How BAC Works
The science behind Blood Alcohol Content — and why most apps get it wrong
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.
BAC Risk Levels
| BAC Level | Risk | Typical Effects |
|---|---|---|
| 0.00 – 0.02% | Safe | Little to no effect |
| 0.02 – 0.05% | Low | Slight euphoria, relaxation, mild impairment |
| 0.05 – 0.08% | Caution | Reduced coordination, impaired judgment |
| 0.08 – 0.15% | Danger | Legally impaired, significant motor skill loss |
| 0.15%+ | Severe | Severe impairment, risk of blackout, medical danger |
How Your Body Processes Alcohol
Absorption
Alcohol is absorbed through the stomach lining (~20%) and small intestine (~80%). This process takes 15–45 minutes on an empty stomach, longer with food. Only about 80% of consumed alcohol reaches your bloodstream — the rest is lost to first-pass metabolism in the stomach.
Distribution
Once in your blood, alcohol distributes throughout your body. This is where most calculators get it wrong. The old Watson-Widmark formula assumes alcohol distributes evenly in body water. It doesn’t. Research by Norberg et al. (2003) proved that alcohol distributes 65% in lean tissue and only 14% in fat tissue.
Metabolism
Your liver breaks down alcohol at a relatively constant rate of about 0.015% BAC per hour. This varies based on your weight, age, sex, and liver health. Nothing speeds this up — not coffee, not water, not food, not exercise. 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 Most BAC Calculators Are Wrong
The majority of BAC calculators and apps use the Watson-Widmark formula, developed in the 1980s based on European populations. Here’s why that’s a problem:
Watson-Widmark (1980s)
- Assumes alcohol distributes evenly in body water (TBW)
- Calibrated on European populations
- Doesn’t account for body fat differences
- Treats a 200 lb athlete and 200 lb sedentary person identically
- Can underestimate BAC by up to 25% in people with higher body fat
Deurenberg + Norberg (Alcophone)
- Accounts for different alcohol distribution in lean vs. fat tissue
- Calibrated for US body composition data
- Adjusts for the 42.4% US obesity rate
- Gives different results for different body compositions at the same weight
- Based on peer-reviewed research (Norberg et al., 2003)
A Real Example
Consider two people who both weigh 180 lbs (82 kg), are male, and are 35 years old. They each drink 3 beers (12 oz, 5% ABV).
| Person A (Athletic, 12% body fat) | Person B (35% body fat) | |
|---|---|---|
| Watson-Widmark BAC | 0.065% | 0.065% (same!) |
| Alcophone BAC | 0.063% | 0.082% |
| Difference | Minimal | Watson underestimates by 26% |
| Legal status | Under limit | Over the legal limit |
These are estimates for illustration. Actual BAC varies based on many individual factors.
What Makes Alcophone Different
Alcophone uses a two-step approach based on published scientific research:
Deurenberg Body Fat Estimation
First, Alcophone estimates your body fat percentage using the Deurenberg formula (1991), with corrections calibrated specifically for the US adult population. This accounts for the significantly higher average body fat in American adults compared to the European populations used in older formulas.
Norberg Distribution Volume
Then, it calculates your alcohol distribution volume using the Norberg method (2003), which treats lean mass and fat mass separately: Vd = 0.65 × lean mass + 0.14 × fat mass. This gives a much more accurate picture of how alcohol actually distributes in your specific body.
The result? Alcophone provides BAC estimates that account for your actual body composition — not just your weight.
Premium takes it further
Add your body measurements (neck, waist, hips) for Navy Method precision — up to 30% more accurate than the free tier.
See Your Real BAC
Download Alcophone — science-backed BAC tracking, free on Google Play.
Alcophone provides BAC estimates only. It is not a breathalyzer and cannot measure your actual blood alcohol content. Never drive after drinking, regardless of what any app says. Must be 21+ to use. © 2026 SafeMetrics4U. All rights reserved.