How the Siply BAC calculator actually works.

1. The formula we use

Siply uses the Widmark formula, developed by Swedish chemist Erik Widmark in 1932. It's the same equation that appears in toxicology textbooks, forensic reports, and most online BAC calculators. After nearly a century of use it remains the standard starting point for BAC estimation — not because it's perfect, but because alternatives that improve accuracy require data most people don't have (gastric emptying time, recent food composition, enzyme activity).

To get A — grams of ethanol — we multiply the volume of each drink by its alcohol-by-volume percentage, then by 0.789 (the density of ethanol in grams per milliliter). A 12-ounce beer at 5% ABV contains roughly 14 grams of ethanol. A 1.5-ounce shot of 40% ABV spirit contains about 14 grams as well. A standard 5-ounce glass of 12% wine, the same.

2. What you tell us — and what we assume

The calculator asks you for four things directly: biological sex, body weight, what you drank, and how long ago you started. From those four inputs, here's what we infer or assume:

Distribution ratio (r)

We use 0.68 for male users and 0.55 for female users. This reflects the average difference in body water content — men have more, which dilutes alcohol more effectively. These are population averages; individuals vary significantly. A very lean woman may have a distribution ratio closer to the male average, and vice versa. The calculator has no input for body composition because asking visitors for body fat percentage felt invasive, and self-reported body composition is unreliable anyway.

Drink volumes and ABV

Each preset drink in the calculator uses standard volume and a typical ABV: beer at 5%, IPA at 7%, wine at 12%, spirits at 40%, hard seltzer at 5%. Your actual drink may differ substantially. A craft beer can range from 4% to 12%; a heavy-pour cocktail at home may contain double the spirit of a bartender's measured pour. When in doubt, choose the closest higher option rather than guess low.

Metabolism rate (β)

We use 0.015% per hour, the most commonly cited average. Real rates range from roughly 0.010% to 0.025% per hour depending on liver enzyme activity, recent eating, regular drinking habits, and several genetic factors. Faster metabolizers will see their BAC drop quicker than the calculator predicts; slower metabolizers will stay elevated longer.

3. What the calculator cannot account for

This is the most important section on this page. The Widmark formula assumes alcohol has been fully absorbed and distributed evenly throughout body water. Reality is messier:

• Food in your stomach. Eating before or during drinking can slow alcohol absorption by 30 to 50 percent and lower peak BAC meaningfully. Drinking on an empty stomach can produce a sharp peak the formula won't show.
• Drinking pace. Four drinks consumed over 30 minutes hits very differently than four drinks over four hours, even if the time-since-first-drink input is the same.
• Carbonation. Sparkling drinks (champagne, hard seltzers, mixed drinks with tonic or soda) speed alcohol absorption.
• Medications. Many prescription and over-the-counter drugs interact with alcohol metabolism in ways the formula doesn't model — sometimes dramatically.
• Hydration, sleep, illness. Dehydration concentrates alcohol in blood. Fatigue and illness slow metabolism. The formula assumes a baseline well-rested healthy adult.
• Tolerance. Regular drinkers may show functional tolerance — appearing less impaired at a given BAC than a non-drinker would. BAC is a measure of chemistry, not behavior. The number doesn't change; how you experience it might.
• Recent vomiting, kidney or liver conditions. Any of these can throw the estimate off significantly in either direction.

4. How accurate is the estimate, really?

Published studies comparing Widmark predictions to measured breathalyzer or blood alcohol concentrations typically find the formula accurate within roughly ±0.02% BAC for most adults under typical conditions. That margin sounds small until you realize the legal driving limit in the US is 0.08% — a 0.02% error spans a quarter of that distance.

In plain terms: if Siply estimates your BAC at 0.060%, your actual BAC could reasonably be anywhere between 0.040% and 0.080%. Use the estimate to understand the rough shape of your night, not to make decisions where precision matters.

5. A note on legal limits

In all 50 US states, the legal limit for driving is 0.08% BAC for drivers 21 and over. Utah is the exception at 0.05%. For commercial drivers the limit is 0.04%, and for drivers under 21 most states enforce a zero-tolerance limit of 0.01% or 0.02%.

Many countries set lower limits. The UK is 0.08% in England, Wales, and Northern Ireland but 0.05% in Scotland. Most of Europe is 0.05%. Sweden is 0.02%. Several countries enforce 0.00% for any driver.

Worth saying again: the only safe BAC for driving is 0.00%. Legal limits are not safety thresholds — they are enforcement thresholds. Impairment begins at the first drink.

6. If you need real support

Siply is a self-awareness tool, not a clinical resource. If your drinking is affecting your health, your relationships, or your daily life, the most useful step is talking to a professional — not opening an app.