Tracking Drinks: Standard Units of Alcohol

writing with computer or paper

One of the simplest concepts in alcohol reduction and moderation, tracking one’s alcohol-containing drinks, can also be one of the most confusing to understand.

Perhaps part of the confusion comes from the often touted and complete oversimplification of one shot = one beer = one glass of wine. Unfortunately, this one equation could be part of the reason people are generally bad at figuring out exactly how much alcohol they really drink. This way of counting drinks fails to account for differences in the alcohol content by volume, as drinks within the same category can have vastly different strengths.

Standard units of alcohol are not standard 

Another issue is that, on a global scale, there is little standardization to how a “standard unit” of alcohol is defined. The so-called standard unit changes based on what country you are looking to for your definition. That is because each country uses a different base measurement as the “standard.” These differences can be significant. Some countries, such as Iceland and the U.K. count as few as 8g of pure alcohol as one standard unit, while in Austria it takes 2.5 times the amount of pure alcohol (20g) to equal one standard unit.

Using technology to track alcohol consumption

The C Three Foundation has developed free drink tracking tools. The TSM Drink Log is available on both iOS and Android devices, and the Excel spreadsheet for tracking drinks can be used by those with Microsoft Office or downloaded as a Numbers spreadsheet on a Mac.

The TSM Drink Log app will automatically calculate the number of standard drinks based on user input. The app allows users to select their country (US or UK). For those who select US, the standard unit is based on 14g of pure alcohol per drink. Those who select the UK will have standard units based on 8g of alcohol.

The spreadsheet, however, does not automatically calculate standard drinks. While it will still show accurate data if the user always drinks the same alcoholic beverage, the only way to track using standard drinks is to manually calculate it.

How to calculate a standard drink

There are four pieces of information you need to know in order to calculate a standard drink:

  1. The volume of your drink,
  2. The alcohol by volume (ABV),
  3. The alcohol density constant (789g/L), and
  4. The standard drink guidelines for your country

Step #1 – Find the pure alcohol mass contained in the drink

This step will stay constant around the world. The formula isn’t incredibly difficult, but it’s not the easiest to find with a simple search.
 
Volume (L) x ABV x pure alcohol density (g/L at 20 degrees C) = pure alcohol mass (g).

Step #2 – Calculate the number of standard drinks in the pure alcohol mass

Once you know the pure alcohol mass in grams, divide that number by the number given by your country for a standard unit of alcohol.

For example, if you have an Imperial pint (568ml) of a popular Irish stout (4.2% ABV), and you live in the US where one standard unit is defined as 14g of pure alcohol, then you would calculate the standard drinks like this:

.568L x 0.042 x 789g/L = 18.8g
18.8g/14g = 1.34 standard units of alcohol

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