Every calculation is grounded in peer-reviewed pharmacokinetics research.
The Widmark Formula: Where It All Starts
In 1932, Swedish scientist Erik Widmark published the foundational equation for estimating blood alcohol concentration (BAC) from the amount of alcohol you drink. His insight was simple but powerful: if you know how much alcohol went in, and how it spreads through your body, you can estimate how much ends up in your blood.
Classic Widmark Equation
BAC = (alcohol_grams) / (body_weight x r) - elimination_rate x time
Here, r is the Widmark distribution factor (how much of your body the alcohol actually reaches), and the elimination rate is how fast your liver clears it. The formula has stood the test of time for nearly a century, though modern implementations improve on it substantially.
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Alcometer does not use the static Widmark equation directly. Instead, it runs a minute-by-minute dynamic simulation that replaces Widmark's simplifying assumptions with evidence-based models for absorption, distribution, and elimination. Think of it as Widmark's core idea, upgraded with 90 years of additional research.
Every drink you enter is converted to grams of pure ethanol using a simple formula:
Alcohol Mass Calculation
alcohol_grams = volume_ml x (ABV / 100) x 0.789
The constant 0.789 g/mL is the density of ethanol at room temperature -- a standard physical constant used in forensic BAC calculations worldwide.
Your Body Shape Matters: The Distribution Factor
Not all of your body absorbs alcohol equally. Bones and fat tissue contain little water and barely absorb any alcohol, while muscle tissue and organs are rich in water and absorb it readily. The distribution factor r captures this: a lower r means alcohol concentrates in a smaller volume, giving you a higher BAC from the same number of drinks.
This is why body composition matters so much. Rather than using a single average value, Alcometer adjusts your r based on your BMI, using the Forrest (1986) interpolation table as compiled by Maskell et al. (2015). [Forrest, 1986][Maskell et al., 2015]
Male distribution factor (r) by BMI
BMI
r value
17.9
0.80
21.9
0.75
24.7
0.72
27.2
0.69
29.6
0.66
Female distribution factor (r) by BMI
BMI
r value
15.6
0.74
20.1
0.69
22.8
0.61
25.3
0.58
27.3
0.53
The inverse relationship tells the story: higher BMI means a lower r, which means less body water to dilute the alcohol, which means higher BAC. Women generally have lower r values than men at the same BMI because of differences in body fat distribution.
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When height isn't provided, the calculator falls back to population averages: r = 0.72 for males and r = 0.61 for females. Adding your height gives you a more personalized -- and more accurate -- estimate.
How Your Body Absorbs Alcohol
The original Widmark formula assumes all alcohol is absorbed instantly -- as if your body processes an entire drink the moment you finish it. In reality, absorption follows a curve that depends on what you drank and whether you've eaten.
Alcometer models this using a logistic (S-shaped) curve for each drink. The key parameter is t50: the time at which 50% of the alcohol has been absorbed. Different beverages have different absorption speeds.
Absorption timing by beverage type (fasting)
Beverage
Time to 50% absorbed
Time to 95% absorbed
Spirits
36 min
75 min
Wine
54 min
95 min
Beer
62 min
105 min
These values come from Mitchell et al. (2014), who measured absorption in 15 healthy men using a crossover study design. Spirits produced the highest peak BAC, followed by wine, then beer -- even when total alcohol was held constant.
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Eating slows everything down by about 30 minutes, but Alcometer deliberately does not reduce the total amount of alcohol absorbed when you eat. This is a safety-conservative choice: underestimating your BAC could be dangerous, so we'd rather overestimate slightly than underestimate.
How Your Liver Clears Alcohol
Think of your liver as a factory that can only process about one drink per hour. Unlike most substances, alcohol is eliminated at a roughly constant rate (called zero-order kinetics) -- your liver works at full speed regardless of how much alcohol is in your blood.
