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BAC Calculator

Estimate Blood Alcohol Content (BAC) based on drinks, weight, and time.

About this tool

Blood Alcohol Content (BAC) is a measure of the percentage of alcohol in your bloodstream and is a critical indicator of alcohol intoxication. This BAC calculator estimates your blood alcohol level based on how much alcohol you've consumed, your body weight, sex, and the time elapsed since you started drinking. Understanding your estimated BAC helps you make informed decisions about whether it's safe to drive, operate machinery, or engage in other activities that require mental clarity.

To use this calculator, enter the number of standard drinks you've consumed, your body weight, select your sex (which affects alcohol metabolism), and specify how much time has passed since drinking began. The calculator uses the Widmark formula, a widely accepted method in forensic science and public health, to estimate your BAC. Keep in mind that this is an estimate—actual BAC varies due to factors like food consumption, metabolism rate, drink strength, and hydration levels.

The results are provided for educational and harm-reduction purposes only and should never be used as a legal defense or as a substitute for a breathalyzer test. Many jurisdictions define legal driving limits (typically 0.08% in many countries), and police breathalyzers are the only reliable method for enforcement. Always prioritize safety: if you've been drinking, use a taxi, designated driver, or ride-sharing service rather than relying on an estimate.

Frequently Asked Questions

Code Implementation

def calculate_bac(
    drinks: list[dict],
    weight_kg: float,
    gender: str,
    hours_elapsed: float
) -> float:
    """
    Estimate Blood Alcohol Content using the Widmark formula.
    BAC = (alcohol_grams / (weight_kg * r * 10)) - (0.015 * hours)
    r = 0.68 for male, 0.55 for female

    drinks: list of {"abv": float (%), "volume_ml": float}
    WARNING: For estimation only. Never use to determine fitness to drive.
    """
    r = 0.68 if gender.lower() == "male" else 0.55
    total_alcohol_grams = sum(
        d["volume_ml"] * (d["abv"] / 100) * 0.789  # ethanol density = 0.789 g/ml
        for d in drinks
    )
    bac = (total_alcohol_grams / (weight_kg * r * 10)) - (0.015 * hours_elapsed)
    return max(bac, 0.0)

# Example: 80kg male, 2 beers (5%, 355ml), 1 hour elapsed
drinks = [
    {"abv": 5, "volume_ml": 355},
    {"abv": 5, "volume_ml": 355},
]
bac = calculate_bac(drinks, weight_kg=80, gender="male", hours_elapsed=1)
print(f"Estimated BAC: {bac:.4f}%")  # ~0.0367%

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