The problem: "I feel fine" is not a BAC reading
The reason people get DUIs after "just a few drinks" is that BAC tracks alcohol in your blood, not how you feel — and tolerance masks impairment without lowering the number. A calculator won't tell you whether you're safe to drive (only a breathalyzer or blood test does that), but it will tell you whether you're even in the ballpark, which is more than most people manage by feel. The math behind it is the Widmark formula, and it's worth understanding because every BAC tool you'll ever use is some variation of it.
Fastest path
Open the BAC Calculator, enter your sex, weight, the drinks, and how long it's been since the first one. It returns an estimated BAC, an impairment description, and a timeline of when you'll hit 0.00%.
Worked example — a 180 lb male, three 12 oz beers (5% ABV) and one 1.5 oz shot (40%), two hours since the first drink, stomach empty:
Alcohol grams: 3 beers × 14.0 g + 1 shot × 14.0 g = 56.0 g
Body weight: 180 lb → 81,647 g
Body water (r): 0.68 (male)
Raw BAC: (56.0 / (81,647 × 0.68)) × 100 = 0.1009%
Minus metabolism: 0.1009 − (0.015 × 2 h) = 0.0709%
Result: 0.0709% → Impaired (under 0.08, over 0.05)
Time to 0.00%: 0.0709 / 0.015 ≈ 4h 44m
Four standard drinks in a 180 lb man, two hours in, lands around 0.07 — impaired, not legally over 0.08 in the US, but over the 0.05 limit that applies in much of Europe and Australia. Same drinks, same weight, a woman would be higher. We'll get to why.
The formula, piece by piece
BAC = (alcohol_grams / (weight_grams × r)) × 100 − (0.015 × hours)
Alcohol grams. A drink's alcohol content is its volume × ABV × the density of ethanol (0.789 g/ml). A 12 oz beer at 5% works out to 14.0 g — which is why the US defines a "standard drink" as 14 g. A 5 oz glass of wine at 12% and a 1.5 oz shot of 40% spirits also land at ~14 g. They're all one standard drink, despite looking very different in the glass. Australia and the UK use 10 g as their standard, so a "three standard drinks" claim from a UK source is 30 g, not 42 — a 40% difference. Don't mix the two.
r, the body water constant. This is where sex enters the formula: r is 0.68 for men and 0.55 for women. Women carry less body water per kilogram, so the same alcohol grams in a 150 lb woman produces a higher concentration than in a 150 lb man. It's not a judgment; it's physiology. The constant also varies with body composition — more body fat means less water, so r drops — but the tool uses the population average for your sex, which is the best you can do without a lab.
The metabolism term. 0.015% per hour is the average rate at which the liver clears alcohol. That's roughly one US standard drink per hour for a typical adult. The formula subtracts 0.015 × hours-since-first-drink linearly, which is the crudest part of the whole calculation — see the gotchas.
The eating modifier. Food in your stomach slows gastric emptying, spreading absorption over a longer period and lowering the peak. The tool multiplies alcohol grams by 0.8 when you tick "eating," a rough 20% reduction. It does not eliminate alcohol; it stretches the curve. You still absorb all of it eventually.
Gotchas
- The formula ignores peak timing. It assumes all the alcohol is absorbed and evenly distributed the moment you enter it. In reality, BAC keeps rising for 30 to 90 minutes after the last drink. Right after finishing a shot, your calculated BAC is lower than your actual BAC, because the alcohol hasn't left your stomach yet. If you're calculating right after drinking, the estimate is optimistic — wait an hour and recalculate.
- 0.015 is an average, not your liver. Metabolism ranges from about 0.012 to 0.020 per hour. Heavy drinkers produce more liver enzymes and clear faster; people with liver damage or certain medications clear slower. The tool can't know which end you're on.
- Coffee, showers, and exercise do nothing. Only time and your liver reduce BAC. Caffeine makes you feel more alert, which is the dangerous combination — alert-feeling but still impaired. A breathalyzer doesn't care how awake you are.
- The eating modifier is not a free pass. It lowers the peak by maybe 20%, not by half. If you were at 0.10 on an empty stomach, you're around 0.08 with food — still possibly over the limit.
- Legal limits vary by country and by driver type. The US general limit is 0.08; Australia and much of Europe are 0.05; Sweden is 0.02; many countries are zero for commercial or new drivers. The tool's "over limit" status assumes 0.08. If you're driving in Germany, 0.05 is the line that matters.
Summary
- BAC = (alcohol grams / (weight × r)) × 100 − (0.015 × hours), where r is 0.68 for men and 0.55 for women.
- A US standard drink is 14 g of alcohol — one 12 oz beer at 5%, one 5 oz wine at 12%, or one 1.5 oz shot at 40%. Australia/UK standard drinks are 10 g; don't confuse them.
- The liver clears about one drink per hour (0.015% BAC), and nothing speeds that up.
- A calculator gives you a ballpark, not a number you can bet a DUI on. If the estimate is anywhere near the legal limit, don't drive — use the BAC Calculator to see the shape of the curve, then make the conservative call. For the related body-math jobs, BMI Calculator and Calorie Calculator cover the rest.