ReckonBoxHow the blood alcohol calculator works
Blood alcohol content is estimated with the Widmark equation: grams of alcohol divided by body weight in grams times a distribution ratio, multiplied by 100, then reduced by an elimination rate for each hour since the first drink. Erik Widmark published the model in the 1920s and every calculator on this search still runs it.
This one returns a range instead of a number, and never tells you whether you can drive. Both of those are deliberate, and the reason is in the arithmetic below.
The constants disagree, and the spread is not small
Every ranked calculator uses Widmark. They do not use the same numbers in it.
| Source | Ratio, male | Ratio, female | Elimination per hour |
|---|---|---|---|
| Indiana University | 0.68 | 0.55 | 0.016 |
| Inch Calculator | 0.68 | 0.55 | 0.015 |
| Team DUI | 0.73 | 0.66 | 0.015 |
The female distribution ratio spans 0.55 to 0.66. That's a fifth of its own value, sitting in the denominator.
Run it through. A 60 kg woman, three US standard drinks, two hours since the first one. At the low ratio with slow elimination the estimate is 0.097%. At the high ratio with fast elimination it's 0.074%. The legal limit in England and Wales is 0.08%, and it sits inside that range.
Identical inputs. Published constants only. One tool would tell her she's over, another that she's under, and both would be quoting the same equation.
That is why this page prints the whole range and the spread between its ends.
The model is weakest exactly where people use it
Wikipedia describes Widmark as a single-compartment model with instantaneous absorption and zero-order kinetics, capable of landing within about 20% coefficient of variation of the true value. Then it lists the conditions.
It performs best a few hours after a single dose taken on an empty stomach. During absorption it is "not at all realistic". Below 0.2 g/L it isn't accurate at all.
An evening out is several doses spread over hours, usually with food, and you're asking the question while still absorbing. Every condition the model wants is absent.
Nothing in the equation knows about food in your stomach, how quickly the drinks went down, medication, hydration, body composition or liver health. All of those move real blood alcohol.
A standard drink is not standard
The World Health Organization defines one as 10 grams of pure alcohol. A US standard drink holds about 14 grams.
Enter "three drinks" and you've described either 30 g or 42 g depending on the convention, a 40% difference in dose before the equation even starts. This calculator asks which one you mean.
Legal limits are a local question
| Jurisdiction | Limit | Detail |
|---|---|---|
| India | 0.03% | Six months and/or Rs 10,000 for a first offence |
| Japan | about 0.03% | Set in breath terms at 0.15 mg/L |
| Scotland | 0.05% | Lower than the rest of the UK |
| Germany | 0.05% | 0.03% if involved in an incident, zero for novice and professional drivers |
| France | 0.05% | 0.02% under three years experience, and for bus drivers |
| Australia | 0.05% | Zero for learner and provisional licences in various states |
| United States | 0.05% or 0.08% | Varies by state, 0.04% for commercial drivers |
| England, Wales, Northern Ireland | 0.08% | |
| Canada | 0.08% federal | Provincial penalties commonly begin at 0.04% to 0.05% |
Several countries set zero for every driver, among them Saudi Arabia, Somalia, Sudan, Yemen, Afghanistan, the Maldives and Mauritania.
Why there is no verdict on this page
No blood alcohol calculator can tell you whether you are fit to drive, and this one does not try.
The worked example above is the whole argument. Published constants alone produce a range that straddles a legal limit, before accounting for food, timing, medication or the absorption phase where the model admits it's unrealistic. A tool that resolved that into "you're under the limit" would be inventing a certainty the mathematics cannot supply.
Worth noting who does what here. Indiana University's alcohol education office runs a BAC calculator with no warning at all about using the result to decide whether to drive, though it does carry a candid note that the formula relies on sex assigned at birth and that validated equations for trans and non-binary people haven't been published. Inch Calculator, a commercial site, says plainly that it's never safe to drink and drive and that the tool cannot substitute for a blood alcohol test.
Alcohol also clears slowly and on its own schedule. At 0.015 to 0.016 percentage points an hour, there is nothing to be done to speed it up: coffee, food, a cold shower and exercise all leave the rate where it was.
What this calculator does not do
There's no tolerance adjustment. Regular drinkers do report feeling less impaired at a given concentration, and no source fetched for this build supplies a coefficient, so nothing here models it.
Breath-to-blood conversion is absent for the same reason, as is any attempt to estimate impairment of reaction time or judgement from a concentration.
Results are estimates and not medical or legal advice. This page exists to explain how the estimate is built and why it can't be trusted for a decision that matters. For other clinical estimates, the eGFR calculator covers kidney function and the sleep calculator handles cycle timing.
Frequently asked questions
How is blood alcohol content calculated? Through the Widmark equation: grams of alcohol divided by body weight in grams times a distribution ratio, multiplied by 100, then reduced by an elimination rate for every hour since the first drink. Erik Widmark published the model in the 1920s and every calculator on this search still runs it.
Why does this show a range instead of one number? Because the published calculators disagree on the constants. Indiana University and Inch Calculator use a distribution ratio of 0.68 for men and 0.55 for women, while other ranked tools use 0.73 and 0.66, and elimination is quoted at both 0.015 and 0.016 per hour. For a 60 kg woman with three US standard drinks over two hours, those constants alone give anything from 0.074% to 0.097%, which straddles the 0.08% limit in England and Wales.
Can this tell me whether I am safe to drive? No, and no calculator can. Wikipedia describes the Widmark model as performing best a few hours after a single dose taken on an empty stomach, and as not at all realistic during absorption, which is most of an evening out. This tool deliberately returns no pass or fail verdict against a legal limit, because the underlying model cannot support one.
What is the legal drink driving limit? It depends entirely on where you are. India sets 0.03%, Scotland 0.05%, England and Wales 0.08%, and most of continental Europe and Australia 0.05%. Germany drops to zero for novice and professional drivers, and France to 0.02% for drivers with under three years experience. Several countries including Saudi Arabia and Somalia set zero for everyone.
How long does alcohol take to leave your system? At the published elimination rates of 0.015 to 0.016 percentage points an hour, clearance is slow and roughly linear. Nothing speeds it up: coffee, food, a cold shower and exercise all leave the rate unchanged, because the liver processes alcohol at its own pace.
Does one standard drink mean the same everywhere? No. The World Health Organization defines a standard drink as 10 grams of pure alcohol while a US standard drink holds about 14 grams. Entering three drinks can therefore mean 30 g or 42 g depending on which convention you use, a difference of 40% in the dose.
What does the model fail to account for? Food in the stomach, how fast the drinks went down, medication, hydration, body composition and liver health all move real blood alcohol without appearing in the equation. Wikipedia puts the model within about 20% coefficient of variation of the true value at best, and worse during absorption.