Alcometer uses elimination rates from the largest systematic review on the topic (Jones, 2010):
Sex
Elimination Rate
Equivalent
Male
0.168 permille/h
~16.8 mg/dL/h
Female
0.190 permille/h
~19.0 mg/dL/h
Women actually eliminate alcohol slightly faster than men in absolute terms, likely due to a proportionally larger liver-to-body-weight ratio.
There is one important exception: when your BAC drops very low (below about 0.005%), your liver shifts from constant-rate processing to a slower, concentration-dependent mode. This is because the liver enzyme responsible (alcohol dehydrogenase) is no longer fully saturated at very low alcohol levels. Alcometer models this transition, which means the "last bit" of alcohol takes disproportionately longer to clear -- a detail most calculators miss.
Why We Show Three Lines, Not One
A single BAC number creates a false sense of precision. In reality, even with perfect inputs, two people of the same sex, weight, and height who drink identical amounts will end up with different BAC levels. Body composition, liver enzyme activity, hydration, and genetics all play a role.
Alcometer addresses this by running three parallel simulations for every scenario, using uncertainty coefficients from Maskell et al. (2015):
Band
Distribution factor (r)
Elimination rate
What it means
Optimistic
r x 1.092 (larger)
Faster by 22%
You metabolize quickly, favorable body composition
Typical
r x 1.0 (average)
Average rate
Most likely estimate for your demographics
Conservative
r x 0.908 (smaller)
Slower by 22%
You metabolize slowly, less favorable composition
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The three bands capture roughly the central 80% of population variation. About 10% of people will fall outside even the conservative band in either direction. This is why we always say these are estimates, not measurements.
Estimating return to a near-zero value
Alcometer runs the simulation forward in time, minute by minute, until your estimated BAC reaches zero or drops below a legal limit. It uses the same dynamic elimination model, including the low-BAC slowdown.
For safety, the "reference threshold timing" display uses the conservative band, deliberately erring on the side of caution. If you're wondering whether you're below the limit, you probably want the worst-case estimate -- not the optimistic one.
Important: Why these are estimates, not guarantees
The model projection assumes no additional drinks, no drug interactions, no medical conditions affecting metabolism, and that your actual BAC matches the model's estimate (which itself is uncertain). These projections should never be used to determine fitness to drive.
What the Model Cannot Account For
Like any model, the BAC calculator makes simplifying assumptions. Here are the most significant factors it does not capture:
Factor
Why it matters
Genetic enzyme variation
ADH/ALDH variants (common in East Asian populations) can dramatically alter metabolism speed
Drug interactions
Many medications affect gastric or hepatic alcohol processing
Very muscular people may have r values outside the modeled range
Chronic heavy drinking
Enzyme induction (CYP2E1) can push elimination rates to 25-35 mg/dL/h
Vomiting
If you vomit soon after drinking, actual absorption may be much lower
Carbonation
Carbonated mixers may speed gastric emptying and absorption
Age, altitude, menstrual cycle
All affect BAC but are not currently modeled
Regulatory and Legal Disclaimer
This calculator provides educational estimates only -- not legal, medical, or forensic determinations.
BAC estimation cannot replace a certified breathalyzer or blood test.
Legal BAC limits vary by jurisdiction, driver class, and vehicle type.
Any BAC above 0.0 indicates alcohol presence and potential impairment.
References
1.
Widmark EMP.Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung.Berlin: Urban & Schwarzenberg (1932). PubMed/PMC
2.
Maskell PD, De Paoli G, Seneviratne C, Pounder DJ.Alcohol calculations and their uncertainty.Medicine, Science and the Law (2015). PubMed/PMC
3.
Jones AW.Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework.Forensic Science International (2010). PubMed/PMC
4.
Hoiseth G, Wiik E, Kristoffersen L, Morland J.Ethanol elimination rates at low concentrations based on two consecutive blood samples.Forensic Science International (2016). PubMed/PMC
5.
Mitchell MC Jr, Teigen EL, Ramchandani VA.Absorption and peak blood alcohol concentration after drinking beer, wine, or spirits.Alcoholism: Clinical and Experimental Research (2014).[n=15]PubMed/PMC
6.
