Health Tool
BAC Calculator UK
Estimate your blood alcohol concentration from UK pints, glasses and measures, and see how long until you are back under the 80 mg limit in England, Wales and Northern Ireland and the 50 mg limit in Scotland. The second tab converts any drink into UK units and keeps a running session log on this device, so a refresh mid-evening does not lose the count.
Looking for the US version? Try our US BAC Calculator.
BAC Estimator
2.8 UK units per serving
How many servings of the selected drink
Weight in kilogrammes
Determines the body water ratio used in the Widmark formula
How to Use This BAC Calculator
Log or pick your drinks
Use the units tab to convert each drink and add it to the session log, or stay on the BAC tab and pick one of 20 UK presets with a serving count.
Describe yourself
Body weight in kilogrammes or stone, biological sex for the Widmark body-water factor, and how long ago the first drink was poured.
Read both limits
You get current and peak BAC in percent and in mg per 100 ml, the hours until you clear the 50 mg Scottish limit and the 80 mg limit elsewhere, and a half-hourly decay curve with both limits drawn on it.
Worked examples
Two Sessions, Step by Step
Three pints of 5% lager, 80 kg man, one hour in
A pint is 568 ml, of which 5% is 28.4 ml of ethanol — that is 2.8 units per pint and 8.5 units for three, over half the weekly guideline in one evening. By weight, 28.4 ml at 0.789 g/ml is 22.4 g, so three pints is 67.2 g.
An 80 kg body is 80,000 g and the male Widmark factor of 0.68 gives 54,400 g of distribution volume, so peak BAC is 67.2 / 54,400 x 100 = 0.124%, or 124 mg per 100 ml. One hour of elimination leaves 0.109% — still 109 mg, well over both limits, with 1h 54min to go before 80 mg and 3h 54min before 50 mg.
Two large glasses of wine, 65 kg woman, two hours in
A 250 ml glass at 13.5% is 3.4 units, so two glasses is 6.8 units — already half the weekly guideline. In grammes that is 250 x 0.135 x 0.789 = 26.6 g each, so 53.3 g in total.
A 65 kg body times the female factor of 0.55 gives 35,750 g, so peak BAC is 53.3 / 35,750 x 100 = 0.149%. Two hours of elimination leaves 0.119%. The identical drinks in an 80 kg man would peak near 0.098% — the same bottle, a third less blood alcohol.
Statutory thresholds
UK Drink Driving Limits
The UK does not have one drink drive limit. Scotland cut its threshold from 80 mg to 50 mg of alcohol per 100 ml of blood on 5 December 2014, so a reading that is legal in Carlisle is an offence forty miles north in Gretna. Both figures are absolute: at or above them the reading alone establishes the offence, with no need to prove that your driving was affected. Below them you can still be prosecuted for driving while unfit through drink, which has no numerical threshold at all.
England, Wales & Northern Ireland
The drink drive limit is 80 milligrammes of alcohol per 100 millilitres of blood (0.08% BAC), 35 microgrammes per 100 millilitres of breath, or 107 milligrammes per 100 millilitres of urine.
England, Wales, Northern Ireland
Scotland
Scotland reduced its drink driving limit in December 2014 to 50 milligrammes of alcohol per 100 millilitres of blood (0.05% BAC), 22 microgrammes per 100 millilitres of breath, or 67 milligrammes per 100 millilitres of urine.
Scotland
New & Young Drivers
There is no separate statutory limit for new or young drivers in the UK, but a ban of six months or more within two years of passing revokes a new licence entirely. THINK!, the Department for Transport road safety campaign, advises no alcohol at all before driving.
All UK jurisdictions (advisory)
Commercial & Professional Drivers
Professional drivers (HGV, PSV, taxi) are subject to the same statutory limits, but employers and licensing authorities routinely impose stricter contractual policies. Many operators require zero alcohol.
All UK jurisdictions
Consequences
Drink Driving Penalties
Driving or attempting to drive while over the limit carries up to six months imprisonment, an unlimited fine and a driving ban of at least one year. A second conviction within ten years raises the minimum ban to three years. Being in charge of a vehicle while over the limit — which includes sleeping in the driver seat with the keys — carries up to three months, a fine of up to 2,500 pounds and a possible ban.
Refusing to provide a specimen of breath, blood or urine is punished on the same scale as driving over the limit, so declining the test is not a way out. The most serious offence in this family, causing death by careless driving while under the influence, carries a maximum of fourteen years imprisonment, an unlimited fine, a minimum five-year ban and a mandatory extended retest.
The financial tail outlasts the sentence. A drink driving endorsement stays on your licence for eleven years from the date of conviction, insurers price against it for the whole of that period, and some employers treat it as a bar to any role that involves driving. All of the figures in this section come from the gov.uk drink driving penalties guidance linked in the sources below.
