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A Blood Alcohol Concentration (BAC) calculator uses the Widmark formula to estimate how much alcohol is in your bloodstream based on what you've drunk, your body weight, sex, and how much time has passed. It's a useful educational tool — but it is an estimate only. Do not use it to decide whether you are safe to drive.
This tool uses the classic Widmark equation:
BAC = (A × 5.14) / (W × r) − 0.015 × H
In all 50 US states, the standard legal limit is 0.08% BAC for drivers 21 and over. Utah lowered theirs to 0.05% in 2018. For commercial drivers the limit is 0.04%, and for drivers under 21 most states enforce a zero-tolerance limit of 0.01–0.02%. A "standard drink" in the US contains about 0.6 fl oz of pure alcohol — that's roughly a 12 oz beer at 5% ABV, a 5 oz glass of wine at 12%, or a 1.5 oz shot of 40% spirits.
England, Wales and Northern Ireland: 80 mg per 100 ml of blood (equivalent to 0.08% BAC). Scotland has a lower limit of 50 mg/100 ml (0.05%) since 2014. The UK also measures alcohol in units: 1 unit = 10 ml (8 g) of pure alcohol. A pint of 4% beer is about 2.3 units; a 175 ml glass of 12% wine is 2.1 units. The NHS recommends drinking no more than 14 units per week, spread over 3+ days.
The liver metabolises alcohol at a roughly fixed rate — about 0.015% BAC per hour, or roughly 1 UK unit per hour. Coffee, a cold shower, food, and exercise will not speed this up. Only time does. If your peak BAC is 0.10%, it takes approximately 6–7 hours to return to 0.00%, and more than 2 hours just to drop below the 0.08% US limit.
No. This is an educational estimate using the Widmark formula. Police in the US and UK use calibrated breathalyzers or blood tests. Never use an online BAC calculator as a go/no-go for driving. If in doubt, do not drive.
If you drank heavily until 2am, a BAC of 0.12% would still be over 0.05% at 7am and over 0.00% well past 10am. Many drink-drive convictions in the UK happen the morning after. When in doubt, take a taxi or public transport.
For an average 180 lb man: roughly 4 standard drinks over 2 hours. For a 140 lb woman: roughly 2–3 standard drinks over the same period. But these numbers vary heavily by individual. The safest rule: if you drive, drink zero.
It means 0.08 grams of alcohol per 100 ml of blood, or 80 mg/100 ml. UK law typically states it as mg per 100 ml (breath: 35 µg/100 ml; blood: 80 mg/100 ml; urine: 107 mg/100 ml).
No. Only time metabolises alcohol. Food eaten before or during drinking slows absorption and lowers the peak, but once alcohol is in your bloodstream only the liver can remove it.
This BAC tool provides a rough estimate for informational purposes only. Real alcohol absorption varies by body composition, food intake, drink size, medications, and health status. Never use this estimate to decide whether you are safe or legal to drive or operate equipment.
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Blood Alcohol Concentration (BAC) is a measure of the amount of alcohol present in a person's bloodstream, expressed as a percentage by volume or as milligrams per 100 millilitres of blood. A BAC of 0.08% means there are 0.08 grams of alcohol per 100 mL of blood. BAC is the legal standard used by law enforcement in both the United States and United Kingdom (and most of the world) to determine alcohol impairment for driving purposes. Understanding your estimated BAC helps you make responsible decisions about whether it is safe and legal to drive after drinking.
It is critical to understand that BAC calculators, including this one, provide estimates only. Individual responses to alcohol vary significantly based on factors including food consumption, hydration, medication, metabolic rate, liver health, and genetic variations in alcohol dehydrogenase enzymes. The only way to know your exact BAC is with a certified breathalyser or blood test. Never use a BAC estimate to justify driving – if you have been drinking, the safest choice is always to find alternative transport.
