Understanding Blood Alcohol Concentration

1. What blood alcohol concentration is

Blood alcohol concentration (BAC) is the amount of pure alcohol carried in the blood. The same physical quantity is written three ways, and the calculator can show any of them: grams per litre (g/L, the European "per mille"), a percentage in grams per decilitre (g/dL, the US convention), and milligrams per 100 millilitres (mg/100 mL). They are exact conversions of one another: 0.5 g/L is 0.05 g/dL is 50 mg/100 mL.

2. How the calculator estimates it

It uses the Widmark method, the model used in forensic practice. The pure alcohol in each drink is its volume times its strength times 0.789, the density of alcohol in grams per millilitre. In the United States one standard drink holds about 14 grams of pure alcohol — roughly a 355 mL beer at 5%, a 148 mL glass of wine at 12%, or a 44 mL shot of spirits at 40% (NIAAA). That mass of alcohol is divided by your body weight times a Widmark factor, which reflects how much of the body is water. The factor is adjusted for sex and body mass index using the relationship published by Forrest, so height and weight both refine the estimate.

3. Why it shows a range, not one number

Absorption, body composition, and the rate the body clears alcohol all differ between people and between days. Because of that, any Widmark estimate carries real uncertainty. The forensic literature concludes that a BAC estimate should be reported with roughly ±40 percent around it (Gullberg, 2007). The calculator carries the published uncertainty of every input through the calculation and shows the resulting band. A calculator that displays one exact-looking figure is not more accurate — it has simply hidden the uncertainty that is genuinely there.

4. How long until it reaches zero

Once the alcohol is absorbed, the body removes it at a steady pace. The model uses 14.8 mg per 100 mL per hour — about 0.15 g/L per hour — the elimination rate used in the forensic uncertainty literature. The calculator shows two times: a typical time using the average rate, and a cautious later time that allows for slow eliminators at the edge of the range. Nothing speeds this up. Sleep does not clear alcohol faster, and coffee, a cold shower, or a walk do not lower BAC — they only make a person feel more awake while still impaired (NIAAA).

5. What food changes, and what it does not

Eating before and while drinking slows how fast alcohol is absorbed and can lower the peak level in the body by about one third (NIAAA). So after a large meal the true peak is usually below the estimate. The calculator assumes all the alcohol is absorbed, which errs on the cautious side. What food does not change is how much alcohol was drunk, and the time until zero depends mostly on that amount — so a full stomach lowers the peak but does not meaningfully shorten the wait.

6. Legal limits and what "under" really means

Most European countries set the standard drink-drive limit at 0.5 g/L of blood, with lower limits for novice and professional drivers; Czechia, Hungary, Romania and Slovakia allow none at all (European Transport Safety Council). In the United States most states set 0.08 g/dL, and Utah sets 0.05 g/dL (CDC). The calculator shows the limits of the country you select next to the estimate. Being under a limit is not proof of being safe to drive. Impairment begins below the legal thresholds — the CDC states plainly that it starts at lower BAC levels — and at 0.08 g/dL a driver is about four times more likely to crash than a sober one (NHTSA).

7. The bottom line

This calculator is an educational tool. It is not a breath or blood test, it is not legal advice, and it can never tell you that driving is safe. The estimate carries roughly ±40 percent uncertainty, and impairment starts below every legal limit. If you plan to drive, the only safe amount of alcohol is none. Decide the plan first — a designated driver, a taxi, or public transport — and let that decide the drinks, rather than the other way round.

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