Blood Alcohol Calculator

About Blood Alcohol Calculator

Most people who open a BAC calculator want one answer: how long until I can drive? This calculator cannot give you that answer, and it should not be used to decide whether to drive. Its numbers describe an average body. Your real blood alcohol concentration can differ from them in either direction, and it can differ for you from one night to the next. If you have been drinking, the decision is simple: don't drive.

What the calculator is good for is understanding the arithmetic. It shows why two glasses of wine hit a small person much harder than a large one, how long a night out actually lingers, and why a late session can still be in your blood on the morning commute. It uses the Widmark formula to estimate peak BAC, current BAC, the time you reach zero, and when you would pass below the reference limits for Korea, Japan, Germany, the US and the UK.

What the Widmark formula actually does

Swedish chemist Erik Widmark worked out the model in the 1930s, and it is still the backbone of forensic alcohol calculations. It has two parts. First, assume the alcohol spreads evenly through the body's water: peak BAC (‰) = alcohol in grams ÷ (body weight in kg × r). Then assume the liver removes a fixed amount every hour: BAC now = peak − β × hours elapsed.

Grams of alcohol come from volume × ABV × 0.789 g/ml, the density of ethanol. A 355 ml can of 5% beer holds about 14.0 g, a 150 ml glass of 12% wine about 14.2 g, and a 44 ml shot of 40% whiskey about 13.9 g. That is why those three are roughly one US 'standard drink' each (14 g). A 500 ml pint of 5% beer is about 19.7 g, closer to one and a half.

The calculator works in per mille and divides by 10 to show a percentage, the same notation US law uses when it says 0.08%.

The r factor: why the same drink hits people differently

r is the fraction of body weight that alcohol can dissolve into. Ethanol mixes with water but barely enters fat, so the more water your body holds, the more diluted each drink becomes. The calculator uses the widely cited averages of 0.68 for men and 0.55 for women; the lower value for women reflects a higher average body-fat share.

These are group averages. A muscular woman may have an r well above 0.55; a man with a high body-fat percentage may sit below 0.68. Whatever the error in r, the peak estimate is off by the same proportion.

The 0.015% per hour rule and its spread

The familiar rule of thumb that BAC falls about 0.015% per hour is the calculator's 'average' setting (0.15‰/h). A 2010 review by forensic toxicologist A. W. Jones put the physiological range much wider, from about 0.010% to 0.035% per hour. For moderate drinkers 0.015% is a fair population average; people who drank on an empty stomach tend toward 0.010–0.015%, and people who had eaten toward 0.015–0.020%. Heavy drinkers can clear alcohol faster, which is one reason 0.019% is used for arrested drivers in that review.

Slow (0.010%/h), average (0.015%/h) and fast (0.020%/h) are three points in that range. Nothing in the calculator tells you which one you are, and the slow setting is not a guaranteed worst case.

Why the estimate can't be trusted for driving

With the drinking duration left at 0, the model treats every drink as absorbed the instant you start. Enter a duration (start to finish) and the alcohol is spread evenly over that time, with the peak BAC and its time shown separately. The delay of absorption from the gut is still not modeled: in reality BAC often keeps rising for 30 to 60 minutes or more after the last drink, so the estimate can be too low right after you stop.

• r and the elimination rate are averages; your own values can differ substantially.
• People rarely remember exactly how much they drank, especially with topped-up glasses or mixed drinks.
• Medication, fatigue and lack of sleep worsen impairment at any given BAC.
• A legal limit is an enforcement line, not a safety line. Reaction time and judgment are already affected below 0.08%.
• Police and courts go by measured breath or blood results. A calculator printout is not a defense.

Legal limits in the table

In the United States it is illegal to drive with a BAC of 0.08% or higher in every state except Utah, where the limit is 0.05%. Many states also have zero-tolerance rules for drivers under 21 and lower limits for commercial drivers. The calculator's US row uses 0.08%, so if you live in Utah, read the Germany row (0.05%) instead.

The other rows are Korea 0.03%, Japan 0.03% (0.3 mg per ml of blood), Germany 0.05%, Spain 0.05% (0.03% for novice and professional drivers) and the UK 0.08% (England and Wales; Scotland is 0.05%). The same person at the same moment can be legal in one country and over the limit in another.

Worked examples

How to use it

1. Enter your weight (kg or lb), sex, and an elimination rate; if unsure, look at the slow result.
2. Add every drink with the preset buttons or a custom volume and ABV.
3. Set the time you started drinking and how long you drank to see current BAC, peak BAC and the estimated time under each reference limit.

Key terms

BAC

Blood alcohol concentration, shown here as a percentage (grams per 100 ml).

Per mille (‰)

Parts per thousand. 1‰ = 0.1%.

r (Widmark factor)

Share of body weight alcohol distributes into: 0.68 for men, 0.55 for women in this calculator.

β (elimination rate)

How much BAC falls per hour: 0.10, 0.15 or 0.20‰/h here.

Frequently asked questions

Notes

Do not use this calculator to decide whether you can drive. If you have been drinking, don't drive.

Legal limits change. The values here were checked in September 2026.