Reference
UK Alcohol Units Guide
A unit is 10 ml, or 8 g, of pure alcohol, and the arithmetic is volume in millilitres times ABV divided by 1,000. The UK Chief Medical Officers advise no more than 14 units a week for both men and women, spread over three or more days, with several drink-free days. That guidance dates from 2016, when the previous separate male allowance of 21 units was withdrawn after the evidence on cancer risk was reviewed.
Note how little of the weekly allowance a normal evening leaves. Four pints of 4% lager is 9.1 units, roughly two thirds of the week, and a single bottle of 12% wine is 9 units on its own. The 14-unit figure is a weekly ceiling for lower-risk drinking, not a target and not a per-session allowance.
What the number means
Impairment at Each BAC Band
Impairment is continuous rather than a switch that flips at 50 or 80 mg. Measurable degradation of divided attention and steering control appears from about 20 mg, well below either statutory limit, which is the evidence base behind the Scottish reduction and behind the THINK! advice to drink nothing at all before driving. Tolerance changes how impaired you feel, not how impaired you are.
Sober / Minimal Effect
Little to no impairment. Normal behaviour expected.
Mild Impairment
Relaxation, slight euphoria. Judgement and inhibitions begin to decrease. Reaction time may slow. Below the Scottish limit, but not by much.
Moderate Impairment
Reduced co-ordination, impaired peripheral vision and reaction time. Over the Scottish limit of 50 mg, approaching the 80 mg limit in England, Wales and Northern Ireland.
Significant Impairment
Poor muscle control, possible nausea, major loss of judgement and self-control. Over the legal limit in every UK jurisdiction. Do not drive under any circumstances.
Dangerous Level
Risk of loss of consciousness, memory blackout, respiratory depression, and alcohol poisoning. Seek medical attention immediately.
Methodology
Widmark Formula Details
BAC = (A / (W x r)) x 100 - (0.015 x t)Each drink is first converted into grammes of pure alcohol:
A (grammes) = volume (ml) x ABV x 0.789 (g/ml density)Then the Widmark variables are:
- A = total grammes of pure alcohol consumed
- W = body weight in grammes (kg x 1000)
- r = Widmark factor: 0.68 for males, 0.55 for females
- t = hours since the first drink
- 0.015 = average metabolic elimination rate, in BAC percentage points per hour
To convert between the two ways the UK talks about this: a BAC of 0.08% is 80 mg of alcohol per 100 ml of blood, so multiplying the percentage by 1,000 gives the milligramme figure the law uses. Breath readings are roughly 2,300 times lower than blood readings, which is why 80 mg of blood alcohol corresponds to 35 microgrammes of breath alcohol.
The Widmark factor is a body-water fraction, not a fudge factor. Alcohol distributes into water rather than fat, and the average male body is about 68% water by weight against about 55% for females, which is why the same drinks give a woman a higher reading than a man of the same weight. The 0.015 elimination rate is a population average; individuals run from roughly 0.010 to 0.020 percentage points per hour.
Known limitations
What This Estimate Cannot See
Absorption. There is no absorption phase in the model: every gramme is credited to your bloodstream at hour zero. Real absorption takes 30 to 90 minutes and is slower after a meal, so the first hour of the curve reads high and the real peak is later and flatter.
Body composition. Weight is a poor proxy for body water. Two people at 80 kg with different muscle-to-fat ratios can differ by 15% or more in distribution volume, and that error passes straight into the BAC figure. Published forensic r values range from about 0.60 to 0.75 for men and 0.50 to 0.62 for women.
Pub measures. The presets assume nominal servings. A generous free pour of wine at home is routinely 250 ml rather than 175, cask ales vary by a full percentage point of ABV between batches, and a double-and-mixer ordered without thinking is 2 units rather than 1.
Medication and health. Liver disease, some antibiotics, antihistamines, opioids and benzodiazepines all interact with alcohol, either by changing metabolism or by adding their own impairment on top of it. You can be unfit to drive at a reading this page would call mild.
Frequently Asked Questions
What is the drink driving limit in the UK?
In England, Wales and Northern Ireland the limit is 80 milligrammes of alcohol per 100 millilitres of blood, which is 35 microgrammes per 100 millilitres of breath or 107 milligrammes per 100 millilitres of urine. Scotland has run a lower limit since 5 December 2014: 50 milligrammes of blood alcohol, 22 microgrammes of breath alcohol, 67 milligrammes of urine alcohol. There is no safe amount that guarantees you stay under either figure, because how much a given drink raises your reading depends on your body water, and THINK! advises none before driving.
What is a UK alcohol unit?