The most widely accepted formula for estimating BAC is the Widmark formula, developed by Swedish physician Erik Widmark in the 1930s:
BAC = (Alcohol consumed in grams) / (Body weight in grams × r) − (Elimination rate × hours since drinking)
Where r is the Widmark factor: approximately 0.73 for men and 0.66 for women (reflecting the different proportions of body water in which alcohol distributes). The elimination rate is typically 0.015% per hour (approximately 0.15g per 100 mL blood per hour), though this varies between approximately 0.010% and 0.020% depending on the individual.
To convert to grams of alcohol: 1 UK unit = 8g pure alcohol = 10mL. A standard 330mL can of 5% beer contains 330 × 0.05 × 0.789 (density of ethanol) = approximately 13g of alcohol. In the US, a "standard drink" contains approximately 14g (0.6 fl oz) of pure alcohol.
In England, Wales, and Northern Ireland, the legal drink-drive limit is:
This limit – 80 mg/100 mL – is higher than most of Europe and has been criticised by road safety experts and medical bodies including the British Medical Association, which recommends reducing it to 50 mg/100 mL (the limit in Scotland). Research consistently shows that even at BAC levels below the current English/Welsh limit, driving ability is measurably impaired.
Scotland has a lower limit of 50 mg of alcohol per 100 mL of blood (22 micrograms per 100 mL of breath), introduced in December 2014. This aligns Scotland with most European Union countries including France, Germany, and Spain. At 50 mg/100 mL, the risk of a fatal crash is approximately twice that of a sober driver; at 80 mg/100 mL, the risk is approximately four times higher.
In the United States, all 50 states have a per se BAC limit of 0.08% (80 mg/100 mL) for drivers aged 21 and over. Driving with a BAC at or above this level constitutes DUI (Driving Under the Influence) or DWI (Driving While Impaired) regardless of observed impairment. Utah lowered its limit to 0.05% (50 mg/100 mL) in December 2018, and the National Transportation Safety Board (NTSB) has recommended all states follow Utah's lead.
Zero tolerance laws for under-21 drivers: All US states enforce zero tolerance laws for drivers under 21, meaning any detectable BAC (typically 0.01-0.02% depending on state) results in license suspension and penalties. Commercial vehicle operators (CDL holders) have a limit of 0.04% nationwide.
The table below describes typical effects at different BAC levels. Individual responses vary considerably:
| BAC Level | Typical Effects | Driving Risk |
|---|---|---|
| 0.02–0.03% | Slight relaxation, mild mood elevation, reduced inhibitions | Slight reduction in visual tracking |
| 0.04–0.06% | Lowered alertness, relaxed judgement, impaired coordination beginning | Reduced concentration and steering ability |
| 0.07–0.09% | Impaired balance, speech, reaction time, and judgment | At or near legal limit in most jurisdictions; significantly impaired |
| 0.10–0.12% | Slurred speech, poor coordination, significantly slowed reaction time | Severely impaired; crash risk approximately 7× higher |
| 0.13–0.15% | Gross motor impairment, blurred vision, dysphoria | Very severely impaired; crash risk 25× higher |
| 0.16–0.19% | Nausea, feeling dazed, disorientation | Extremely dangerous to drive |
| 0.20–0.30% | Loss of consciousness possible; memory blackout likely | Do not drive; medical emergency territory |
| 0.30–0.40% | Possible death from respiratory failure | Medical emergency; call 999 (UK) or 911 (US) |
In the UK, alcohol is measured in units. One unit equals 10 mL or 8 grams of pure alcohol. Common drinks and their unit content:
| Drink | Approximate Units |
|---|---|
| Pint of 4% beer/lager | 2.3 units |
| Pint of 5% lager | 2.8 units |
| 330 mL can of 5% beer | 1.7 units |
| 175 mL glass of 13% wine | 2.3 units |
| 750 mL bottle of 13% wine | 9.75 units |
| 25 mL single of 40% spirits | 1 unit |
| 330 mL bottle of 5% cider | 1.7 units |
The UK Chief Medical Officers recommend drinking no more than 14 units per week to keep health risks low (applicable to both men and women). They also advise spreading consumption over 3 or more days rather than bingeing, having several alcohol-free days per week, and avoiding alcohol completely during pregnancy. There is no safe level of alcohol consumption during pregnancy.
