BAC Calculator Explained: How Blood Alcohol Content Is Calculated

What is blood alcohol content?

Blood alcohol content (BAC), sometimes called blood alcohol concentration, is the share of your bloodstream that is pure ethanol at a given moment. It is reported as a percentage of grams of ethanol per 100 millilitres of blood — 0.08% means 0.08 grams of ethanol per 100 mL, or 80 milligrams per decilitre. Forensic toxicologists, traffic officers and medical examiners use the same unit, which is why it appears on breathalyser readouts and in the legal limits printed in every drink-driving statute. The BAC calculator on this site estimates that number from four inputs: body weight, biological sex, the number of standard drinks consumed, and the time elapsed since the first drink.

BAC matters because it is the variable that nearly every impairment guideline is written in terms of. The US federal per se driving limit is 0.08% in all 50 states. England, Wales and Northern Ireland share that 0.08% threshold; Scotland lowered its limit to 0.05% in 2014, in line with most of continental Europe. Some countries (Czech Republic, Hungary, Slovakia, Romania, and others) operate a 0.00% limit for drivers. Below those numbers, measurable impairment in reaction time, depth perception and decision-making starts much earlier — the US National Highway Traffic Safety Administration cites detectable effects from around 0.02%, well below any legal threshold. BAC is not a binary safe/unsafe number; it is a continuous risk scale that the law happens to cut at one or two convenient points.

The single most important caveat is that any BAC calculator, including this one, produces a population-average estimate. Your actual BAC at any moment depends on physiology no formula can capture without a blood draw. Use the BAC calculator to understand how the math works and to compare scenarios, not as permission to drive after a particular number of drinks. The only reliably safe BAC for operating a vehicle is zero.

How BAC is calculated

The standard model is the Widmark formula, first published by Swedish physician-chemist Erik Widmark in 1932 and still the workhorse of forensic alcohol calculations almost a century later. The formula has two parts: a peak BAC term based on how much alcohol entered the body and how it distributes, and an elimination term that subtracts the steady rate at which the liver clears alcohol over time.

BAC (%) = (A × 100) / (BW × r) − (β × t)

Each symbol is a measurable quantity:

• A — grams of pure ethanol consumed. One US standard drink (the National Institute on Alcohol Abuse and Alcoholism definition) is 14 grams, equivalent to a 12 fl oz can of 5% beer, a 5 fl oz glass of 12% wine, or a 1.5 fl oz shot of 40% spirits.
• BW — body weight in grams. A 70 kg adult is 70,000 g in the formula. Many textbook presentations show BW in pounds with a different constant; the math is identical once units are tracked carefully.
• r — the Widmark distribution factor, dimensionless. It is the fraction of body mass available for alcohol to dissolve into — essentially the share of body water. The population averages are 0.68 for males and 0.55 for females; modern refinements (Watson, Forrest and Seidl) tune r individually using height, age and weight, but the classic factors are within a few per cent for most adults.
• β — the elimination rate, 0.015 g/100 mL per hour as a population mean. Individual rates range from roughly 0.010 to 0.020. Alcohol is eliminated by zero-order kinetics at typical doses, which means the rate is constant rather than proportional to concentration — the liver is running flat out, not paced by how much is left.
• t — hours since the first drink. Time runs from when ethanol first entered the bloodstream, not when the most recent drink was finished.

Read in plain English, the formula says: take all the ethanol consumed, dilute it into the share of the body it can actually reach, convert to grams per 100 mL of blood, then subtract the steady clearance the liver has done in the time since. The BAC calculator runs that arithmetic for you and reports both the current estimate and the hours until BAC reaches zero.

Two simplifying assumptions are baked in. First, ethanol is treated as instantly absorbed at t = 0 — in reality absorption takes 30 to 90 minutes, with the curve peaking and then declining, which is why a Widmark estimate can slightly overshoot in the first hour and slightly undershoot as the curve rises. Second, the elimination rate is held constant regardless of how much alcohol is present, which is accurate at typical drinking concentrations but breaks down at the very high end (above ~0.4%) where saturated liver enzymes can no longer maintain the linear rate.

