Blood Alcohol (BAC) Calculator

BAC: Blood Alcohol Concentration

BAC (Blood Alcohol Concentration) is the mass of ethanol per volume of blood, usually written as g/L or g/100 mL. The Widmark formula puts it as BAC = A / (P · r) − β · t. Here A is the alcohol consumed in grams, P is body weight in kg, r is the distribution factor (≈ 0.68 for men, 0.55 for women), β is the elimination rate (≈ 0.15 g/L per hour), and t is the time since drinking. To put numbers on it: a 350 mL beer at 5% ABV holds about 14 g of ethanol, which is one standard drink, and the body clears roughly one drink every hour.

Legal limits and applications

In Brazil, the Federal Traffic Code (Law 9.503/1997) runs on zero tolerance. Any BAC > 0 is an administrative offense, and once it reaches BAC ≥ 0.6 g/L of blood (or ≥ 0.3 mg/L on the breathalyzer) it becomes the crime defined in art. 306, carrying a fine of R$ 2,934.70 (5× for severe offenses, up to 10×), a 12-month license suspension, and 6 months to 3 years of prison. For comparison, the US limit sits at 0.08 g/dL, most of Europe falls between 0.02 and 0.05 g/L, and Sweden goes as low as 0.02. Keep in mind the formula only gives an estimate. An empty stomach, medication or a medical condition can shift the real result by a lot.

FAQ

How long until I'm sober? The body clears about 0.15 g/L per hour, so starting from a BAC of 0.8 g/L you're looking at roughly 5–6 hours to reach zero. Coffee and cold showers won't speed any of that up.

Is the estimate reliable? Not really. Widmark is an approximation, and an empty stomach, your body composition, genetics and any medications you're on all push the actual BAC around. Don't decide to drive based on the calculator.

Does the breathalyzer measure BAC directly? It doesn't. What it reads is the alcohol in alveolar breath (mg/L), then converts that figure. In Brazil, hitting ≥ 0.3 mg/L on the breathalyzer is criminal under art. 306 of the CTB.