One UK unit is 10 millilitres, or 8 grammes, of pure alcohol. Units are volume multiplied by strength divided by 1,000, so a pint of 4% lager is 568 x 4 / 1000 = 2.3 units and a 175 ml glass of 12% wine is 2.1 units. A single 25 ml measure of 40% spirits is exactly 1 unit, which is where the system anchors. The UK Chief Medical Officers advise no more than 14 units a week for both men and women, spread over three or more days.
Can I drive the morning after drinking?
Often not, and it is the single most common way otherwise careful drivers get convicted. Your body clears roughly 0.015 BAC percentage points an hour, about one unit an hour for an average-sized man, and the clock starts when you stopped drinking rather than when you went to bed. Ten units, a bottle of wine plus a pint, finished at midnight puts a 65 kg woman at roughly 0.22% (220 mg): she is still over the 80 mg limit until about 09:20 and over the 50 mg Scottish limit until about 11:20. Nothing accelerates it: not coffee, not a shower, not breakfast.
Does this calculator account for absorption time?
No. The Widmark model credits your bloodstream with every gramme of alcohol at hour zero, whereas real absorption from the stomach and small intestine takes roughly 30 to 90 minutes and is slower on a full stomach. The practical consequence is that the first hour of the projected curve reads high and your genuine peak arrives later and flatter than the chart shows. From about 90 minutes after your last drink the elimination line tracks reality much more closely.
What are the penalties for drink driving in the UK?
Driving or attempting to drive while over the limit carries up to 6 months imprisonment, an unlimited fine and a driving ban of at least one year, rising to three years if you are convicted twice in ten years. Being in charge of a vehicle while over the limit carries up to 3 months, a fine of up to 2,500 pounds and a possible ban. Refusing to give a specimen is punished as heavily as driving over the limit. Causing death by careless driving when under the influence carries a maximum of 14 years, a minimum five-year ban and a mandatory extended retest. The endorsement stays on your licence for 11 years from the date of conviction.
How accurate is a BAC calculator?
It is an estimate with a wide error band, not a measurement, and no court would accept it. The Widmark r factor varies by roughly 15% between people of the same weight because body composition differs, and the elimination rate ranges from about 0.010 to 0.020 percentage points per hour with genetics, liver health and drinking history. Food, medication, hydration and drinking pace move it further. Only a calibrated evidential breath test or a blood sample gives a real figure.
How many units are in a bottle of wine?
A 750 ml bottle at 12% ABV is 9 units, which is close to two thirds of the 14-unit weekly guideline in one bottle. At 14% ABV the same bottle is 10.5 units. Pub and restaurant glasses have grown alongside the strength: a large 250 ml glass of 13.5% wine is 3.4 units, more than a pint of 4% lager, and three of them exceed the weekly guideline on their own. The session log on this page tracks the running total and the share of the 14-unit guideline it represents.
Authoritative Sources
- gov.uk - The drink drive limit
Source for the 80 mg blood / 35 microgramme breath / 107 mg urine limits in England, Wales and Northern Ireland, and the 50 / 22 / 67 figures in Scotland.
- gov.uk - Drink driving penalties
Source for every penalty figure on this page: the 6-month and 3-month maxima, the 1-year and 3-year bans, the 14-year maximum for causing death, and the 11-year endorsement.
- The Road Traffic Act 1988 (Prescribed Limit) (Scotland) Regulations 2014
The statutory instrument that cut the Scottish limit to 50 mg with effect from 5 December 2014.
- NHS - Calculating alcohol units
Source for the 1 unit = 10 ml (8 g) definition, the volume x ABV / 1000 formula, and the published unit values for pints, glasses and measures.
- UK Chief Medical Officers - Low Risk Drinking Guidelines
The 2016 guideline setting 14 units a week for both sexes, spread over three or more days.
- THINK! - Drink driving
Department for Transport road safety campaign; the basis for the none-before-driving advice rather than a per-drink rule of thumb.
Disclaimer
Not medical or legal advice, and not evidence of sobriety. This page runs a population-average model on the numbers you type in; it cannot see your stomach contents, your medication, your liver or how fast you drank, and the plausible error on any single estimate is wide enough to cross 50 mg or 80 mg in either direction. Only a calibrated evidential breath test or a blood sample gives a figure that means anything. If you have been drinking, do not drive. All calculations run locally in your browser and the session log stays on this device; nothing is uploaded.
Looking for the US Version?
Our US BAC Calculator uses US standard drinks of 14 g, fluid ounces, and the 0.08% per-se limit that applies in 49 states and D.C. This UK version uses millilitres, the 10 ml UK unit, and the separate 80 mg and 50 mg drink-drive limits.
Published by Kalcify · Last updated
Calculators are built against primary sources — government tax authorities (IRS, HMRC, CRA, ATO) for finance and the World Health Organization for health metrics. Updated when rates or rules change. View methodology and data sources.
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