You’re at a party, halfway through a nice bottle of pinot, when you discover your designated driver forgot it was his turn behind the wheel. You think, If I stop drinking now, I should be OK to drive by the time the evening ends, right? Maybe. But maybe not.

Legally, you’re considered intoxicated if your blood alcohol content (BAC) reaches .08% or above. That can occur after two drinks for an average-size woman and after three drinks for an average man. A rule of thumb states you’ll stay within safe limits if you restrict your drinks to one per hour—with a drink defined as a 12-ounce beer, 5 ounces of wine, or 1.5 ounces of 80-proof liquor.

Try the If I Drink BAC calculator with first person POV videos to see how you would drive, ride a bike, or drive a car after drinking.

But that bit of conventional wisdom doesn’t work for everyone, not only because each person processes alcohol differently but because a wide range of factors comes into play whenever we take a drink. Here are just a few of the variables:

What’s in your stomach. Not even the biggest meal can negate the effects of alcohol, but food can slow its absorption. A person who drinks on an empty stomach can reach a peak BAC in as little as 30 minutes. In contrast, a person who has eaten (protein is best) will generally reach a peak BAC in one to six hours. But don’t depend on food to keep you sober. Its only result may be to give you more to clean off your shoes later.

Your body type. You may think those extra pounds you’ve been carrying around will finally do you some good. After all, everyone knows the more you weigh, the less you’re affected by alcohol. But, in one of those annoying metabolic twists, it turns out that muscle tissue absorbs alcohol faster than fat tissue. That means if two people weigh the same and drink the same amount, the person with more body fat will have a higher BAC than the person with less fat.

What’s in your drink. Not all drinks are created equal. Drinks with high alcohol content or multiple types of alcohol can quickly boost BAC. In addition, carbonated alcoholic drinks, such as champagne or alcohol mixed with soft drinks, and hot drinks, such as specialty coffee drinks, are generally absorbed rapidly, which will, in turn, increase your BAC. And don’t forget that mixed drinks vary with the heavy-handedness of the pourer: One gin and tonic concocted by crazy Uncle Billy may pack the punch of three from Aunt Barbara.

Your gender. Women are at a natural disadvantage when it comes to holding their liquor, due to several factors that boost BAC levels. They generally have more body fat, they have smaller amounts of body water (body water dilutes the effects of alcohol), and they have less alcohol dehydrogenase, an enzyme that breaks down alcohol in the stomach. This is particularly true of young women. They also have hormonal fluctuations that can cause a higher BAC prior to their period.

Your mood. “I need a drink,” you may hear yourself saying at the end of a stressful day. But this can be the worst time of all to turn to alcohol. Stress can affect the stomach’s ability to process alcohol, which can in turn boost BAC. Although small amounts of alcohol can temporarily seem to improve mood, that feeling can easily turn to depression or anxiety as BAC increases. And that makes it harder to recognize when it’s time to put down the glass.

Your tolerance level. If you drink regularly, you may develop a tolerance for alcohol. This doesn’t mean you’ll be any less intoxicated, only that you won’t show it as much, leading to a false sense of sobriety for you and those around you. That’s an especially dangerous mindset when determining whether you’ve had too many. Saying you “feel fine” won’t do you much good when an officer pulls you over.

Medications. Countless medications, including antidepressants, sedatives, antibiotics, pain killers, cold medications and even aspirin can enhance the effects of alcohol as well as lead to dangerous side effects. Keep in mind that even herbal remedies and supplements can interact with alcohol. Just because it says “natural” on the label doesn’t mean it’s harmless.

Dehydration and/or fatigue. Dehydration and fatigue can impede the liver’s ability to process alcohol, which can in turn boost BAC. That’s why too much time in the hot sun and too much cold beer can be a bad combination. Alcohol also acts as a diuretic, meaning as you drink, your kidneys are directing fluid straight to the bladder, which can cause dehydration. In essence, the more liquid you drink, the more you lose.

Age. As we age, our body composition changes. We lose muscle tissue and body water, and our metabolism slows. We also are typically taking more medications than when we were younger. These changes, separately and collectively, can affect our level of impairment and BAC. If you need proof, just ask any 50-year-old how his body handles a six-pack of beer now compared to when he was in college. 

Adding Up Your BAC

While there’s no magic calculator that can crunch your personal data and tell you exactly what your BAC would be in any given situation, there are tools that can offer up eye-opening peeks into what we look like after a few drinks. One is a simulated drinking app that allows you to plug in some details—such as gender, weight and time period—and then see how you might drive, ride a bike or walk a line after drinking.

video bac calculator
BAC Calculator with First Person POV Videos

The simulation also serves as a reminder that DUIs come in many different forms. It’s possible, for example, in some states to get cited for drunk bicycling. Although cycling while impaired might seem like a lesser danger than driving while impaired, cycling drunk is nonetheless dangerous. Nearly 25 percent of bicycling deaths involve an intoxicated rider, according to a 2012 government survey. In addition, researchers at Johns Hopkins University report that having a BAC level at or above the legal limit raises a bicyclist’s risk of serious injury by 2,000 percent. Yes, 2,000.

Even walking drunk carries often-overlooked risks. In 2011, pedestrian deaths accounted for 14 percent of all traffic fatalities, and more than one-third of pedestrians killed in traffic accidents in 2011 had blood BAC levels above the legal driving limit. Remember this when you decide to walk home after that party.

Unfortunately, the moment when we most need to make an accurate assessment of our BAC is the exact time when we are least capable of doing so—when we’re drinking. It becomes the equivalent of advanced math: Calculate body mass times the number of drinks consumed per hour plus age times the level of mood divided by amount eaten … And we expect ourselves to do all this mental activity after three margaritas?

Why put yourself through it? Call a cab. After all, the only truly safe level of intoxication—whether driving, biking or walking—is no intoxication at all.

David Sack, M.D., is board certified in Addiction Medicine and Addiction Psychiatry.  As CEO of Elements Behavioral Health he oversees treatment centers such as Promises in California, the Ranch rehab in Tennessee, and Florida rehab The Recovery Place. Dr. Sack regularly appears in the major media as an expert on addiction and recovery.

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