How many drinks does it take to get to .08? This is a common topic of conversation whenever I’m drinking with friends. In Washington, as in other states, having a BAC (blood alcohol concentration) above .08 within two hours of driving is one way in which a person can commit a DUI. But how many drinks does it take to get there?

The short answer is that is varies from person to person depending on his weight, gender, experience with alcohol and many other factors. In Washington, official BAC readings are obtained using a BAC Datamaster machine. So unless you’re packing one of these machines around with you when you go out drinking, you’re really only guessing. With that said, there are techniques you can use to get an estimate.

One such technique is using Widmark’s Formula, which is a mathematical equation that estimates a person’s blood-alcohol concentration based on weight, gender, the amount of alcohol he’s had and the amount of time since he started drinking. The formula says that a person’s BAC % = ((liquid alcohol consumed) x 5.14 / (weight) x (.73 for men OR .66 for women)) – (.015 x (hours since the person started drinking)).

Here’s an example to illustrate. Let’s say I go to dinner at 6:00 and finish at 8:00. While there I have three 12-ounce beers, each of which contains 5 % alcohol. What would my BAC be when I’m driving home? I consumed 1.8 ounces of liquid alcohol (each beer is 12 ounces, so I consumed 36 ounces of beer, and 5% of that beer was alcohol, equaling 1.8 ounces of liquid alcohol). I weigh 160 pounds. And 2 hours have passed since I started drinking. So the formula for me would be ((1.8 x 5.14) / (160 x .73) – .015 x 2), which equals a BAC % of .049.

A word of caution: just because this formula says my BAC should be below .08 doesn’t mean I should be driving. In reality, my BAC could be higher. And even if it was accurate, studies have shown that a person’s ability to drive is negatively affected well before his BAC reaches .08. Moreover, while having a BAC above .08 is one way a person can commit a DUI, a person is also guilty of DUI if his or her ability to drive is ‘appreciably affected’ by alcohol, regardless of his or her BAC. So not only could I be exposing myself to a potential DUI charge, but I could also be putting myself and others at serious risk.

This formula may seem a little impractical to use while you’re out drinking. For whatever reason, one’s ability to perform complex math equations tends decrease proportionally to the amount of alcohol one consumes. There are a variety of iPhone apps available that will do the calculation for you. Or you can simplify the formula further on your own, although this will make it less reliable. To do this, just assume each drink (1.25 ounce shot of hard alcohol, 12 ounce beer, 5 ounce glass of wine) is .6 ounces of liquid alcohol. Assuming this, you can figure out roughly based on your weight and gender how much each typical drink will raise your BAC. For me, its .026 (.6 x 5.14 / 160 x .73). For a 120-pound woman, it would be .039 (.6 x 5.14 / 120 x .66). Once you figure this number out for yourself based on your weight and gender, you can get a rough estimate of your BAC by just multiplying this number by the number of drinks you’ve had, then remembering to subtract .015 for every hour since you started drinking to account for the alcohol your body burns off.

While this can be a useful tool for trying to determine how much alcohol you have in your system, keep in mind that it’s only an estimate based on the population in general. Your body is unique, so you don’t really know how closely it conforms to these averages. And you also may not know exactly how many ounces are in your drink or its precise alcohol concentration. Ultimately there’s no way to know exactly how alcohol will affect your body, which is why it’s better to error on the side of caution. The inconvenience of limiting the number of drinks you consume or arranging a designated driver pales in comparison to the hassle, expense and humiliation of a DUI charge.

You can find more information about DUIs or contact me through my website if you have any questions at www.jeffstapleslaw.com