Q: Are online blood alcohol calculators a good tool for determining how much alcohol you can drink before reaching the legal limit for driving?
A: No, don’t count on them. These free online calculators—including one from the Cleveland Clinic, and another from the website DrivingLaws.org—provide only a rough idea of how much alcohol it takes for you to reach the maximum legal blood alcohol concentration (BAC) for driving. They are typically based on your sex (women reach a higher BAC more quickly, on average, than men for an equal amount of alcohol consumed), age (alcohol’s intoxicating effects increase as you get older), body weight, how many drinks you had and their percent alcohol, and how fast you consumed them. You plug in the information and the calculator provides a BAC number, sometimes indicating the corresponding level of impairment you might feel.
The legal BAC limit is 0.08% for drivers ages 21 and older in all U.S. states except Utah, where the limit was lowered to 0.05%. Drivers under 21 face zero or near-zero tolerance limits (0.0% to 0.02%, depending on the state).
But there are many disclaimers since these simple calculators don’t take into account the many other factors that can affect BAC, including how much body fat and muscle you have, as well as your metabolism, food and fluid intake, health status, and use of medications. That is, the measurement may not reflect real-life drinking conditions. One website says outright that the reading is not accurate. Thus, you should not use these calculators to determine if it is safe for you to drive after drinking alcohol—or let them give you a false sense of security. If you consume even a small amount of alcohol, it’s best to have someone else do the driving.
More practical, perhaps, is the other information some of these websites provide, such as about DUI legislation, myths about alcohol use, and how to drink responsibly. A table that gives BAC estimates corresponding to number of drinks consumed and body weight can be found at dui.drivinglaws.org/drink-table.php.
This article first appeared in the UC Berkeley Wellness Letter.
Also see Alcohol: Your Questions Answered.