The body eliminates alcohol at a fixed rate of approximately 1 unit per hour (or 0.015% BAC per hour). No amount of coffee, water, cold showers, food, or exercise will speed this up. These measures may make you feel more alert, but they do not reduce BAC or restore driving ability. A person who goes to sleep drunk may still be over the legal limit when they wake up. If you had 10 units before bed at midnight, you would not be below the English/Welsh 80 mg/100 mL limit until approximately 8-10 hours later (depending on body weight, gender, and food consumed). The morning-after risk is real and frequently results in prosecutions.
The drink drive limit in England and Wales is 80 mg of alcohol per 100 mL of blood, or 35 micrograms per 100 mL of breath. This is higher than in Scotland (50 mg/100 mL) and most of Europe (also 50 mg/100 mL). Exceeding the limit is a criminal offence that carries a minimum 12-month driving ban, an unlimited fine, and up to 6 months in prison for a first offence.
Scotland's drink drive limit is 50 mg of alcohol per 100 mL of blood, or 22 micrograms per 100 mL of breath – introduced in December 2014. This is lower than in England, Wales, and Northern Ireland (80 mg/100 mL) and aligns Scotland with most European countries. Road safety evidence suggests the lower limit reduces fatal crashes. Penalties are the same as elsewhere in the UK: minimum 12-month ban for a first offence.
There is no fixed rule because the answer depends on how much you drank, your body weight, sex, whether you ate, and your individual metabolism. A rough guide: the body eliminates approximately 1 unit of alcohol per hour. If you had 4 pints of 4% beer (approximately 9 units) over an evening, you need approximately 9 hours after your last drink to be below the English/Welsh limit. For Scotland (50 mg/100 mL), allow even more time. The safest approach is to not drive at all after drinking or to wait until the following day.
In all 50 US states, the BAC limit for drivers aged 21 and over is 0.08% (80 mg per 100 mL of blood). Utah has a lower limit of 0.05%. For drivers under 21, zero tolerance laws apply in all states, typically meaning any BAC above 0.01-0.02% results in penalties. Commercial vehicle drivers (CDL holders) face a 0.04% limit nationwide. Penalties for DUI/DWI vary by state but typically include fines, license suspension, and possible jail time.
No. Coffee, water, food, exercise, and cold showers do not speed up alcohol metabolism. Your liver processes alcohol at a largely fixed rate regardless of external interventions. These measures may reduce some symptoms (coffee increases alertness, food slows alcohol absorption) but they do not lower BAC. The only reliable way to lower BAC is time. Drinking coffee while drunk just makes you a wide-awake drunk driver with the same impaired reaction time and judgement.
The UK Chief Medical Officers (CMOs) recommend that adults drink no more than 14 units of alcohol per week to keep health risks from alcohol low. This applies to both men and women. They recommend spreading consumption over 3 or more days and having several alcohol-free days each week. There is no safe level during pregnancy. The 14-unit guideline represents approximately 6 medium glasses of wine, 6 pints of 4% beer, or 14 single measures of spirits per week – less than many people assume.
Body weight affects BAC because alcohol distributes through body water, and larger people have more body water. A person weighing 80 kg will have a significantly lower BAC than a 60 kg person consuming identical amounts of alcohol. This is why the Widmark formula includes body weight as a key variable. However, body composition also matters: muscle contains more water than fat, so a muscular person will have a lower BAC than a person of the same weight with higher body fat percentage, all else being equal.
Women typically have a higher BAC than men for the same quantity of alcohol for two main reasons: first, women on average have a higher proportion of body fat relative to total weight, meaning less body water for alcohol to distribute into; second, women generally have lower levels of alcohol dehydrogenase (the enzyme that begins breaking down alcohol in the stomach), meaning more alcohol reaches the bloodstream. The Widmark r factor reflects this: 0.73 for men versus 0.66 for women. These are population averages and vary individually.