What this does that a Widmark calculator doesn't
- Absorption takes time. Each drink enters your blood gradually, so the peak usually comes some time after your last drink rather than at it. How long depends on how full your stomach is, how strong the drink is and how much of it there is.
- Food changes the curve twice. The stomach passes its contents on at roughly 2-8 kcal per minute (Hunt and Stubbs 1975), so a bigger meal holds alcohol back for longer. And the longer alcohol waits in a full stomach, the more of it is broken down before it reaches your blood (Jones and Jönsson 1994). The model caps that first-pass loss at 30%. Caloric drinks count as food as well: a regular cola slows whatever you drink after it, a diet cola doesn't. A meal also speeds up elimination for a while (Ramchandani et al. 2001); the model leaves that out, so its tail errs on the long side.
- Strength isn't a straight line. Absorption is fastest at around 22% ABV and slower for both beer and neat spirits (Roine et al. 1991; Mitchell et al. 2014). I wrote up the details in why a Maß takes longer to hit you than it should.
- Body water, not just body weight. How far the alcohol is diluted comes from the Seidl et al. (2000) regression on sex, height and weight, instead of one fixed factor of 0.68 or 0.55 for everybody.
- Elimination is steady. The liver clears alcohol at a roughly constant rate. The calculator uses the population average of 0.15‰ per hour (Jones 2010).
What it can't see
Plenty. Your own elimination rate is the big one. It typically sits somewhere between about 0.10 and 0.20‰ per hour and runs higher in heavy drinkers, and in the model going from 0.10 to 0.25 lowers the peak by about a third and brings the time back to zero forward by more than half. Genetics, medication, how quickly your stomach empties and what exactly you ate are invisible to it too. That's what the band is for, and it's why every figure on this page says estimate.
How big the food effect is depends on how much you drink. In the study the model can be checked against, ten men drank 0.80 g of alcohol per kg of body weight (for someone weighing 80 kg, about what is in three half-litre beers) straight after a substantial breakfast. Their peak was 36% lower and their total exposure 39% lower than on an empty stomach (Jones and Jönsson 1994). At that dose the model gives about 31% and 39%. For a single beer it shows a much larger relative drop, because slower uptake lets your liver clear more of a small dose before it builds up. No study has measured that case, so read the gap between the two lines as an estimate with the same uncertainty as everything else, not as something to plan around.
Why a range instead of one number
Two people with the same height, weight and drinks can end up roughly 20-30% apart in the blood alcohol they actually reach. A calculator that prints a result to two decimal places is claiming a precision it doesn't have. The shaded band is ±25% around the estimate, and the app draws the same band by default.
Can I use this to decide whether I can drive?
No. Not this calculator, not the app, not any calculator. It doesn't know your real elimination rate, and a ±25% error can be the difference between two very different legal situations. That's why there's no driving limit on the chart. If you've been drinking and you're not sure, don't drive.
Questions
Does eating food lower your blood alcohol?
Yes, in two ways. A meal slows gastric emptying, so alcohol reaches the small intestine, where most of it is absorbed, more gradually and the peak comes later and lower. And the longer alcohol sits in a full stomach, the more of it is broken down by first-pass metabolism before it ever reaches your blood. The model caps that loss at 30% of the dose. Caloric drinks count as food too: a regular cola slows the drinks after it, a diet cola doesn't.
Does drinking water lower your BAC?
No. Alcohol spreads through all of your body water, around 42 litres for a 75 kg adult, so a 500 ml glass changes the volume it's diluted in by about one percent, and your kidneys clear the extra fluid within the hour. The liver breaks alcohol down at a roughly fixed rate that more fluid can't speed up. Water is good for you on a night out; it just doesn't move this curve, so the calculator ignores it on purpose.
How long does it take for alcohol to leave your body?
Once absorption is over, blood alcohol falls at a roughly constant rate, on average about 0.15‰ (0.015% BAC) per hour. At that rate 0.5‰ takes a bit over three hours to clear. Your own rate may be well above or below the average. Coffee, water and a cold shower do not speed it up; a meal does raise it for a while, but clearing a few drinks still takes hours, and the calculator leaves that effect out.
Why does the peak come after the last drink?
Because absorption takes time. Each drink enters your blood gradually, over anything from about half an hour to two hours or more, depending on how full your stomach is, the drink's strength and its volume. While a drink is still being absorbed faster than your liver clears alcohol, the curve keeps rising.
Can I use this to decide whether I can drive?
No. It doesn't know your real elimination rate, and individual results vary by roughly ±20-30%, which can be the difference between two very different legal situations. That's why there is no driving limit on the chart. If you've been drinking and you're not sure, don't drive.
What do ‰, g/l, % BAC and mg/100 ml mean?
They are the same quantity written four ways. In this calculator ‰ and g/l are numerically identical, % BAC is the per mille value divided by ten (0.5‰ is 0.05%), and mg/100 ml is the per mille value times 100 (0.5‰ is 50 mg/100 ml).
Is anything I enter stored or sent anywhere?
No. The calculation runs entirely in your browser and nothing you type leaves the page.
Sources
- Widmark, E.M.P. (1932). Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung.
- Seidl, S., Jensen, U., Alt, A. (2000). The calculation of blood ethanol concentrations in males and females. Int J Legal Med 114:71-77.
- Jones, A.W. (2010). Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework. Forensic Sci Int 200:1-20.
- Hunt, J.N., Stubbs, D.F. (1975). The volume and energy content of meals as determinants of gastric emptying. J Physiol 245:209-225.
- Jones, A.W., Jönsson, K.Å. (1994). Food-induced lowering of blood-ethanol profiles and increased rate of elimination immediately after a meal. J Forensic Sci 39:1084-1093.
- Ramchandani, V.A., Kwo, P.Y., Li, T.-K. (2001). Effect of food and food composition on alcohol elimination rates in healthy men and women. J Clin Pharmacol 41:1345-1350.
- Roine, R.P. et al. (1991). Effect of concentration of ingested ethanol on blood alcohol levels. Alcohol Clin Exp Res 15:734-738.
- Mitchell, M.C. et al. (2014). Absorption and peak blood alcohol concentration after drinking beer, wine, or spirits. Alcohol Clin Exp Res 38:1200-1204.
Make the estimate yours
This page uses population averages because it knows nothing about you. The CheersCount app draws the same curve while you log a night, and if you own a breathalyzer it can do something no formula can: two readings at least 20 minutes apart on the way down give your own elimination rate, and every curve after that uses it.
The numbers, the estimated sober time, calibration and every finished night's curve are free. Watching the curve live while a night is still running is part of Pro, which starts with a 30-day trial and then costs from $2.99 a month.