Blood Alcohol (BAC) Calculator
Estimate blood alcohol concentration with the Widmark equation, and how long until you are sober.
What you drank
Time since your first drink
From when you started, not when you stopped, your body begins clearing alcohol from the first sip.
This is a Widmark estimate for information only, and it is not evidence of fitness to drive. Real BAC varies with body composition, food, medication, liver function and how fast you drank, easily by 30% either way. The only safe amount before driving is none.
This estimates blood alcohol concentration from the number of drinks, your body weight and sex, and the time elapsed.
It is an estimate based on population averages, and it cannot tell you whether you are safe or legal to drive.
How blood alcohol concentration works
Blood alcohol concentration is the percentage of alcohol in your bloodstream. 0.08% is the legal limit for driving in most US states, and Utah is lower at 0.05%.
This calculation cannot tell you whether you are safe to drive. It uses population averages for how bodies absorb and clear alcohol, and individual variation is large. Food, medication, fatigue, health and drinking pace all shift the real figure. If you have been drinking, do not drive.
What an estimate is genuinely useful for is understanding the pattern. The same four drinks over the same two hours puts a 160 lb man at about 0.076 and a 140 lb woman at about 0.104. Identical drinking, and one is over the limit.
Two factors drive the gap: less body weight to distribute the alcohol through, and a lower proportion of body water. Both are averages, and both vary between individuals.
What to enter
- Number of standard drinks
- A US standard drink is 0.6 fl oz of pure alcohol: a 12 oz beer at 5%, 5 oz of wine at 12%, or 1.5 oz of spirits at 40%. Poured drinks are frequently larger than this.
- Body weight
- More weight means more body water to distribute alcohol through, so a lower concentration from the same drinks.
- Sex
- The Widmark equation uses a distribution ratio of about 0.73 for men and 0.66 for women, reflecting average differences in body water proportion.
- Time since drinking started
- The body clears roughly 0.015 BAC per hour, and that rate cannot be increased.
What a standard drink is
- Beer
- 12 fl oz at 5% ABV. Craft beers are often 7-9%, which makes one glass closer to two drinks.
- Wine
- 5 fl oz at 12% ABV. A generous restaurant pour is often 8-9 oz.
- Spirits
- 1.5 fl oz at 40% ABV. Free-poured measures are routinely larger.
- The practical problem
- Almost nobody drinks in standard drinks. Undercounting is the single largest source of error in any BAC estimate.
What this assumes
The Widmark equation uses average distribution ratios. Real values vary substantially between individuals with the same weight and sex.
Absorption is treated as instant. In reality it takes 30-90 minutes to peak, and food slows it considerably, so a short-interval estimate can be well out in either direction.
How to calculate blood alcohol concentration
The Widmark equation: alcohol consumed relative to body water, less what has been eliminated since.
- A
- Fluid ounces of pure alcohol, so 0.6 per standard drink
- W
- Body weight in pounds
- r
- Distribution ratio: about 0.73 for men, 0.66 for women
- H
- Hours since drinking began
Count standard drinks honestly. 0.6 fl oz of pure alcohol each. A 9% craft beer in a 16 oz glass is 2.4 standard drinks, not one.
Divide by body weight and the distribution ratio. This is what makes the same drinks affect two people differently.
Subtract 0.015 for each hour elapsed. That is the elimination rate, and nothing changes it. Coffee, cold showers and food do not speed it up.
Treat the result as an estimate only. It is not a measurement and it has no legal standing. Do not use it to decide whether to drive.
See a worked example: the same drinks, two very different results
- Drinks
- 4 standard drinks over 2 hours
- Person A
- 160 lb male
- Person B
- 140 lb female
Alcohol consumed: 4 × 0.6 = 2.4 fl oz.
A: (2.4 × 5.14) ÷ (160 × 0.73) = 0.1056, less 0.03 eliminated over 2 hours = 0.076.
B: (2.4 × 5.14) ÷ (140 × 0.66) = 0.1335, less 0.03 = 0.104.
One is under 0.08 and one is well over, from identical drinking. And a fifth drink would take Person A to 0.102 as well.
0.076 and 0.104 from the same four drinks
Frequently asked questions
No. This is an estimate from population averages, not a measurement of your blood. Individual variation is large, and a figure below 0.08 here does not mean you are below 0.08 or safe to drive.
Impairment also begins well below the legal limit. Reaction time and judgement are measurably affected from around 0.02. If you have been drinking, arrange another way home.
About 0.015 BAC per hour, and that rate is essentially fixed. From 0.10 it takes roughly 6.7 hours to reach zero.
Nothing speeds it up. Coffee, cold showers, exercise and food make you feel more alert without changing your BAC at all, which is exactly what makes them dangerous.
Alcohol distributes through body water. More body weight means more water and a lower concentration from the same drinks. Body composition matters too, since muscle holds more water than fat.
Beyond the equation, absorption is affected by food, drinking pace, medication, tolerance, liver health and genetics. Two people matching weight and sex can still end up meaningfully apart.
It slows absorption, which lowers the peak, so food genuinely helps. It does not reduce the total alcohol absorbed or speed up elimination.
A full stomach can spread the peak out over hours rather than minutes. You still end up processing the same amount of alcohol.
0.6 fl oz of pure alcohol: a 12 oz beer at 5%, 5 oz of wine at 12%, or 1.5 oz of spirits at 40%.
Real drinks are often larger. A 16 oz craft beer at 9% is 2.4 standard drinks. Undercounting here is the biggest reason estimates come out too low.
0.08% in most US states for drivers 21 and over. Utah is 0.05%. Commercial drivers are typically held to 0.04%.
Most states also have zero-tolerance laws for drivers under 21, where the limit is 0.00 or 0.02. And you can be charged with impaired driving below any of these limits if impairment is evident.
Problems people actually run into
Trusting an estimate over a decision not to drive
The equation uses averages. Your actual absorption, elimination rate and body composition can put you well above what it reports, and it cannot see how much you actually poured.
There is no version of this calculation that makes driving after drinking a reasonable risk. Treat the number as education, never as permission.
Counting glasses instead of standard drinks
A home-poured spirit is often two to three times a 1.5 oz measure, and a restaurant wine pour is frequently 8-9 oz rather than 5.
Someone who counts four glasses may have consumed seven standard drinks. That is the difference between an estimate of 0.076 and one well above 0.15.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Sources
- What is a standard drink? · National Institute on Alcohol Abuse and Alcoholism
- Impaired driving: get the facts · Centers for Disease Control and Prevention
Last updated: September 4, 2026