Alcohol units: how much can I drink and drive?

This is what you need to know about alcohol units and driving

alcohol units and driving
(Image credit: Getty)

The issue of alcohol units and driving can be confusing, with most people researching the phrase 'how much can I drink and drive' wanting to know just how many units of alcohol will keep them within the legal limit. 

How many units can I drink and drive?

The legal limit for alcohol and driving is defined not in units, but in the amount of alcohol in your system. The current legal limit is:

  • 80 milligrams of alcohol per 100 millilitres of blood, 35 micrograms per 100 millilitres of breath, or 107 milligrams per 100 millilitres of urine
  • If you're in Scotland, it's 50mg of alcohol per 100ml of blood, 22 micrograms of alcohol per 100ml of breath, or 67mg of alcohol per 100ml of urine

How does this translate into units? There's no one-size fits all. The main factors that affect how much alcohol you can consume safely before driving are your weight, age, and sex. However, other factors will also affect how you metabolise alcohol, including whether you also consumed any food, what your stress levels were prior to drinking, and the type of alcohol you were drinking. 

With so many variables, it's nearly impossible to say how much alcohol is safe to drink before driving for any given person. Even a small amount of alcohol can affect your reactions and judgment while driving, so the best option is to not drink any alcohol if you're driving. 

How many units are in what I'm drinking? 

Remember, units are not a reliable way to work out how much alcohol is safe for you to drink if you're driving. However, for your general information:

  • A standard 175 ml of 13 per cent wine has 2.3 alcohol units
  • A pint of 4 per cent lager also has 2.3 units
  • A single shot of a 40 per cent spirit equals one unit of alcohol

How long should I wait before driving after drinking?

Again, this is a complex matter. On average, it takes a human body one hour to process one unit of alcohol. So technically, if you drank half a pint of medium-strength beer or lager (4 per cent), you would consume one unit of alcohol and would need to wait at least an hour before driving safely. 

However, individual metabolic rates come into play here; how fast your body breaks down alcohol may be slower than the average, so going by these estimates, especially if you've had more than one drink, is not safe. 

It is not true that you can sober up by taking a shower, drinking coffee, or sleeping: you cannot speed up the metabolic processes involved in breaking down alcohol in this way. 

The conclusion? If you're driving, always swap alcohol for soft drinks or alcohol-free beer. 

Anna is Content Editor at Real Homes. She moved to the world of interiors from academic research in the field of English Literature and photography. She is the author of London Writing of the 1930s and has a passion for contemporary home decor and gardening. At Real Homes, she covers a range of topics, from practical advice to interior and garden design.