BreathometerBAC Calculator

Our Methodology

A comprehensive breakdown of our scientifically-backed BAC calculation approach

Overview

Our BAC calculator uses the Watson equation (1980) for Total Body Water (TBW) calculation, combined with a two-compartment pharmacokinetic model to accurately simulate alcohol absorption and metabolism.

This approach is significantly more accurate than traditional simplified methods, accounting for individual physiology including age, height, weight, and biological sex.

Step 1: Total Body Water (Watson Equation)

The Watson equation calculates Total Body Water (TBW) based on your physical characteristics. TBW represents the total amount of water in your body, which is crucial because alcohol distributes in body water.

Males

TBW = 2.447 - (0.09156 × age) + (0.1074 × height_cm) + (0.3362 × weight_kg)

Females

TBW = -2.097 + (0.1069 × height_cm) + (0.2466 × weight_kg)

Why Watson? The Watson equation is widely recognized in pharmacokinetics and provides more accurate body water estimates than simple weight-based calculations, especially across different body types and ages.

Step 2: Volume of Distribution

Alcohol doesn't distribute uniformly throughout all body water. We convert TBW to the effective volume of distribution (Vd) using blood water content factors.

Vd = TBW / bloodWaterContent
Males: bloodWaterContent = 0.825 (82.5%)
Females: bloodWaterContent = 0.670 (67.0%)

Critical Factor: This conversion is essential for accurate BAC calculation. Blood has a slightly different water content than average body tissues, and alcohol concentrates more in blood plasma.

Step 3: Two-Compartment Pharmacokinetic Model

We simulate alcohol movement through two compartments: stomach (absorption) and blood (metabolism). This models how alcohol is absorbed over time and metabolized simultaneously.

Stomach Compartment

  • • Alcohol enters when drinks are consumed
  • • Absorbs into blood at varying rates
  • • Food level affects absorption speed
  • • Empty stomach: 1.0x (fastest)
  • • Light meal: 0.7x (moderate)
  • • Full meal: 0.5x (slowest)

Blood Compartment

  • • Receives absorbed alcohol from stomach
  • • Liver metabolizes at constant rate
  • • Standard elimination: 0.015 BAC/hour
  • • Metabolism is independent of BAC level
  • • Zero-order kinetics (constant rate)

Calculation Per Time Interval (5 minutes)

// Absorption from stomach to blood
alcoholAbsorbed = min(alcoholInStomach, absorptionRate × timeInterval)
// Metabolism (removal from blood)
alcoholMetabolized = min(alcoholInBlood, metabolismRate × Vd × 10 × timeInterval)
// Update compartments
alcoholInStomach -= alcoholAbsorbed
alcoholInBlood += alcoholAbsorbed
alcoholInBlood -= alcoholMetabolized

Step 4: BAC Calculation

At each time point, we calculate BAC from the amount of alcohol currently in the blood and your volume of distribution.

BAC = (alcoholInBlood / Vd) / 10

Division by 10 converts from g/L to g/dL (percentage)

< 0.02
Sober
0.02-0.05
Mild Impairment
0.05-0.08
Moderate Impairment

Step 5: Distribution Patterns

We distribute your drinks over the session time according to realistic consumption patterns.

Steady

Drinks evenly distributed throughout the session

Frontloaded

Most drinks consumed early in the session

Backloaded

Most drinks consumed late in the session

Bell Curve

Drinking peaks in the middle of the session

Timeline Calculation

We calculate your BAC every 5 minutes from 30 minutes before your session starts until 30 minutes after BAC reaches zero, providing a complete picture of your alcohol metabolism journey.

  • Padding: 30 minutes before and after for context
  • Interval: 5-minute increments for smooth curves
  • Peak Detection: Automatically identifies maximum BAC and time
  • Zero Time: Calculates when BAC drops below detectable levels
  • Legal Limit: Tracks time spent above your country's legal driving limit

Accuracy & Limitations

Strengths

  • ✓ Uses peer-reviewed Watson equation for body water calculation
  • ✓ Two-compartment model accurately simulates absorption and metabolism
  • ✓ Accounts for individual physiology (age, height, weight, sex)
  • ✓ Considers food consumption effects on absorption
  • ✓ Models realistic drinking patterns over time

Limitations

  • ⚠ Individual metabolism varies (genetics, liver health, medications)
  • ⚠ Estimates assume standard absorption and elimination rates
  • ⚠ Does not account for individual tolerance or impairment perception
  • ⚠ Not a substitute for professional breathalyzer measurements
  • ⚠ Should not be used to determine driving fitness

⚠️ IMPORTANT: This calculator is for educational purposes only. Never drink and drive. Always use a designated driver or rideshare service if you've been drinking.