Drink-Drive / BAC Calculator
Estimate your blood alcohol concentration (BAC) and total alcohol units using a Widmark-style formula. This is an educational estimate only, not a measurement and not a driving decision tool.
Used only for the Widmark body-water distribution constant. It does not make the estimate personal to you – see the caveats below.
Used to estimate how much alcohol may have been broken down since you began drinking. If you drank very recently, your actual BAC could still be rising and may be higher than this estimate suggests.
Drinks consumed
Drink-by-drink breakdown
UK legal drink-drive limits (reference information)
| Nation | Blood alcohol limit |
|---|---|
| England, Wales & Northern Ireland | 80 mg per 100 ml of blood |
| Scotland | 50 mg per 100 ml of blood |
These figures are shown for reference only. This calculator does not compare your estimated BAC against these limits, calculate a personal margin, or suggest how much you could drink and remain under them, because no calculator can reliably do this for an individual. Only an evidential breath, blood or urine test administered by police can establish your actual legal status. A UK Government consultation on lowering the England & Wales limit closed in May 2026; no decision has yet been published, and the current limits remain in force – check GOV.UK for the current position.
How this estimate is calculated
This calculator uses a simplified version of the Widmark formula, a long-established method for estimating blood alcohol concentration:
BAC (permille) = alcohol consumed (g) / (body weight (kg) x distribution constant) – (0.15 x hours elapsed)
- Alcohol per unit: 8g of pure alcohol per UK unit (the official UK definition of one unit).
- Distribution constant: 0.68 for men and 0.55 for women – commonly cited average values representing the proportion of body weight that alcohol distributes into. Individual values vary with body composition.
- Elimination rate: a fixed average of 15 mg/100 ml per hour is used. Published research shows this can realistically range from around 10 to 20 mg/100 ml per hour depending on the person.
- The estimate assumes all alcohol has been fully absorbed. In reality, absorption typically continues for 30-90 minutes or longer after drinking, so BAC can still be rising during and shortly after drinking.
Because of these assumptions, the figure this calculator produces is a rough approximation and can differ meaningfully from a person’s actual BAC.
Why this cannot tell you if you’re fit or legal to drive
A calculated BAC estimate is not the same as a legal or medical measurement, for several reasons:
- Individual variation is large. Metabolism, liver function, medication, hormonal factors, genetics, and general health all affect how alcohol is absorbed and broken down, often by considerably more than this calculator can account for.
- Food, hydration and drinking pace matter. Eating before or during drinking slows absorption; drinking quickly raises BAC faster than the same amount spread over time. This calculator does not model these effects.
- Impairment can occur below the legal limit. Reaction times and judgement can be affected by alcohol well before someone reaches the legal limit in any part of the UK.
- Only an evidential test is legally conclusive. Your actual legal status while driving can only be established by a breath, blood or urine test administered by police using calibrated equipment – not by any online calculator.
If you have been drinking, the safest and only reliable choice is not to drive – arrange a lift, call a taxi, use public transport, or wait until the next day.
UK Drink-Drive / BAC Calculator
Our Drink-Drive Calculator is an educational tool that estimates blood alcohol concentration (BAC) and total alcohol units from the drinks you’ve had, using a simplified Widmark-style formula. It is designed to help you understand roughly how alcohol units and BAC are calculated – it is not a tool for deciding whether you are safe or legal to drive. The only way to be certain you are not putting yourself or others at risk is not to drink alcohol before driving.
What is BAC and how is it estimated?
Blood alcohol concentration (BAC) is the amount of alcohol present in a person’s blood, usually expressed in the UK as milligrams of alcohol per 100 millilitres of blood (mg/100ml). This calculator estimates BAC using the Widmark formula, which combines the amount of alcohol consumed, body weight, biological sex (via a distribution constant), and time elapsed since drinking began. Because the formula relies on population averages rather than your specific physiology, any BAC estimate – from this tool or any other – carries meaningful uncertainty and should never be treated as a precise reading.
How to use this calculator
- Select your biological sex – this affects the body-water distribution constant used in the formula.
- Enter your body weight in kilograms or stone.
- Enter how many hours have passed since you started drinking.
- Add each drink you’ve had – choose from common UK drink presets (pint of beer, glass of wine, spirit shot, etc.) or enter a custom volume and ABV, and set the quantity for each.
- Click Calculate Estimate to see your estimated BAC, total units, and a full drink-by-drink breakdown.
What is a UK alcohol unit?
In the UK, one alcohol unit is defined as 10ml, or 8 grams, of pure alcohol. Because different drinks have different strengths (ABV) and serving sizes, the number of units in a drink varies widely:
| Drink | Typical serving | Approx. units |
|---|---|---|
| Beer/lager (4% ABV) | 1 pint (568ml) | ~2.3 units |
| Wine (12% ABV) | Medium glass (175ml) | ~2.1 units |
| Wine (12% ABV) | Large glass (250ml) | ~3.0 units |
| Spirits (40% ABV) | Single shot (25ml) | 1.0 unit |
| Cider (4.5% ABV) | 1 pint (568ml) | ~2.6 units |
These figures are approximate and vary with the exact ABV and serving size of the specific drink.