Worked example

Take a 70 kg adult male who has had two US standard drinks and is sitting at the bar one hour after the first sip. Plugging into Widmark:

1. Ethanol consumed: 2 drinks × 14 g = 28 g.
2. Peak BAC (at t = 0): (28 × 100) / (70,000 × 0.68) = 2,800 / 47,600 = 0.0588%.
3. Elimination over 1 hour: 0.015 × 1 = 0.015%.
4. Current BAC: 0.0588 − 0.015 = 0.044%.
5. Hours until zero: 0.044 / 0.015 = 2.93 hours.

The result — 0.044% — sits well below the 0.08% US federal driving threshold, but it is still measurable impairment. NHTSA flags reduced visual tracking, lower coordination, and difficulty with divided-attention tasks at 0.05%, and detectable effects on multitasking and decision-making as low as 0.02%. The 0.08% line is not the edge of safety; it is the edge of legality. Drop the same person on a 0.05% jurisdiction’s roads — Scotland, most of the EU, Australia, much of Latin America — and they are within 0.006% of an arrest.

Re-run the same numbers for a 60 kg female with the same two drinks and one hour elapsed: peak BAC = (28 × 100) / (60,000 × 0.55) = 0.085%, minus 0.015 = 0.070%. Almost the same dose, but the smaller female body and the lower distribution factor combine to push BAC just under the 0.08% threshold — and well above the 0.05% Scottish or European limit. Try other combinations in the BAC calculator to see how each input shifts the result.

Factors that affect BAC

Body weight and composition

Alcohol dissolves in body water, not in fat. A heavier person has more body water to dilute the same dose into, which is why the same number of drinks produces a lower peak BAC in a 100 kg adult than in a 60 kg one. Composition matters too: at the same body weight, a leaner individual carries more water than a less lean one, so two adults of equal mass can land at different BAC numbers from the same drinks. The Widmark r factor folds this into a single coefficient, which is why it is a population average rather than an individual prediction. The lean body mass calculator gives a rough sense of where you sit on the composition axis.

Biological sex

On average, women carry a higher proportion of body fat and a lower proportion of total body water than men of the same weight. The Widmark factor reflects that: 0.68 for males, 0.55 for females. Two consequences follow. First, the same dose lifts a female BAC higher than a male BAC at equal body weight. Second, women on average have lower gastric alcohol dehydrogenase activity, which means more of an oral dose reaches the bloodstream rather than being broken down in the stomach lining. Both effects compound, which is why drink-driving guidance often presents women’s thresholds in a separate column. These are population averages; individuals vary substantially and the formula cannot capture that.

Food in the stomach

Food slows gastric emptying, which delays the alcohol from reaching the small intestine where most absorption happens. The same dose taken with a substantial meal can produce a peak BAC 30 to 50 per cent lower than the same dose on an empty stomach, with the peak shifted later by an hour or more. The Widmark formula does not model this directly — it assumes uniform absorption — so a calculator result for someone who has just eaten a steak dinner should be read as a slight overestimate near the start, while the same result for someone drinking on an empty stomach during a happy hour is more likely to be a slight underestimate of the peak.

Drinking speed

Four drinks spread across four hours produces a very different trajectory than four drinks in the first hour. The total ethanol is the same, but in the second case absorption outpaces elimination dramatically, the peak is much higher, and the dwell time above any given threshold is longer. The BAC calculator averages across the time elapsed, so very fast drinking will tend to push true peak BAC above the figure shown; very slow drinking pulls it below. Pacing is one of the few controllable variables.

Genetics and medication

A common variant of the ALDH2 gene, present in roughly 40 per cent of people of East Asian descent, produces a much less active form of the enzyme that breaks down acetaldehyde — the toxic intermediate between ethanol and harmless end-products. Carriers feel flushing, nausea and rapid heart rate from small amounts of alcohol and face substantially higher long-term cancer risk at any given intake. Medications matter too: many sedatives, opioids, antihistamines, sleep aids and certain antidepressants amplify the central-nervous-system effects of alcohol independent of BAC. The number on the readout is the same; the functional impairment is much worse.

