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Using percentages – Alcohol and equivalent “standard drinks”

These notes will help you with the activities in the Booze in blood CD. They show how to work out standard drinks using a calculator.

If you’re having a big night out, you need to keep track of the amount of alcohol you’ve had and the time you’ve been drinking. Factors like gender, weight and metabolism change how each person is affected by alcohol. They alter how quickly the alcohol is absorbed into the bloodstream and thus the individual’s blood alcohol concentration (BAC).

In Australia, alcoholic drinks are labeled to tell you the volume (how much liquid they contain) and the alcoholic strength (what percentage of the liquid is alcohol). In Australia, “a standard drink" contains 10 grams (about 12.5ml) of alcohol.

In order to drive a car legally in Australia, a person’s blood alcohol concentration (BAC) must be below 0.05. These guidelines are recommended to keep BAC below 0.05.

  • Men can drink 2 standard drinks in the first hour and one standard drink each hour after that.
  • Women can have one standard drink per hour.
  • When drinking stops, most people get rid of about one standard drink per hour.

To predict whether someone is OK to drive, you need to

  • calculate how much alcohol they’ve had in millilitres(mls) using the liquid volume and alcoholic percentage strength for each drink
  • change the mls of alcohol into an equivalent number of standard drinks
  • consider the time factor

Here is an example to show how it works.

Gill drinks 2 stubbies and a shot of bourbon over a 3-hour period. How much alcohol has she drunk altogether? What number of standard drinks is this? Is her blood alcohol concentration likely to be above or below 0.05?

To work this out we break the calculation up. Let’s deal first with the beer.

How many mls of alcohol in the stubbies? / A Stubby holds 375 ml of beer. The beer is 4.9 % alcohol in strength.
To work out how many milliliters of alcohol in one beer, we multiply the volume (375ml) by the alcoholic strength (4.9%)
375 x 4.9% = 18.4
So in one stubby, there is 18.4ml of alcohol.
If Gill drinks more than one stubby, we multiply the amount of alcohol in one stubby by the number of stubbies she’s had.
She drank 2 stubbies, so she’s had 18.4 X 2 = 36.8 ml alcohol from the stubbies
How many mls of alcohol in the bourbon shot? / Now let’s find out how much alcohol in the shot of bourbon.
A shot glass holds around 30ml of liquid. Spirits like bourbon, vodka, whiskey and rum are much stronger than beer and are around 40% alcohol.
To work out how many milliliters of alcohol in one shot of bourbon, we multiply the volume (30ml) by the alcoholic strength (40%)
30 x 40% = 12
So in one shot of bourbon, there is 12ml of alcohol.
What total amount of alcohol? / To find out the total amount of alcohol Gill had, add up the mls of alcohol in all the drinks she had.
2 stubbies 36.8 ml
1 shot 12.0 ml
Total 48.8 ml
Change mls of alcohol into equivalent standard drinks / In Australia, the standard drink contains 10 gram or about 12.5ml of alcohol. So to find how many standard drinks Gill had, we divide the total volume of alcohol (mls) by 12.5
48.8 ÷ 12.5 = 3.9
Gill has drunk the equivalent of 3.9 or, rounding off, 4 standard drinks.
OK to drive?
Let’s think about BAC and time / Remember, to stay below a BAC of 0.05, men can drink 2 standard drinks in the first hour and one standard drink each hour after that. Women can have one standard drink per hour. If they stop drinking, most people get rid of about one standard drink per hour.
From the above calculations, Gill had about 4 standard drinks over 3 hours. Because Gill is female, the recommendation is one standard drink per hour. Gill would probably be over BAC 0.05 and thus unable to legally drive.
In contrast, a man who drank 4 standard drinks in 3 hours would probably be under 0.05 BAC and a legal driver.

Try this problem

Jarred had 3 cans (375ml) of pre-mixed Rum and Coke (alcohol strength 5%) and 4 glasses(120 ml) of wine (alcohol strength 12%) over a 4 hour period. How much alcohol has he drunk altogether? What number of standard drinks is this? Is his blood alcohol concentration likely to be above or below 0.05?

Rum & coke / 375 X 5% = 18.75
18.75 X 3 drinks = 56.25 ml
Wine / 120 X12% = 14.4
14.4 X 4 = 57.6 ml
Total / 56.25 + 57.6 = 113.85 So total alcohol is 113.85 ml
Equiv st drinks / 113.85 ÷ 12.5 = 9.1 So 9.1 standard drinks
Time?? / Jarred has drunk 9.1 standard drinks in 4 hours. Since Jarred is male, the recommendation is 2 +1+1+1 = 5 standard drinks over 4 hours to be below BAC 0.05. So Ben is probably well over BAC 0.05

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