The number that knows your night.

In 1932, a Swedish chemist named Erik Widmark published an equation that should not have aged well. He was working before MRI machines, before the human genome, before anyone had a phone in their pocket — let alone an app that could calculate their blood alcohol content between sips. And yet, almost a century later, the formula he wrote is still the equation your phone uses, the equation forensic experts cite in courtrooms, and the equation that every BAC calculator on the internet — including ours — quietly leans on.

That's a long shelf life for a piece of math. The reason it has lasted is not that the formula is perfect. It isn't. The reason is that the alternatives that would improve it — measuring stomach contents, liver enzyme activity, real-time absorption rates — require data nobody has. Widmark's formula survives because it asks for things ordinary people can answer. How much do you weigh. What did you drink. How long ago did you start.

The equation, in plain English

Widmark's insight was that alcohol distributes itself through the water in your body, not the fat. Two people who weigh the same amount can have very different BACs from the same drink if one is leaner than the other, because the leaner person has more body water to dilute the alcohol into.

Reduced to its essentials, the formula does three things at once. It calculates how much pure ethanol you consumed. It divides that ethanol by the volume of your body water. Then it subtracts what your liver has metabolized since you started drinking. What's left is your blood alcohol content, expressed as a percentage.

The distribution ratio is the only piece that isn't measured directly. Widmark proposed 0.68 for men and 0.55 for women — numbers derived from population averages of body water content. These ratios still appear in toxicology textbooks today, though every researcher who uses them adds a footnote: they are averages, not predictions. A very lean woman may have a ratio closer to the male number. A man with substantial body fat may have a ratio closer to the female one. The equation has no input for body composition because, frankly, nobody knows their body composition with the kind of precision the equation pretends to need.

What the formula leaves out

This is the part of the conversation most BAC calculators skip, and it is the most important part. The Widmark formula assumes that all the alcohol you drank has been fully absorbed into your bloodstream and distributed evenly through your body water. Reality is messier than that, and the messiness matters.

What you ate before you started drinking changes the entire shape of your night. Food in your stomach can slow alcohol absorption by thirty to fifty percent, lowering your peak BAC meaningfully and stretching it out over a longer window. Drinking on an empty stomach produces a sharp peak the formula simply will not show. The pace at which you drank matters too. Four drinks consumed over thirty minutes hits very differently than four drinks consumed over four hours, even if your "time since first drink" input is the same.

Then there are the variables that are harder to even talk about:

• Carbonation — sparkling drinks like champagne, hard seltzer, or a cocktail with tonic accelerate alcohol absorption
• Medications — many prescription and over-the-counter drugs interact with alcohol metabolism in ways no formula can model
• Hydration, sleep, illness — dehydration concentrates alcohol in your blood; fatigue and sickness slow metabolism
• Tolerance — regular drinkers may show functional tolerance, appearing less impaired at a given BAC, though the chemistry is unchanged
• Genetic variation — liver enzyme activity differs from person to person, sometimes dramatically

Published studies comparing Widmark predictions against measured breathalyzer or blood draws typically find the formula accurate to within roughly two-hundredths of a percent. That sounds tiny until you realize the legal driving limit in most of the United States is eight-hundredths of a percent. The margin of error spans a quarter of the legal threshold.

So what is the number actually for?

If the formula can be off by twenty-five percent of the legal limit, what good is it? The honest answer is that the number is useful for one specific purpose: understanding the rough shape of your night. It is not useful for deciding whether you are safe to drive. The only safe blood alcohol content for driving is zero, and Siply will never tell you otherwise.

What the number gives you is calibration. Most people, asked to estimate their own BAC, are wildly off in one direction or the other. They think they had "just a couple" when they had four. They think they're at a one-and-a-half-drink buzz when their body is processing closer to three. The number, even imperfectly, replaces vibes-based estimation with something concrete. It does not change what you drink. It changes whether you noticed.

That is, in the end, what Siply is for. The Widmark formula gives you a calibrated guess. Your honest engagement with that guess — what you do with it, what it tells you about your night, what pattern it joins on the seventh or fortieth time you see it — is the part no equation can do. That part is yours.

The Siply Editors write about the small chemistry and large culture of how we drink. Read more in Issue 01.