How to interpret your BAC reading

• Treat any positive reading as impaired. NHTSA logs measurable reductions in tracking and divided-attention performance from 0.02%. Legal limits are political compromises, not safety guarantees.
• Add an honest error margin. The Widmark formula returns a single number, but the underlying population variation is wide. An estimate of 0.05% might plausibly correspond to a true value anywhere from 0.035% to 0.065%, especially for fast drinking on an empty stomach or for body compositions far from the population mean.
• Check the jurisdiction you are in. The calculator flags the 0.08% US federal threshold, but half the developed world drives on 0.05% or lower. Scotland, France, Germany, Italy, Spain, Australia, Japan and many others are all at or below 0.05%. A reading that is “legal” in Texas is an arrest in Edinburgh.
• Read the hours-to-zero figure carefully. The estimate of when BAC will reach 0.00% assumes a population-average 0.015% per hour. Slow metabolisers can take 30 to 50 per cent longer. The morning after a heavy session is often the highest-risk drive of the week because residual BAC is invisible and the driver feels sober.
• Do not chase a target BAC. Drinking up to a calculator-predicted 0.07% in a 0.08% jurisdiction assumes a single-digit error margin the formula does not deliver. Plan around not driving, not around the estimated number.

Common mistakes

Conflating “a drink” with one ethanol unit

A craft IPA at 7.5% in a 16 fl oz pint glass is roughly 2.4 US standard drinks — not one. A double whisky is two drinks, not one. A large home pour of wine is often a drink and a half. Counting “drinks consumed” as glasses or rounds rather than 14 g ethanol equivalents is the single most common source of low estimates. If you drink mostly in pints or large wine pours, multiply your count.

Forgetting time of first drink, not last

The Widmark formula counts t from the first sip, because that is when elimination begins. People often enter time since the last drink instead, which under-counts elimination at the start of the session and over-counts absorption at the end. Anchor t to whichever drink reached your bloodstream earliest.

Assuming “sobering up” is faster than it is

Cold showers, coffee, food after the fact, and exercise do not change the elimination rate. The liver clears ethanol at roughly 0.015% per hour and nothing else moves that number meaningfully in the short term. A 0.10% reading at midnight is still 0.04% at 4 am no matter what you eat or drink in between. The only treatment for elevated BAC is time.

Trusting a breathalyser only on the way home

Personal breathalysers vary widely in accuracy — a cheap model can be 30 to 50 per cent off, and even a well-calibrated unit reflects alcohol in deep lung air, which lags blood concentration in both directions. Police evidential units cost thousands and are calibrated to a traceable standard. If your decision hinges on a 0.07 versus 0.08 reading, the decision is already wrong.

When to seek professional advice

The arithmetic of BAC is the easy part. The decisions it prompts are not. Contact a clinician or a service such as SAMHSA’s National Helpline (1-800-662-HELP in the US), Drinkline (0300 123 1110 in the UK), or your local equivalent if any of the following apply:

• You are using the calculator to decide whether you can drive after drinking. The answer is no — not at any non-zero BAC, regardless of the legal threshold. Plan transport before you start.
• You notice withdrawal symptoms (tremor, sweating, anxiety, racing heart) on days you have not been drinking. Alcohol withdrawal can be medically dangerous and benefits from clinical supervision.
• You drink to manage anxiety, low mood, sleep, or physical pain. The unit count is the symptom, not the underlying issue.
• You take prescribed medication. Many common drugs — paracetamol, sedatives, antidepressants, blood thinners, some antibiotics — interact with alcohol in ways the BAC number alone does not capture.
• You are pregnant or trying to conceive. Major health authorities (CDC, NHS, WHO) all advise no alcohol at all in pregnancy; the BAC formula on this page does not apply in that context.

