How not to lose your licence: the arithmetic

The four variables

Blood alcohol concentration at any moment depends principally on:

• how much alcohol you have consumed;
• over what period you consumed it;
• your sex; and
• your body weight.

Medical conditions, medication, food and individual metabolism affect it as well, but those four do most of the work.

The effect of the last two is larger than people assume. A 100kg man who has four drinks over two hours will generally show a materially lower reading than a 50kg woman who has the same four drinks over the same two hours. Matching someone drink for drink is not the same as matching their reading.

A standard drink is not a drink

Most people know that alcohol is measured in standard drinks, and that a standard drink is 10 grams of pure alcohol.

Almost all the rules of thumb people carry around are expressed in standard drinks. The difficulty is that nothing you actually buy comes in standard drinks. Wines vary in strength and glass size, beers vary in strength and vessel, and spirits vary in pour.

This is where people come unstuck. It is the "I only had one glass of wine" problem. One glass of wine is sometimes less than one standard drink and sometimes more than two.

Two questions matter every time:

1. How much alcohol is in the thing you are drinking?
2. How much of it are you drinking?

Both answers vary enormously:

Nobody is suggesting you examine bottles with a magnifying glass on a Friday night. Two habits cover most of it. Know roughly what strength you are drinking, and know roughly how much of it you have had. That is easy with cans and bottles and considerably harder with poured drinks in glasses of varying size, especially when somebody keeps topping you up.

Three worked examples

These use a publicly available BAC calculator and are illustrative only. Individual readings vary, and a calculator is not a defence.

Adam

Adam is a 100kg man who has three schooners of full strength beer in an hour after work. He may not feel particularly affected. His estimated reading is around 0.06, which puts him over the general alcohol limit. He should not drive, and it will take roughly four hours to clear.

Jennifer

Jennifer is a 55kg woman who goes round for round with Adam. Same three schooners, same hour. Her estimated reading is around 0.134, which is more than double his and sits in the mid range band. It will take her nearly nine hours to return to zero.

Same drinks, same time, very different legal position. That is the point of the example.

Michael

Michael is a 60kg man who has three glasses of wine over two hours without paying attention to the pour. The venue was serving large glasses generously, so he has actually consumed around 5.4 standard drinks. Despite having had "only three glasses", his estimated reading is around 0.096, close to double the general limit. It will take around six and a half hours to clear.

Michael is the most common version of this. Nothing about his evening felt excessive to him.

The morning after

The clearance times above are the reason a great many drink driving charges are laid before 9am.

Alcohol clears at a roughly fixed rate that you cannot speed up. Coffee, food, a shower and a night’s sleep make you feel better without changing the reading. Six hours of sleep after a heavy night frequently leaves a person over the limit at breakfast.

If you were at the high end of consumption, work on the basis that eleven hours or more may be needed. Plan the next morning before you start drinking, not while you are looking for your keys.

What is at risk

Queensland splits drink driving above the general limit into three bands, each with its own consequences. We cover them separately in our guides to low range, mid range and high range drink driving.

Whichever band applies, you are exposed to licence suspension or disqualification, a fine, and in more serious cases imprisonment. Beyond that there is the ordinary business of explaining to an employer, a family and a partner why you cannot drive anywhere for the next several months.

And there are worse outcomes than losing a licence. Keeping the arithmetic in mind is mostly about avoiding those.