Ramchandani VA, Kwo PY, Li TK.Effect of food and food composition on alcohol elimination rates in healthy men and women.Journal of Clinical Pharmacology (2001). PubMed/PMC
7.
Jones AW, Hahn RG, Stalberg HP.Disposition of ethanol in blood and cerebrospinal fluid after oral administration.Journal of Studies on Alcohol (1991). PubMed/PMC
8.
Holford NHG.Clinical pharmacokinetics of ethanol.Clinical Pharmacokinetics (1987). PubMed/PMC
9.
Norberg A, Jones AW, Hahn RG, Gabrielsson JL.Role of variability in explaining ethanol pharmacokinetics.Clinical Pharmacokinetics (2003). PubMed/PMC
10.
Forrest ARW.The estimation of Widmark's factor.Journal of the Forensic Science Society (1986). PubMed/PMC
The Watson equation: a body-water alternative to Widmark r
The Widmark distribution factor r is the workhorse of BAC estimation, but it has a well-known structural weakness: it treats body-water fraction as a coefficient tied to sex and a crude BMI lookup, when what actually matters for ethanol dilution is the individual's total body water (TBW). In 1981 Watson, Watson and Batt published regression equations that estimate TBW directly from age, height, and weight, and the forensic literature has spent four decades demonstrating that TBW-based BAC estimates are more accurate than the static Widmark r in subjects who sit at either extreme of body composition.
A parallel equation for females replaces the coefficients with values fitted to female body composition. Once TBW is known, BAC at peak absorption can be written as:
The practical difference between Widmark and Watson shows up most clearly in two groups: obese subjects, whose excess adipose tissue lowers their water fraction well below the population average that Widmark's r assumes, and very lean, muscular subjects, who carry more water than the Widmark lookup predicts. In both cases, using a static r can bias BAC estimates by 15–25%. Maskell and colleagues in 2015 conducted a detailed uncertainty analysis of BAC calculations and explicitly recommended Watson over Widmark for forensic precision, noting that Watson propagates individual anthropometry into the TBW term rather than averaging it away.
Alcometer uses a hybrid of the two — the Forrest-Maskell BMI-to-r table for the typical band, with a Watson-style correction at the optimistic and conservative bounds. The result is narrower uncertainty intervals for users with unusual body composition without sacrificing the interpretability of the classic Widmark output.
The 2100:1 partition ratio: why breathalyzers diverge from BAC
Every breathalyzer ever sold assumes a fixed relationship between the ethanol concentration in deep-lung (alveolar) air and the ethanol concentration in the bloodstream supplying those alveoli. That relationship is the blood-breath partition ratio, and by convention it is set to 2100:1: 2100 millilitres of breath contain the same mass of ethanol as 1 millilitre of blood. The ratio is a direct consequence of Henry's Law — at body temperature, ethanol partitions between the blood and the alveolar air phase at a roughly constant equilibrium — and it is the reason a breath reading can be converted to a BAC-equivalent figure at all.
The problem is that 2100:1 is a population mean. Individual partition ratios measured in controlled clinical studies range from roughly 1,500:1 to 3,000:1, and the distribution is broad enough that a single breath reading can under- or over-estimate true BAC by a substantial margin. Three variables drive the variance. Body temperature is the largest: for every +1 °F (+0.55 °C) above the 34 °C alveolar calibration temperature, the reading increases by roughly 7%. Haematocrit, breathing pattern, and time-since-last-drink also matter.
The second category of error is mouth alcohol. A recent sip, a burp, regurgitation, or gastro-oesophageal reflux can deposit ethanol directly in the mouth and upper airway, and that residual alcohol evaporates into the breath sample without ever passing through the lungs. Mouth-alcohol artefacts can spike a reading by five to ten times the true alveolar value. For this reason, evidential breath-testing devices such as the Intoxilyzer 9000 require a 15-minute deprivation period — no food, drink, smoke, or vomit — before a valid sample can be taken, and they run slope detection on the breath curve to flag samples with a mouth-alcohol signature.
