In Denmark, where I live, we unfortunately hold a record within Europe: our kids are the heaviest drinkers of alcohol on the continent. Because of this, there’s a strong focus on reducing youth alcohol consumption and educating kids about the effects of alcohol.

I put together an alcohol calculator in HTML and JavaScript for our local school to show students how alcohol affects the body and how blood alcohol content (BAC) is calculated.

How is BAC Calculated?

To estimate BAC or "promille," you need a few key pieces of information:

• Your body weight – since a larger body can dilute alcohol more than a smaller one.
• Biological gender – due to different body water content levels, which affect alcohol distribution in the body.
• The number of alcohol units consumed – because each type of drink has a different alcohol concentration.

Calculating Alcohol Units

Different drinks contribute different amounts of alcohol to your bloodstream, depending on their volume and alcohol concentration. To standardize this, the unit of alcohol calculation is typically done as follows:

volume (cl) * alcohol percentage * 0.8 / 120

Structure

The entire "app" is structured as a <form> with organized fieldsets and named controls, making it easy to extract the necessary elements with:

const { add, addbeverage, etc. } = app.elements;

Since BAC limits vary by region, we need to first select a region, adjust the weight slider, and choose a biological gender:

Next, we can start adding beverages:

This is a simple <fieldset>, where the individual fields for units, volume and alcohol percentage are disabled (and hidden via CSS), until you make a selection from the beverage-selector:

The visibility of these fields is controlled from a enableElements-function:

[percentage, units, volume].forEach(el => el.disabled = !bool);

When we then add the beverage, a list of consumed beverages is created, and the BAC, estimated hours to sober up, etc., are calculated:

You can then continue to add more beverages, and the BAC updates accordingly:

It was very surprising for me to learn, that the legal BAC in the U.S. is 4x greater than that of Sweden or Norway!

In other words, in Sweden, you could lose your license for drinking an amount that would be legally permissible in the U.S.

Breaking Down the BAC Calculation

The core calculation for BAC is handled in the calculateAnswer function:

function calculateAnswer() {
  const alcoholMetabolismRate = 0.015;
  const currentBAC =
    ((total * 10) /
      (weight.valueAsNumber *
        1000 *
        parseFloat(bodywater.value))) *
    100;
  const hoursToSober = (
    currentBAC / alcoholMetabolismRate
  ).toFixed(1);
  const canDrive =
    currentBAC <= parseFloat(limit.value);
  answer.value = `Current BAC: ${currentBAC.toFixed(2)}%. 
  Time to sober: ${hoursToSober} hours. ${
    canDrive
      ? "You are fit to drive."
      : "You are NOT fit to drive."
  }`;
}

Let’s break it down:

1. Alcohol Metabolism Rate: alcoholMetabolismRate = 0.015

   • This value represents the average rate at which the body reduces BAC per hour (approximately 0.015%).

2. Calculating Current BAC: currentBAC = ((total * 10) / ((weight.valueAsNumber * 1000) * parseFloat(bodywater.value))) * 100

   • total * 10: This converts the total alcohol units consumed into grams (since one unit equals about 10 grams of pure alcohol).
   • weight.valueAsNumber * 1000: Converts body weight from kilograms to grams for calculation purposes.
   • bodywater.value: A decimal factor based on biological gender, affecting how alcohol distributes in the body (e.g., 0.58 for males, 0.49 for females).
   • The result is then multiplied by 100 to convert BAC to a percentage.

3. Estimating Hours to Sober: hoursToSober = (currentBAC / alcoholMetabolismRate).toFixed(1);

   • Divides the currentBAC by the metabolism rate to estimate the time needed for BAC to reach zero.

4. Determining Legal Driving Status: canDrive = currentBAC <= parseFloat(limit.value);

   • Compares currentBAC to the selected legal BAC limit. If the current BAC is within the limit, the user is considered "fit to drive"; otherwise, they are not.

The calculateAnswer function then combines these values into a message that shows the current BAC, hours until sober, and whether the user is fit to drive.

… And that concludes this tutorial. Feel free to experiment with the CodePen if you’d like — perhaps by creating a more child-friendly UX. Just remember, this tool provides an estimate and doesn’t account for factors like the time you consumed each beverage. And keep in mind, it will most likely be used by sober people!

Demo

Cover by DALL·E, from the prompt: Generate a drunk car in a cartoonish style similar to early Disney and Cupheads, as an image I can crop to 1000x420px.