🍷 BAC Calculator
A rough estimate of blood alcohol concentration using the Widmark formula. It is an average of a population, not a measurement of you, and it must never be used to decide whether to drive.
The Widmark formula, and everything it cannot see
Blood alcohol concentration is the percentage of alcohol in your bloodstream. The Widmark formula, published in the 1930s, estimates it from the alcohol consumed, body weight and a distribution factor, then subtracts the amount your liver has processed. It is the basis of almost every BAC calculator in existence, and it is an estimate of a population, not a measurement of you.
BAC % = (grams of alcohol / (body weight in g × r)) × 100 − (0.015 × hours)
The r factor is roughly 0.68 for men and 0.55 for women, reflecting differences in body water as a proportion of mass. The 0.015 is the typical rate of elimination in percentage points per hour.
What a standard drink is
| Country | Grams of alcohol |
|---|---|
| United States | 14 g |
| Canada, Australia | 10–12 g |
| United Kingdom (one unit) | 8 g |
| Most of Europe | 10 g |
| Japan | 20 g |
In the US, 14 g is roughly 350 ml of 5% beer, 150 ml of 12% wine or 44 ml of 40% spirits. In practice a restaurant glass of wine is often 250 ml and a craft beer is often 6.5%, so a "couple of drinks" can easily be four standard drinks. This is the single biggest source of error when people estimate their own intake.
Elimination is slow and nothing speeds it up
The liver processes alcohol at a roughly constant rate of about one standard drink an hour, which is where the 0.015 points per hour comes from. It does not accelerate. Coffee makes you an awake drunk rather than a sober one; food slows absorption but does not increase elimination; cold showers, exercise and fresh air do nothing measurable. Time is the only mechanism.
This matters most the morning after. Four drinks finishing at 1am leaves a measurable BAC at 7am for many people. Morning drink-driving arrests are common for exactly this reason, and the driver is usually genuinely surprised.
Legal limits
| BAC limit | Where it applies |
|---|---|
| 0.08% | United States, Canada, England and Wales |
| 0.05% | Scotland, most of the EU, Australia |
| 0.03% or lower | China, India, Japan, Norway, Sweden |
| 0.00% | Czechia, Hungary, Slovakia, and most countries for novice or professional drivers |
Many jurisdictions apply a lower or zero limit to drivers under 21, newly qualified drivers and commercial licence holders. Impairment of reaction time and judgement is measurable well below every one of these limits, which is why the limits are not a target.
Why this estimate can be badly wrong for you
- Body composition. The r factor assumes average body water. Muscular and lean people distribute alcohol differently from the average.
- Food. Eating slows absorption substantially, lowering the peak but not the total.
- Medication. Many drugs interact with alcohol or affect its metabolism.
- Drinking pace. Widmark assumes full absorption; drinking over several hours changes the curve.
- Individual variation. Liver enzyme activity varies considerably between people and with regular drinking.
Validation studies routinely find errors of 30% or more against measured breath or blood tests. That is the margin between a confident calculation and a criminal offence.
A worked example
A 78 kg man drinks three US standard drinks, 42 g of alcohol, over two hours. The peak estimate is 42 / (78,000 × 0.68) × 100 = 0.079%. Two hours of elimination removes 0.030, leaving about 0.049%. That is below the 0.08% US limit and above the 0.05% limit in Scotland and most of Europe, and his reaction time is measurably slower either way. He reaches zero roughly 3.3 hours later.
Never use this, or any calculator, to decide whether you are fit to drive. The honest answer after any alcohol is to use another way home.
If the number here raised another question, it is probably one of these: the calories burned calculator or the BMI calculator.
Frequently Asked Questions (FAQs)
How long does alcohol stay in your system?
Roughly one standard drink per hour, at about 0.015 percentage points of BAC. Four drinks take around five to six hours to clear. Breath and blood tests detect it for that window; urine tests for considerably longer, and hair tests for months.
Can I speed up sobering up?
No. Coffee, food, water, exercise and cold showers change how awake you feel without changing your blood alcohol at all. The liver processes alcohol at a fixed rate and nothing available to you accelerates it.
How accurate is a BAC calculator?
Within perhaps 30% for an average person under average conditions, and considerably worse outside them. Body composition, food, medication, drinking pace and individual metabolism all shift the real figure. It is an educational estimate, never a fitness-to-drive test.
Why do women reach a higher BAC than men on the same drinks?
Lower average body water as a proportion of mass, which means the same alcohol is distributed through less fluid, plus differences in the stomach enzyme that begins breaking alcohol down. The Widmark r factor encodes this as 0.55 against 0.68.
Can I be over the limit the morning after?
Yes, and it is common. Finishing six drinks at midnight can leave a measurable BAC at 8am. Because you feel rested rather than drunk, this is one of the most frequent circumstances for an unexpected drink-driving charge.
Does eating before drinking lower my BAC?
It lowers the peak by slowing absorption, which is worth doing, but it does not reduce the total alcohol absorbed or speed up elimination. You reach a lower maximum and take a similar time to return to zero.