Blood Alcohol Content (BAC) is a measure of the amount of alcohol present in a person's bloodstream. It's typically expressed as a percentage (grams of alcohol per 100 milliliters of blood). BAC is a crucial indicator of intoxication and is used by law enforcement to determine if a person is legally fit to drive. Factors influencing BAC include the amount of alcohol consumed, body weight, gender, metabolism, and the time elapsed since drinking.
The Widmark Formula
A common method for estimating BAC is the Widmark formula. There are several variations, but a widely used one is:
BAC = (A * 5.14 / W * r) - (0.015 * H)
Where:
BAC is the Blood Alcohol Content.
A is the total amount of alcohol consumed, measured in ounces.
W is your body weight in pounds. (Note: Our calculator uses kilograms and converts internally).
r is the alcohol distribution ratio (or Widmark factor), which is approximately 0.68 for men and 0.55 for women. This accounts for the difference in body water content between genders.
H is the number of hours since the first drink.
How Our Calculator Works
Our calculator simplifies this by allowing you to input your weight in kilograms, select your gender, the number of standard drinks, and the time elapsed. Here's how the conversion and calculation happen internally:
Weight Conversion: Your weight in kilograms is converted to pounds: Weight (lbs) = Weight (kg) * 2.20462.
Alcohol Amount (A): A standard drink in the US contains about 0.6 ounces of pure alcohol. So, the total alcohol consumed in ounces is: A = Number of Drinks * 0.6.
Alcohol Distribution Ratio (r): Based on your selected gender, the appropriate 'r' value is used (0.68 for male, 0.55 for female).
Metabolism Rate: The formula subtracts 0.015% from the BAC for each hour that has passed since the first drink, representing the approximate rate at which the body metabolizes alcohol.
Final Calculation: The modified Widmark formula is applied using the converted values. The result is then multiplied by 100 to express it as a percentage.
Factors Affecting Accuracy
It's important to remember that this is an estimation. Actual BAC can vary due to many factors, including:
Food intake (drinking on an empty stomach leads to faster absorption)
Type of alcohol and mixers
Hydration levels
Medications
Individual metabolism variations
Recent consumption patterns
Disclaimer: This calculator is for informational and estimation purposes only. It is not a substitute for professional medical advice or a legal determination of sobriety. Never drink and drive. Always consult the legal limits in your jurisdiction.
function calculateBAC() {
var weightKg = parseFloat(document.getElementById("weight").value);
var gender = document.getElementById("gender").value;
var drinks = parseFloat(document.getElementById("drinks").value);
var hours = parseFloat(document.getElementById("hours").value);
// Clear previous error messages
document.getElementById("result-value").innerText = "–";
// Input validation
if (isNaN(weightKg) || weightKg <= 0) {
alert("Please enter a valid weight in kilograms.");
return;
}
if (isNaN(drinks) || drinks < 0) {
alert("Please enter a valid number of drinks.");
return;
}
if (isNaN(hours) || hours < 0) {
alert("Please enter a valid number of hours.");
return;
}
// Constants and conversions
var ouncesPerStandardDrink = 0.6; // Standard alcohol content in ounces
var weightLbs = weightKg * 2.20462; // Convert kg to lbs
var alcoholConsumedOunces = drinks * ouncesPerStandardDrink;
// Alcohol distribution ratio (Widmark factor)
var r = (gender === "male") ? 0.68 : 0.55;
// Calculate BAC using Widmark formula
// BAC = (A * 5.14 / W * r) – (0.015 * H)
var bacEstimate = (alcoholConsumedOunces * 5.14 / (weightLbs * r)) – (0.015 * hours);
// Ensure BAC doesn't go below zero due to metabolism
if (bacEstimate < 0) {
bacEstimate = 0;
}
// Display the result, formatted to 3 decimal places
document.getElementById("result-value").innerText = bacEstimate.toFixed(3);
}