When is it safe for me to drive after drinking?

Question: When is it safe for me to drive after drinking? How long do I have to wait before I can safely drive? How long does it take alcohol to leave my body?

Answer: It is safest to drive when all the alcohol has left your body. Alcohol is metabolized at the rate of 0.015 of blood alcohol concentration (BAC) every hour, regardless of age, weight, gender, or activities done after the alcohol was consumed (such as eating or sleeping). 

To calculate the amount of time it takes for the alcohol to get out of your system, you need to calculate your BAC and then divide the answer by 0.015. 

How do I calculate my body’s BAC? 

This is a tricky calculation, because so many variables influence the outcome, such as

  • your age
  • your weight
  • your gender
  • how long you have been drinking
  • and of course, how much you drank (as measured in Standard Drinks) 

To obtain an estimate of your BAC, you can use the blood alcohol calculator here

Step Two: Divide by 0.015

After you divide your BAC by 0.015, the resulting answer is the amount of time it takes to metabolize that much alcohol. 

For example, a 150 pound male drinking 1 beer in 15 minutes will have a BAC of 0.023. 

0.023 divided by 0.015 is about 1.5. Thus, it would take an hour and a half for his BAC to fall to zero. 

Or say the same person drinks 8 cans of beer in 4 hours. It would then take approximately 10 hours for his BAC to reach zero. 

When will it be legal for me to drive after drinking?

Since the legal driving limit is a BAC of 0.08, you will not have to wait as long compared to those waiting for their BAC to reach zero.

But how long exactly do you have to wait? To find out, subtract 0.08 from your calculated BAC. Then, divide that number by 0.015.

The result is the approximate amount of time you must wait to have a BAC of 0.08. Since all these numbers are approximations, I strongly advise you to wait another hour or so, to make sure your BAC actually falls BELOW 0.08. 

Keep in mind that just becuase a BAC of 0.08 is legal, it's not garaunteed safe. Studies have shown impairment begins when the BAC can be as low as 0.02—after just one drink.

According to the United States Drunk Driving Car Accident Statistics (2009), there is a higher percentage of accidents involving people with a BAC lower than the legal limit of 0.08 than those with a BAC that exceeds it. The longer your wait, the safer you--and other drivers on the road--will be. 

Remember: Only Time Can Sober You Up

Let’s assume our 150 pound male friend was at a late night party. 

He arrived at 9:30, partied hard, finished off 20 shots of hard liquor, with his last drink at 1:30 am. Collapsing into bed, he slept for 10 hours, took a shower, took some aspirin, ate some breakfast, and got into the car by 12:30 pm. 

Even though he had slept and had something to eat, he could still be pulled over for drunk driving: his BAC would be 0.309--still way above the legal driving limit. (Calculation: BAC of 0.474 minus amount metabolized during the next 11 hours)

The fact that he ate and slept between the party and the driving has minimal effect on the body’s rate of metabolism: only time can sober you up. 

Sources

http://www2.potsdam.edu/hansondj/drivingissues/1127227453.html

http://bloodalcoholcalculator.org/

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