about 50% to 80% of body weight. A muscular person has more, and reaches a lower BAC from the same drinks; someone with more body fat has less, and reaches a higher one.

Elimination ranges from about 0.01% to 0.025% an hour between people.

Food in the stomach slows absorption and lowers the peak, sometimes substantially. Drinking on an empty stomach produces a faster, higher peak.

How fast you drink changes the shape of the curve: the formula assumes all the alcohol is absorbed at once.

Taken together, a real BAC can be half or double the estimate. That is why the calculator reports a range, and why it cannot tell you whether you are safe or legal to drive.

Drink-Drive Limits

Many countries set lower limits, or zero, for new and professional drivers. But limits mark where the law steps in, not where impairment starts. Reaction time and judgement are measurably affected from around 0.02% to 0.05%, and the risk of a crash rises steadily from there.

How Long Until It Is Gone

At the average rate, BAC falls about 0.015% an hour — very roughly, one UK unit an hour for many adults. Nothing speeds this up. Coffee may make you feel more alert but does not lower BAC; neither do water, a cold shower, food after drinking, or sleep.

After a heavy evening, it is common to still be over the limit the next morning, which is when many drink-drive convictions happen.

Standard Drinks and BAC

A common rule of thumb is that one standard drink raises BAC by about 0.02% for an average man, and a little more for an average woman. It is a useful sense of scale: two or three drinks in an hour can put many people near 0.05%. But it is only an average, and the same person can reach very different levels on different nights depending on food, fatigue and how quickly they drink.

Understanding Your Result

Estimated BAC is the Widmark estimate for now.

Realistic range shows how far real BAC could plausibly differ, given normal variation in body water and elimination.

Against drink-drive limits compares the estimate with the 0.08% and 0.05% limits — for information only.

Time to zero gives the hours until the estimate reaches zero at the average rate.

Alcohol drunk shows the grams, UK units and US standard drinks.

Worth knowing explains what the number can and cannot tell you.

When Should You Use This Calculator?

To understand how alcohol builds up and clears, and why the morning after matters.

To see how body size and sex change the effect of the same drinks.

Out of curiosity about the Widmark formula and its limits.

Never to decide whether to drive. If you plan to drive, do not drink.

Common Mistakes

Treating the estimate as your real BAC. It can be half or double.

Believing coffee or water sobers you up. Only time does.

Forgetting the morning after. Alcohol can still be in the blood.

Counting drinks, not grams. Large glasses and strong beers carry far more.

Assuming "under the limit" means unimpaired. Impairment starts well below it.

If someone has drunk heavily and cannot be woken, is breathing slowly or irregularly, is cold or pale, or is having a seizure, call emergency services. Nothing here is medical or legal advice.