Frequently asked questions

What is a normal BAC? Sober is 0.00%. Anything above that is alcohol in the bloodstream, with functional effects detectable from around 0.02%. Legal per se limits range from 0.00% (some Eastern European countries) through 0.05% (Scotland, most of the EU, Australia, Japan) to 0.08% (US, England/Wales/NI, Canada). Levels above 0.30% are typically associated with severe intoxication and risk of unconsciousness; 0.40% and above is potentially fatal.

How is BAC measured in practice? Three ways. Direct blood draw is the gold standard but invasive. Breath alcohol measured by an evidential breathalyser is the common roadside test, validated against blood at a fixed ratio (the Henry’s Law partition coefficient, around 2,100:1). Urine alcohol is used in some workplace and clinical contexts but lags blood by 30 to 90 minutes and is less precise. The BAC calculator models the blood number directly.

Why does the same number of drinks produce different BACs for different people? Three main reasons: total body water (which scales with weight and leanness), biological sex (which sets the Widmark distribution factor and gastric ADH activity), and individual elimination rate. A 90 kg man and a 55 kg woman drinking identical quantities can land an order of magnitude apart on BAC. Body composition, age and genetic variation in alcohol-metabolising enzymes add further spread.

How fast does BAC drop? The population mean elimination rate is 0.015 g/100 mL per hour, with a normal range of roughly 0.010 to 0.020. That works out to roughly one US standard drink’s worth of BAC per hour for an average adult. A reading of 0.10% takes around 6.5 hours to reach zero at the population-mean rate, plus the absorption time still in progress from the most recent drink. The calculator’s hours-to-zero figure is built on the 0.015% rate.

Does coffee, cold water or exercise lower BAC? No. None of them change the rate at which the liver metabolises ethanol. Caffeine can mask the feeling of intoxication, which is dangerous because it leaves impairment intact while removing the cues that would otherwise discourage driving. The only intervention that lowers BAC is time.

Is a calculator estimate good enough for legal purposes? No. A Widmark estimate is a teaching tool. Legal BAC determinations rely on evidential breath or blood samples taken by trained officers using calibrated equipment with documented chain of custody. The numbers from this page will not stand up in court in either direction — never as a defence and never as an accusation.

How accurate is the Widmark formula? For typical drinking doses in average adults, Widmark estimates fall within roughly ±0.02% of the true value when inputs are honest. Accuracy improves for body compositions close to the population mean, for moderate doses, and for time points well past the absorption phase. It degrades for very small or very large individuals, for very fast drinking, for full-stomach absorption, and for atypical metabolisers. Treat the result as a single point in a range, not a single precise number.

Why does eating before drinking lower peak BAC? Food slows gastric emptying, which spreads absorption over a longer time window. The same total dose produces a lower, later peak because elimination is chipping away at it from below while absorption rises more slowly from above. Solid, fat- and protein-rich meals slow emptying most; light or liquid meals less so. Eating after drinking does nothing because absorption has already largely happened.

Related calculators

Use these alongside the BAC calculator to put alcohol intake in the context of body composition, daily energy balance and overall health.

• Alcohol units calculator — converts any drink into UK alcohol units (8 g of ethanol each) and compares the total against the 14-unit weekly low-risk ceiling. Pairs naturally with the BAC calculator for anyone counting in UK units.
• BMR calculator — basal metabolic rate via Mifflin–St Jeor. Alcohol metabolism contributes a small share of daily energy expenditure but adds about 7 kcal per gram of ethanol to dietary intake.
• Lean body mass calculator — estimates the fat-free portion of body weight, which is the volume alcohol actually dissolves into. The same total weight at higher leanness lowers peak BAC for a given dose.
• Calorie calculator — maintenance, cut and bulk targets. Alcohol calories are routinely under-counted; a bottle of wine adds around 600 kcal before any food, and a single spirit-and-mixer can match a chocolate bar.
• Water intake calculator — ethanol is a diuretic, and adequate hydration is one of the few variables shown to modestly improve next-day recovery from alcohol consumption.