Jones's 2010 systematic review concluded that the 2100:1 convention is fit-for-purpose for population-level DUI enforcement but should never be interpreted as a precise per-individual conversion. Alcometer models blood alcohol directly and does not attempt breath calibration; when you compare our estimate to a breathalyzer reading, disagreement within roughly ±20% is expected and does not indicate an error in either number.
BAC units: a Rosetta stone
Blood alcohol concentration has more units than any other common lab value, and the resulting confusion is one of the top sources of error in popular writing about drinking. The table below reconciles the main systems.
Notation
Equivalents
Typical use
1 ‰ (per mille)
= 0.1% = 1 g/L = 100 mg/100ml = 100 mg/dL
EU, most of the world (legal)
0.1%
= 1 ‰ = 1 g/L
United States (legal and common)
1 g/L
= 1 ‰ = 0.1% = 100 mg/dL
Clinical labs, SI-strict countries
1 mg/L BrAC × 2100
= 1 mg/L BAC = 1 g/L BAC
Breath to blood conversion
A worked example: the US legal driving limit of 0.08% converts to 0.8 g/L and 0.8 ‰. The EU default of 0.5 ‰ converts to 0.05% and 0.5 g/L. The numbers look very different; the underlying physiology is identical.
Why three systems coexist is a minor historical accident. The United States uses percent because early twentieth-century FDA labelling required alcoholic-beverage strength to be stated as a percentage, and that convention carried over into forensic BAC reporting when the first breath-testing laws were drafted in the 1940s. Europe uses per-mille (‰) because Widmark's original 1932 paper expressed BAC in grams per kilogram of blood, which rounds cleanly to parts-per-thousand at blood density. Japan uses per-mille for legal proceedings but percent in casual discussion, mirroring the two-track approach common in much of Asia.
Back-extrapolation: the forensic use of Widmark
The situation arises often enough to have its own name. A driver is stopped at 01:00, and by the time a blood sample is drawn at a police station at 02:30 the measured BAC is 0.062%. The prosecution wants to know what the driver's BAC was at the time of driving, which is the legally relevant quantity. The procedure that answers this question is retrograde extrapolation, and mechanically it is nothing more than Widmark's elimination term run backwards:
BAC(T) = BAC(T + N) + (N · elimination_rate)
If we assume the driver had been in the post-absorptive phase at the time of driving — typically defined as more than 60 minutes after the last drink, once absorption has plateaued — and we use a sex-appropriate elimination rate from Jones (2010), 1.5 hours of extrapolation at 0.168 ‰/hour adds roughly 0.025 ‰ to the measurement. The 0.062% reading at 02:30 becomes approximately 0.087% at 01:00, enough to cross the 0.08% US limit.
The forensic literature treats back-extrapolation as admissible but demands that three conditions hold. First, the subject must be post-absorptive at the time of interest; if absorption was still ongoing, BAC was still rising, and extrapolation in the wrong direction will overstate the historical level. Second, the elimination rate must be individually appropriate, which in practice means a range is calculated, not a single number. Third, the measurement and the extrapolation interval must be documented precisely, because small errors in time compound.
Maskell and colleagues in 2015 quantified the residual uncertainty in well-executed retrograde Widmark at roughly ±15–20% of the back-extrapolated value. That is a large band, and it is the primary reason Alcometer displays three lines (optimistic, typical, conservative) rather than one. A single-number estimate in a domain with ±20% intrinsic uncertainty communicates false precision; the three-band display makes the uncertainty visible, which is exactly what a defensive forensic presentation would do.
ADH1B*2 and ALDH2*2: why Widmark needs a genetic correction
The Widmark framework assumes a single elimination rate per sex, a simplification that hides one of the largest real-world sources of variance in alcohol metabolism: the alleles of the two hepatic enzymes that handle ethanol. Edenberg's 2007 review in Alcohol Research & Health remains the definitive overview.
