How the estimate works
Every drink contributes a known mass of ethanol: volume × alcohol by volume × 0.789 g/ml (the density of ethanol). Two 0.5 L beers at 5 % contain about 39 g. What that mass does to your blood depends on how much water your body has to dilute it in, how much of it actually reaches the blood, and how fast your liver removes it. The calculator models those three steps.
Widmark and the distribution factor
Erik Widmark showed in 1932 that blood alcohol after full absorption is the alcohol mass divided by body weight times a factor r — the share of body weight that behaves like water for alcohol. He measured averages of 0.68 for men and 0.55 for women. Those constants are still widely used, but they ignore that a tall lean 25-year-old and a short heavy 60-year-old of the same weight carry very different amounts of water.
Watson, Watson and Batt (1980) published equations that estimate total body
water (TBW) from sex, age, height and weight. Seidl, Jensen and Alt (2000)
validated their use for blood alcohol against measured values and derived r = TBW / (0.8 × weight), because blood is about 80 % water.
This is the form forensic institutes use and the one this calculator uses
whenever age and height are given. If you leave them blank it falls back to
Widmark's averages and says so under the result.
Absorption deficit and food
Not all ingested alcohol shows up in the blood: part is metabolised in the stomach wall and liver before it circulates (first-pass metabolism). Widmark already put this deficit at 10–30 %; it is smallest on an empty stomach and largest after a full meal, which slows gastric emptying. The stomach toggle sets the deficit to 10, 20 or 30 %. Blank means 20 %.
Time: absorption and elimination
Alcohol keeps entering the blood for roughly 30–60 minutes after the last sip. The calculator lets it rise linearly from the first drink until 45 minutes after the last one, then treats absorption as complete — which is when the peak occurs. From the first drink onward the liver removes alcohol at a nearly constant rate β. Jones (2010) surveyed the evidence and found 0.10–0.20 ‰ per hour across healthy adults, with a mean around 0.15.
That spread is why you get a corridor instead of a number: the lower bound assumes a fast eliminator (0.20 ‰/h), the upper bound a slow one (0.10 ‰/h), and the typical value sits at 0.15 ‰/h. The time back to 0.0 is reported the same way. If anything depends on being below a level, the upper bound is the one that matters.
What the model cannot know
- Your personal elimination rate — regular drinkers tend toward the fast end, but there is no way to tell from the outside.
- Your drinking pace within the evening; counting every drink from the start is a simplification that can shift the peak.
- Illness, medication, dehydration, fatigue — all change how alcohol affects you at a given concentration.
- Real glass sizes and strengths: a "0.5 L" beer is often 0.4 L of liquid, craft beers run 6–9 %, and pours at home are generous.
The estimate is a planning aid, not a measurement. Only a breath or blood test tells you your actual level, and no calculator should decide whether you drive.
Units
Continental Europe states blood alcohol in per mille by mass (g of alcohol per kg of blood). The USA uses percent by volume (g per 100 ml), the UK and Ireland milligrams per 100 ml. The calculator converts with the common forensic rounding 1.0 ‰ ≈ 0.10 % ≈ 100 mg/100 ml; the exact factor differs by a few percent because blood is slightly denser than water.
Sources
- Widmark EMP. (1932). Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung . Urban & Schwarzenberg, Berlin. The original formula: blood alcohol = alcohol mass / (r × body weight) − β × time. Still the backbone of forensic BAC estimation.
- Watson PE, Watson ID, Batt RD. (1980). Total body water volumes for adult males and females estimated from simple anthropometric measurements . American Journal of Clinical Nutrition 33(1):27–39. The body-water equations from sex, age, height and weight that replace Widmark's fixed r constants.
- Seidl S, Jensen U, Alt A. (2000). The calculation of blood ethanol concentrations in males and females . International Journal of Legal Medicine 114(1–2):71–77. Validated Watson-based distribution factors against measured blood alcohol; basis for r = TBW / (0.8 × weight).
- Posey D, Mozayani A. (2007). The estimation of blood alcohol concentration — Widmark revisited . Forensic Science, Medicine, and Pathology 3(1):33–39. Review of the Widmark method, its refinements and error sources for forensic use.
- Jones AW. (2010). Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework . Forensic Science International 200(1–3):1–20. Population range of elimination rates, 0.10–0.20 ‰ per hour with a mean around 0.15 — the source of the corridor.