UK legal drink-drive limits
The legal blood alcohol limits for driving in the UK are set separately for different nations:
| Nation | Blood alcohol limit |
|---|---|
| England, Wales & Northern Ireland | 80 mg per 100 ml of blood |
| Scotland | 50 mg per 100 ml of blood |
These limits are provided for reference only. This calculator does not compare your estimated BAC against them or tell you whether you are within the legal limit, because individual variation means no calculator can determine this reliably. A UK Government consultation on lowering the England & Wales limit closed in May 2026. No decision has yet been published, and the current limits remain in force. Always check GOV.UK for the current legal position.
Why estimates like this can’t tell you if you’re safe or legal to drive
- Everyone processes alcohol differently. Body composition, liver function, medication, general health, stress and genetics all affect absorption and elimination rates, sometimes substantially.
- Food and drinking pace change absorption speed. Drinking on an empty stomach, or drinking quickly, both raise BAC faster than this simplified model assumes.
- Impairment doesn’t wait for the legal limit. Reaction times, judgement and coordination can be affected by alcohol at levels below the legal limit in any part of the UK.
- Only a police-administered evidential test is legally conclusive. Breath, blood or urine tests using calibrated equipment are the only reliable way to establish BAC for legal purposes.
How long does alcohol take to leave your system?
On average, the body eliminates alcohol at a rate equivalent to roughly 10-20 mg/100ml of blood per hour, though this varies from person to person. As a very rough guide, it can take about an hour for an average adult to process a single unit of alcohol – but this varies enough between individuals that it should never be used to plan or time driving. The safest approach after drinking is to wait considerably longer than you think you need to, and to remember that alcohol consumed the evening before can still leave you over the limit the following morning.
Frequently Asked Questions
Can this calculator tell me if I’m safe or legal to drive?
No. This calculator provides a rough educational estimate of blood alcohol concentration only. It cannot confirm whether you are safe, legal or fit to drive. The only reliable way to avoid drink-driving is not to drink alcohol before driving.
What does this calculator estimate?
It estimates your total alcohol units and an approximate blood alcohol concentration (BAC) using a simplified Widmark-style formula, based on the drinks you enter, your body weight, biological sex and the time elapsed since you started drinking.
What are the UK drink-drive limits?
In England, Wales and Northern Ireland, the legal limit is 80 milligrams of alcohol per 100 millilitres of blood. In Scotland, the limit is lower, at 50 milligrams per 100 millilitres of blood. These figures are provided for reference only and are not compared against your personal estimate by this calculator.
How accurate is a BAC estimate?
BAC estimates based on formulas like Widmark’s are approximations, not measurements. They rely on population-average constants and cannot account for individual differences in metabolism, food intake, medication, drinking pace, and other factors. Actual BAC can differ meaningfully from any calculated estimate, in either direction.
What is a UK alcohol unit?
A UK alcohol unit is defined as 10 millilitres, or 8 grams, of pure alcohol. The number of units in a drink depends on its volume and alcohol by volume (ABV) percentage – for example, a pint of 4% beer contains roughly 2.3 units.
How long does alcohol take to leave your system?
On average, the body processes alcohol at a rate of roughly 10 to 20 milligrams per 100 millilitres of blood per hour, though this varies between individuals. Because this rate is so variable, it should never be used to plan or time driving after drinking.
Does food or water affect how quickly I sober up?
Eating before or during drinking can slow how quickly alcohol is absorbed into the bloodstream, and drinking water can help with hydration and general wellbeing, but neither food nor water speeds up how quickly the liver actually breaks down alcohol already in your system. Only time allows BAC to fall.
Is the drink-drive limit the same across the whole UK?
No. England, Wales and Northern Ireland currently share an 80 milligrams per 100 millilitres of blood limit, while Scotland has a lower limit of 50 milligrams per 100 millilitres of blood. A UK Government consultation on lowering the England and Wales limit closed in May 2026; no decision has yet been published, and the current limits remain in force.
What should I do if I’ve been drinking and need to travel?
Do not drive. Arrange an alternative such as a taxi, rideshare, public transport, a lift from a sober friend or family member, or simply wait until you have had a full night’s sleep and, ideally, longer. If you’re unsure, the safest choice is always to leave the car and travel another way.
Official UK guidance
- GOV.UK: The drink drive limit – official legal limits and penalties
- THINK! road safety campaign – official UK road safety guidance on drink-driving
- NHS: Alcohol advice – guidance on units, health risks and lower-risk drinking
- Drinkaware – independent alcohol education charity