Blood alcohol content (BAC) is calculated using a formula that accounts for how much you drank, your body weight, your biological sex, and how much time has passed. The most widely used method is the Widmark formula, developed in the 1930s and still the foundation of forensic alcohol calculations today. With a few numbers and some simple math, you can get a reasonable estimate of BAC at any point after drinking.
The Widmark Formula
The simplified version of the Widmark equation that most people use looks like this:
BAC = (alcohol consumed in grams / (body weight in grams × r)) × 100 − (elimination rate × hours since first drink)
Each variable breaks down as follows:
- Alcohol consumed in grams: The total pure ethanol you’ve consumed, calculated from the number and size of your drinks.
- Body weight in grams: Your weight converted to grams (1 pound = 453.6 grams).
- r (the Widmark factor): A constant that reflects how water is distributed in your body. The standard values are 0.73 for men and 0.66 for women, though simplified charts often round to 0.7 and 0.6.
- Elimination rate: The rate your body clears alcohol, typically 0.015 per hour for most people.
- Hours since first drink: Time elapsed from when you started drinking.
What Counts as a Standard Drink
Before you can plug anything into the formula, you need to know how much pure alcohol you actually consumed. In the United States, one standard drink contains 0.6 fluid ounces (14 grams) of pure ethanol. That translates to 12 ounces of regular beer at 5% alcohol, 5 ounces of wine at 12%, or 1.5 ounces of distilled spirits at 40%.
This is where most estimates go wrong. A pint of craft IPA at 7.5% is closer to 1.5 standard drinks. A generous pour of wine at a dinner party can easily be 8 or 9 ounces, not 5. If you’re trying to calculate your BAC, measuring honestly matters more than the math itself.
To convert any drink to grams of alcohol: multiply the volume in milliliters by the alcohol percentage (as a decimal) by 0.789, which is ethanol’s density in grams per milliliter. A 12-ounce (355 mL) beer at 5% contains 355 × 0.05 × 0.789 = about 14 grams.
A Worked Example
Say a 180-pound man has three standard drinks over two hours. Here’s the math step by step.
First, convert body weight: 180 pounds × 453.6 = 81,648 grams. Next, calculate total alcohol: 3 drinks × 14 grams = 42 grams. Now plug into the formula:
BAC = (42 / (81,648 × 0.73)) × 100 − (0.015 × 2)
The first part: 42 / 59,603 = 0.000705. Multiply by 100 to get 0.0705. Then subtract the elimination factor: 0.015 × 2 = 0.03. Final BAC: 0.0705 − 0.03 = approximately 0.04%.
Now take the same scenario for a 140-pound woman drinking three standard drinks over two hours: (42 / (63,504 × 0.66)) × 100 − 0.03 = (42 / 41,913) × 100 − 0.03 = 0.1002 − 0.03 = approximately 0.07%. The difference is significant, driven by both lower body weight and a lower distribution factor.
Why the r Factor Differs by Sex
The Widmark factor (r) exists because alcohol distributes through body water, not through fat. Men carry proportionally more water and less fat than women of the same weight, so alcohol dilutes into a larger volume. Erik Widmark’s original experiments found an average r value of 0.68 in men and 0.55 in women, though these values ranged widely: 0.51 to 0.85 in men and 0.47 to 0.64 in women.
Body composition is the real driver here. Two people of the same weight and sex will get different BAC readings if one carries significantly more body fat. A higher body fat percentage means less water for alcohol to dissolve into, which pushes BAC higher. This is one reason the formula produces estimates, not exact readings.
How Your Body Eliminates Alcohol
The 0.015 per hour elimination rate is an average. Most people fall somewhere between 0.010 and 0.020 per hour. Your liver does the vast majority of this work, and the process is essentially fixed: it doesn’t speed up if you drink water, eat food, or exercise after drinking. Those strategies can help you feel better, but they won’t lower your BAC faster in any meaningful way.
What food does affect is absorption. Eating before or while drinking slows how quickly alcohol reaches your small intestine, where most absorption happens. A valve between the stomach and small intestine closes during digestion, especially when processing protein and fatty foods. Drinking on an empty stomach means alcohol passes into the small intestine quickly, and your BAC peaks faster and higher. This doesn’t change the total amount of alcohol absorbed, but it changes the timing, which the formula can’t fully capture.
What Each BAC Level Feels Like
The National Highway Traffic Safety Administration breaks down impairment by BAC level, and the effects start lower than most people expect.
- 0.02%: Slight warmth, mild mood changes, and a small decline in the ability to track moving objects or split attention between two tasks.
- 0.05%: Lowered alertness, reduced coordination, exaggerated behavior, and difficulty steering or responding to sudden driving situations.
- 0.08%: Poor muscle coordination affecting balance, speech, and reaction time. Judgment, self-control, and short-term memory are all noticeably impaired.
- 0.10%: Clear deterioration in reaction time. Slurred speech, poor coordination, and reduced ability to stay in a lane or brake appropriately.
- 0.15%: Major loss of muscle control and balance. Vomiting is common. Visual and auditory processing are substantially impaired.
Legal Limits for Driving
In all 50 U.S. states, the legal per se BAC limit for driving is 0.08%. But impairment begins well below that threshold, which is why more than 100 countries, including Germany, Japan, India, Russia, and South Africa, have set their legal limits at 0.05% or lower. The National Transportation Safety Board has urged all U.S. states to adopt 0.05% as well, though none have done so yet. Utah came closest, lowering its limit to 0.05% in 2018.
For commercial drivers in the U.S., the limit is 0.04%. For drivers under 21, most states enforce zero-tolerance laws, typically setting the limit at 0.02% or lower.
Why These Calculations Are Estimates
The Widmark formula gives you a ballpark, not a lab result. Several factors introduce error. The r value you use is a population average, not your personal number. Your actual elimination rate could be 50% faster or slower than the 0.015 standard. The formula assumes all alcohol has been absorbed, which isn’t true during or shortly after drinking. And it can’t account for medications, liver health, hydration status, or genetic differences in how you metabolize alcohol.
Even clinical measurement tools have limitations. Breathalyzers assume a fixed ratio between alcohol in your blood and alcohol in your breath (2100:1), but the actual ratio varies between 1500:1 and 3000:1 depending on the individual, their age, and their state of intoxication. This means a breathalyzer can overestimate or underestimate your true BAC compared to a direct blood draw.
The practical takeaway: a calculated BAC of 0.07% could easily be 0.05% or 0.09% in reality. If your estimate puts you anywhere near a legal limit, you’re not in a safe margin. The formula is useful for understanding how alcohol works in your body and for making rough comparisons, but it’s not precise enough to tell you whether you’re legally safe to drive.