The first variant is ADH1B*2 (historically called ADH2*2), a single-nucleotide polymorphism in the alcohol dehydrogenase 1B gene that produces an enzyme with roughly 40 times the catalytic activity of the common ADH1B*1 isoform. ADH1B*2 is common in East Asia — carried by approximately 75% of the population — and reaches about 30% prevalence in India, with smaller fractions in Jewish and some Middle Eastern populations. Its carriers convert ethanol to acetaldehyde faster than the Widmark elimination rate predicts.
The second variant is ALDH2*2, a Glu504Lys substitution in aldehyde dehydrogenase 2. The mutant enzyme retains only a small fraction of normal activity, so the acetaldehyde produced by ADH accumulates rather than being cleared. ALDH2*2 reaches prevalences of 30–50% in Japan, Korea and parts of China, and is very rare outside East Asia.
The combination of fast ADH and slow ALDH produces the syndrome familiar as the Asian flushing response: facial erythema, tachycardia, nausea, and headache after even small amounts of alcohol, driven by acetaldehyde concentrations several times higher than a non-carrier would ever experience. For BAC specifically, the allele combination means that a 75-kg ALDH2*2 heterozygote may show an elimination-phase BAC curve that looks unremarkable even while endogenous acetaldehyde is far above normal, because the population-average rate used by Widmark does not capture allele-specific pharmacokinetics.
Alcometer does not currently model these alleles; a single elimination rate per sex is still used. Genetic calibration is a flagged future enhancement — the challenge is that users typically do not know their ADH/ALDH genotype, so any correction would have to be offered as a toggle based on self-reported flushing history or known ancestry. Until that work is done, users who experience strong flushing should interpret the conservative band (slower elimination, longer acetaldehyde exposure) as the more realistic estimate.
Excluded factors and model boundaries
Alcometer intentionally omits or simplifies variables that can move a real BAC curve outside the population bands we show. Examples include: acute illness and pregnancy; prescription or illicit drug interactions with ethanol; individual ADH1B/ALDH2 genotypes (discussed above) unless you manually interpret the conservative band; mouth-alcohol artefacts and device-specific breath calibration; workplace, aviation, or maritime limits that differ from the road-traffic thresholds in our tables; and legal doctrines beyond the numeric limit (e.g. impairment-based offences).
Retrograde extrapolation, field impairment testing, and laboratory chain-of-custody requirements are described for education only — the calculator does not perform admissible forensic reconstructions. When a decision is safety- or law-critical, rely on qualified professionals and statutory testing, not on this site.
Version history and the compliance evidence register
Scientific citations on this page track peer-reviewed pharmacokinetics and official legal sources. Separately, the operator maintains a versioned evidence register that lists every launch-blocking (P0) remediation item for the BAC v7 hardening programme, its recommended fix, acceptance criteria, and resolution phase. Major UX, privacy, accessibility, schema, editorial, monetization, and legal-page changes are logged there so auditors can reconcile on-site copy with internal compliance work.
When the register records a resolved P0 item, the public site is updated in the same release whenever the finding touches user-visible text or structured data. If you need to report a factual error in a citation or jurisdiction row, use the Source corrections page linked from the footer.
Frequently asked questions
Why does Alcometer show 3 BAC lines instead of 1?
Real blood alcohol concentration varies by roughly ±20% around any model estimate because of individual differences in body water, enzyme activity, absorption timing, and hydration — even with perfect inputs. Alcometer runs three parallel simulations (optimistic, typical, conservative) using uncertainty coefficients from Maskell et al. (2015) so that the central ~80% of the population sits inside the band. The three lines exist to make uncertainty visible. They are educational estimates only — not measurements, not court-admissible evidence, and not a fitness-to-drive indicator. If any line is at or near a legal limit, treat it as "do not drive".
Is the Widmark formula accurate?
