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Free BAC calculator: how it works (Widmark explained)

How a blood alcohol (BAC) calculator works: the Widmark formula step by step, a worked UK example in mg per 100 ml, and the limits of any online estimate.

The ConsomAction team Published 5 min read
In this article

Free BAC calculator: how it works (Widmark explained)

Short answer: a BAC calculator estimates the concentration of alcohol in your blood from four things: how much pure alcohol you drank, your weight, your sex and the time since you started. Almost all of them, including ConsomAction’s, use the Widmark formula, developed in the 1930s and still used in forensic science. It gives a useful estimate of your peak and of when you will be sober, but not a legal guarantee: your real level can be 10 to 20 mg higher or lower. Never use it to aim just under the limit.

What BAC means, and how the UK measures it

BAC stands for blood alcohol concentration. Different countries express it differently, which is a common source of confusion:

  • UK and Ireland: milligrams of alcohol per 100 ml of blood. The limit is 80 mg in England, Wales and Northern Ireland, 50 mg in Scotland and Ireland, and 20 mg for learner, novice and professional drivers in Ireland (GOV.UK, Citizens Information).
  • United States: a percentage. 0.08% is the same as 80 mg per 100 ml.
  • Much of Europe: grams per litre. 0.5 g/L is the same as 50 mg per 100 ml.

Roadside tests in the UK usually measure your breath, with matching limits (35 µg per 100 ml of breath in England, 22 µg in Scotland).

The Widmark formula in plain English

Swedish scientist Erik Widmark described the relationship between the alcohol you drink and the level in your blood. In simple terms:

Peak BAC = alcohol drunk ÷ (body weight × r)

  • Alcohol drunk is in grams. One UK unit is 8 g of pure alcohol (NHS).
  • Body weight is in kilograms.
  • r is the share of your body in which alcohol spreads, mostly water: around 0.7 for men and 0.6 for women on average. That is why the same drink gives women a higher level.

Then the level falls over time:

BAC after t hours = peak BAC − (elimination rate × t)

The elimination rate is roughly 0.15 g per litre per hour on average (15 mg per 100 ml per hour), and can be as low as 0.10 for slow eliminators. It is fixed: coffee, water, food after drinking or sleep do not change it.

A worked example

An 80 kg man drinks 3 pints of 4% lager between 7pm and 10pm.

  1. Alcohol: 3 × 2.3 units = 6.8 units, or about 54 g of alcohol.
  2. Peak if drunk all at once: 54 ÷ (80 × 0.7) = 0.97 g/L, or 97 mg per 100 ml.
  3. Elimination during the evening: by 10:30pm, 3.5 hours after starting, he has cleared about 52 mg. Real level around 45 mg.
  4. Back to zero: about 97 ÷ 15 = 6.5 hours after starting, plus absorption, so around 1:30am to 2am.

With a slow elimination rate, the same evening could leave him with alcohol in his blood until almost 5am. For a 65 kg woman drinking the same three pints, the peak would be far higher and zero would come several hours later.

Our tables on how long to wait before driving and how many pints you can drink and still drive apply the same method to common situations.

Why BAC calculators disagree

If you try several calculators, you will get different answers. The reasons:

  • Different r values: some use a single value for everyone, others adjust for height and age.
  • Different elimination rates: 0.10, 0.15 or 0.20 g/L per hour change the “sober” time by hours.
  • Absorption: some assume every drink is absorbed instantly, others spread it out over 30 to 60 minutes.
  • Drink data: a “glass of wine” can be 125 ml at 11% or 250 ml at 14.5%, almost three times more alcohol.

The last point matters most. The best formula in the world gives the wrong answer if the drinks are wrong. That is why it helps to pick drinks from a database with real volumes and strengths, like our UK units table or the ConsomAction drinks list.

What no calculator can tell you

A BAC calculator is an estimate, not a breathalyser. It cannot know:

  • whether you ate, and what;
  • how fast you really drank;
  • how your own liver works that day, or whether medication, illness or tiredness change things;
  • whether a measure was really a single.

It also cannot tell you whether you are fit to drive. Reaction times and judgement are affected below any legal limit. Use a calculator to understand how much you have drunk and how long it takes to clear, not to decide that you are “just under”.

How the ConsomAction calculator works

The ConsomAction calculator applies the Widmark formula to your weight, sex and the drinks you actually log, chosen from more than 240 drinks with their real volume and strength. It shows your full blood alcohol curve over time and the time you will be sober. It is free and works without an account.

FAQ

Is there a free BAC calculator for the UK?

Yes. ConsomAction is free and does not require an account. Enter your weight, sex and drinks, and it estimates your blood alcohol curve and the time you will be back to zero.

How accurate are BAC calculators?

They give a reasonable estimate for an average person, but real levels can differ by 10 to 20 mg per 100 ml or more, depending on food, metabolism and how accurately the drinks are entered. They are not a substitute for a breath test.

What is 0.08 BAC in UK terms?

0.08% is 80 mg of alcohol per 100 ml of blood, the legal limit in England, Wales and Northern Ireland. The Scottish and Irish limit of 50 mg corresponds to 0.05%.

Can I use a BAC calculator to decide whether to drive?

It can tell you that you are clearly not ready. It should never be used to decide that you are “just under”. If there is any doubt, do not drive.

Further reading

Sources