Type "virtual breathalyzer" into a search engine and you will find dozens of them. Enter your sex, your weight, how many beers, what time you started. A number appears, usually to two or three decimal places, and it looks like a measurement. It isn't.
A team of toxicologists at the Medical University of Lodz collected 42 of these calculators from Polish and English search results, discarded the ones that no longer worked, and put the remaining 36 through a controlled test. Joanna Stragierowicz and colleagues had a benchmark to test against: a 2020 Polish forensic study in which 16 people, eight women and eight men, drank measured amounts of alcohol both on an empty stomach and after a meal, with their blood alcohol concentration actually measured afterward. The researchers fed each of those 16 people's real details into each of the 36 calculators and compared what the websites predicted against what the laboratory had recorded.
The websites ran high. Across the board, predicted values averaged 39 percent above the measured peak concentration, 42 percent above the true value at 60 minutes, and 49 percent above at 120 minutes. Some individual sites returned figures approaching double the real number. Only one calculator, Omnicalculator, came in below the reference at peak; three sites underestimated at the later time points.
That overestimation might sound like a safety margin, and in one sense it is. A tool that tells you you are drunker than you are will not put you behind the wheel. But the authors are careful about this. Consistent overestimation across dozens of independently built websites is not caution, it is a shared modelling flaw, and a flaw that runs in one direction on average can still run the other way for an individual user. The paper's blunt phrasing is that the real risk is a calculator that "may falsely indicate sobriety."
What the calculators don't ask
The reason becomes clear from looking at what these sites request. Every single one asked for sex. Nearly all asked for weight. After that, the questions thin out fast: only 30.6 percent asked age, 38.9 percent asked height, and just 8.3 percent asked about body type or metabolic rate. Nearly two thirds ignored whether the person had eaten, even though a meal meaningfully slows alcohol absorption by delaying stomach emptying and increasing the liver's first pass at the alcohol. A third of the sites would not let the user enter the exact amount consumed, and more than 40 percent would not accept the actual strength of the drink.
Nothing asked about medication, ethnicity, or alcohol dependence, all of which the authors note can shift alcohol metabolism substantially. Aspirin and the heartburn drug cimetidine both raise blood alcohol by inhibiting the enzyme that breaks it down in the stomach. Populations differ in the activity of aldehyde dehydrogenase, another enzyme in the pathway. Heavy drinkers metabolise alcohol faster because their livers adapt. None of this fits in a four-field web form.
More data did not reliably mean better answers. Two sites that asked for sex, age, weight, height, meal and drink strength were thrown out of the analysis before it began, because their outputs barely varied at all: they returned nearly the same number regardless of who was asking. On some sites, the researchers noticed, toggling between "empty stomach" and "full stomach" changed nothing in the result. Three tools did survive the statistical comparison. BACscan and Aisko produced results comparable to the measured values at peak concentration and at 120 minutes, and one anonymised site, Website No. 3, matched at 60 minutes and at peak. Even these overestimated, by roughly 0.09 to 0.21 g/l depending on the tool and the time point.
The team also surveyed 103 people about how they use these tools. Just over half had heard of a virtual breathalyzer, and 73.2 percent of those had used one, mostly out of curiosity. Reassuringly, 85.4 percent said they did not act on the results. Less reassuringly, 36.6 percent said one of their reasons for using a calculator was checking their sobriety before driving. Most simply clicked whatever appeared first in the search results.
Why it matters
Around a quarter of road deaths in Europe are estimated to involve alcohol, and impairment starts early: driving skill begins to degrade at a blood alcohol concentration of 0.2 g/l, which is itself the legal limit in Poland, Sweden, Norway and Estonia. The margin for error is thin, and a website with no calibration, no oversight and no obligation to be right is being consulted inside that margin.
The authors are not arguing that these tools should vanish. They can build awareness of how much alcohol a given evening actually involves. The argument is narrower and harder to disagree with: an online calculator is an estimate built on a simplified formula, most likely a 1932 equation by Erik Widmark that assumes fixed metabolism, and it should never be the thing that decides whether someone drives. That decision needs a certified device.
The study has real limits, which the authors state. Sixteen reference subjects is a small sample, and the findings apply only to the 36 tools tested, on the days they were tested. Websites change. But the pattern held across almost all of them, which suggests the problem is not any particular site so much as the idea that a handful of form fields can stand in for a blood test.