The classic 1932 Widmark equation reproduces population-average BAC within about ±20%, with simplifying assumptions (instant absorption, constant elimination, a single distribution factor per sex). Alcometer uses a Widmark backbone improved with evidence-based submodels: logistic absorption curves, BMI-adjusted distribution factors (Forrest/Maskell), and a concentration-dependent elimination rate at low BAC. The result tracks controlled clinical studies more closely than textbook Widmark, but it is still an educational model. Any number it produces is a statistical estimate, not a clinical or forensic measurement, and should not be used to decide whether to drive or to argue a legal case.
What is the difference between breath and blood alcohol?
Blood alcohol concentration (BAC) is the mass of ethanol per unit volume of blood. Breath alcohol concentration (BrAC) is the mass of ethanol per unit volume of exhaled alveolar air. The two are linked by the blood-breath partition ratio, conventionally 2100:1 (Henry's Law). True individual ratios run from about 1,500:1 to 3,000:1, so a real breathalyzer reading can differ from a true BAC by 20% or more. Temperature, mouth alcohol, recent food or drink, and timing add further variance. Evidential breath devices mitigate this with 15-minute deprivation periods, slope detection, and calibration logs. Alcometer is not a breathalyzer and is not a substitute for one.
Can an online estimate replace an evidentiary alcohol test?
No. Court-admissible BAC requires a certified breath or blood test performed by trained personnel on calibrated, type-approved equipment, with documented chain of custody and statutory observation periods. Any calculator — including Alcometer — produces a statistical estimate inside a ±20% uncertainty band built from population averages and your self-reported inputs. It has no evidentiary weight in any jurisdiction, will not be accepted as a defence in DUI proceedings, and cannot be used to certify that you are under a legal limit. If legal exposure is a concern, do not drive on the basis of a calculated BAC. Use an evidential breathalyzer, request a blood test, or simply wait. When in doubt, do not drive.
Do people of different genetics metabolize alcohol differently?
Yes. ADH1B*2 produces an alcohol dehydrogenase about 40x faster than the common variant and is carried by roughly 75% of East Asian and 30% of Indian populations. ALDH2*2 produces a nearly inactive aldehyde dehydrogenase and is carried by 30–50% of Japanese, Korean, and Chinese populations. The combination accelerates ethanol-to-acetaldehyde conversion while slowing acetaldehyde clearance, producing facial flushing, tachycardia, and nausea. BAC curves and hangover severity can differ by a factor of two between carriers and non-carriers at the same sex and weight. Alcometer uses sex-average elimination rates and does not adjust for these alleles, so users with strong flushing should treat the conservative band as more realistic and avoid driving on the basis of any estimate.
How does back-extrapolation work in DUI cases?
Retrograde extrapolation is a forensic technique used by trained toxicologists to estimate BAC at an earlier time from a documented later measurement. It multiplies the elapsed time by a sex-appropriate elimination rate (about 0.168 ‰/h for males, Jones 2010) and is admissible in DUI prosecutions only when the subject is post-absorptive, the measurement is from certified equipment, and the chain of custody is documented; it still carries about ±15–20% residual uncertainty (Maskell 2015). Alcometer is not a forensic tool and does not perform admissible back-extrapolation. The methodology page explains the technique for educational purposes only — the calculator should never be used to argue a DUI case or to "prove" what your BAC was at any prior moment.
Is Alcometer medical advice?
No. Alcometer is an educational pharmacokinetic model. Its output is not a medical diagnosis, treatment plan, or prescription, and it does not establish a clinician-patient relationship. None of the supplements, vitamins, or interventions referenced in the methodology page are recommendations — they are summaries of published research. If you are pregnant, taking medication, have a chronic condition (liver, kidney, cardiovascular, neurological, or mental-health), or are concerned about your drinking, speak to a qualified healthcare professional before changing your diet, taking supplements, or relying on these estimates. If you experience confusion, repeated vomiting, slow or irregular breathing, seizures, or you cannot wake someone after drinking, treat it as a medical emergency and call your local emergency number.