How BAC Works

What is BAC?

Blood Alcohol Content (BAC) is the percentage of alcohol in your bloodstream. In the United States, the legal limit for driving is 0.08% in most states (0.05% in Utah). But impairment begins much earlier — reaction time, judgment, and coordination start declining at just 0.02%.

BAC is measured in grams of alcohol per deciliter of blood (g/dL). A BAC of 0.08% means 0.08 grams of alcohol per 100 milliliters of blood.

How Your Body Processes Alcohol

Absorption

Most alcohol is absorbed in the small intestine, so how fast the stomach empties sets how fast BAC rises. On an empty stomach BAC typically peaks within about an hour; food slows absorption. A small part of the alcohol (a few percent) is broken down before it reaches the blood.

Distribution

Once in your blood, alcohol spreads through the water in your body. Lean tissue — muscle and organs — is mostly water; fat holds very little. So at the same weight, more body fat means less room for the alcohol and a higher BAC.

Metabolism

Your liver breaks down alcohol at a relatively constant rate of about 0.015% BAC per hour on average. It varies from person to person (roughly 0.010–0.025% per hour); sex and drinking history matter, body weight does not. Nothing speeds this up enough to count on — not coffee, not water, not exercise. A real meal makes it run somewhat faster, never enough to make driving safe. Time is the only remedy.

Elimination

About 5–10% of alcohol leaves through breath, sweat, and urine. The remaining 90–95% is metabolized by the liver. A breathalyzer measures the alcohol in your breath to estimate blood concentration.

Why Weight Alone Isn’t Enough

Classic formulas — Widmark’s from 1932, or Watson’s body-water equations from 1980 — work from your weight; Watson’s also from height, age and sex. None of them sees body fat, so two people of the same build get the same number even when one carries far more fat.

What Makes Alcophone Different

Alcophone works in two steps:

1. Body Fat Estimate

First, the app estimates your body fat percentage from your sex, age, weight and height, with an adult body-fat equation based on BMI. At higher BMI it adds a small cautious margin, which raises the estimate — and with it the BAC. For users in India it switches to a South Asian model, because the same BMI carries more body fat and less lean mass there. Your legal limit is country-aware too — see the limit tables for the USA, Canada, Australia, UK, India and Ireland.

2. Distribution Volume

Then it calculates how much of your body the alcohol spreads into, counting lean and fat tissue separately: Vd = 0.65 × lean mass + 0.14 × fat mass (litres, kilograms). Each kilogram of lean tissue counts almost five times as much as a kilogram of fat.

The model is checked against a published controlled drinking study (Jones & Jönsson, 1994): for a typical man it reproduces the study’s average peak BAC.

The result? Alcophone provides BAC estimates that account for your actual body composition — not just your weight.

Frequently Asked Questions

What does a BAC of 0.08% mean?

It means 0.08 grams of alcohol in every 100 milliliters of blood — about one part alcohol in 1,250. It is the driving limit in every US state except Utah, which uses 0.05%; see BAC limits by state. Impairment starts well below it, as What does a BAC of 0.05 vs 0.08 actually feel like? describes.

How fast does BAC go down?

At about 0.015% per hour on average, a pace set by your liver. Coffee, water, a cold shower and exercise do not change it; after a real meal it runs somewhat faster, but never enough to count on (why each one fails). From 0.08% it takes more than five hours to reach zero — long enough to still be over a limit the morning after a late night.

Is a breathalyzer more reliable than a BAC calculator?

They answer different questions. A breathalyzer measures the alcohol in your breath at this moment and converts it to a blood figure; a calculator models the whole evening from what you drank, including alcohol that is still being absorbed. A personal breathalyzer depends on its calibration and on waiting out alcohol left in the mouth. Breathalyzer or BAC calculator: which one should you trust? compares the two.

Why does body fat change the result at the same weight?

Alcohol goes into water-rich lean tissue, not into fat, so the volume it is diluted in follows your lean mass. The free BAC calculator on this site uses the same body-composition method as the app, and the BAC chart shows the averages by weight.