Blood Alcohol Calculator
See what NHTSA’s blood alcohol method estimates from what you entered, and how wide the range around that estimate really is.
Result
Enter weight to see the result.
https://calcgallery.com/health-risk/bac-calculator/
How to use the Blood Alcohol Calculator
- Choose which published body-water figure to apply, and enter your body weight.
- Add a row for each drink, with its serving size in millilitres and its strength.
- Enter how many hours have passed since the first drink.
- Read the range. The width of it is the point of the page.
Read this first
Any amount of alcohol impairs driving, and the only reliable number is zero.
This page will not tell you whether you can drive. It will not compare the estimate to a legal limit, because legal limits differ by country, by state and by driver class, and because the error on this estimate is larger than the gap between common limits. It will not print a time at which you would be sober, because that number would be a guess dressed as a fact.
The reason the page exists is to show how wide the range around such an estimate really is, and therefore why it cannot be used to make that decision.
How it is calculated
The method NHTSA sets out in Computing a BAC Estimate (1994):
alcohol grams = volume in mL × ABV × 0.79body water = body weight × 0.58 for a man, 0.49 for a womanBAC % = alcohol grams ÷ body water in mL × 0.806 × 100 − 0.017 × hoursThe 0.79 is the specific gravity of ethanol, so a serving’s volume and strength give the mass of alcohol in it. Once absorbed, alcohol spreads through the body’s water: on average 58% of a man’s weight and 49% of a woman’s. Blood is about 80.6% water, so the concentration in blood is the concentration in body water times 0.806. The 0.017 per hour is NHTSA’s average fall for moderate drinkers; for heavy drinkers it averages nearer 0.02.
This has the same shape as Widmark’s 1932 equation. NHTSA notes that its body-water figures are not Widmark’s distribution factor R, which folds both steps into one number.
The result is clamped at zero. Elimination over enough hours exceeds what was absorbed, and a negative blood alcohol content is not a thing.
Why the answer is a range
Body water is the weak point. It is not a constant: it reflects body composition, which differs between people and changes within the same person. NHTSA itself notes that younger and leaner people carry more water and older and heavier people less.
So this page reports the same arithmetic with body water fifteen percent either side of the average figure, and treats the resulting range as the answer. A single number, printed to three decimal places, is the misleading part of every other calculator that does this.
And that variation is only one of several. On top of it:
- Food in the stomach slows absorption and lowers the peak, sometimes considerably.
- Drinking rate changes the shape of the curve entirely; the same alcohol over one hour or four produces different peaks.
- Medication interacts with alcohol metabolism in ways this arithmetic knows nothing about.
- Elimination rate varies between individuals, and the 0.017 figure is an average.
None of those is modelled here. Each of them moves the answer.
What a standard drink is
In the US, 14 grams of alcohol: roughly a 355 mL beer at 5%, a 150 mL glass of wine at 12%, or a 44 mL spirit measure at 40%. The definition is not universal — the UK unit is 8 grams, Australia’s is 10 — so the page computes from the volumes and strengths you actually enter and reports the total in US standard drinks for reference.
Practical tips
Do not use this to decide whether to drive. It is the whole reason the page is written the way it is.
The range is the answer. If you take one number away from this page, take the width of the range rather than the midpoint.
Time is the only thing that lowers it. Not coffee, not food after the fact, not a cold shower, not exercise.
Peak comes later than the last drink. Absorption continues after you stop, so a reading taken at the moment of the last drink understates where it is headed.
If you are uncertain, choose the safer option regardless of the number. That is the only advice a page like this can responsibly give.
With your numbers
- 42.1 g of alcohol is about 3 US standard drinks
- Spread through 176 lb of body, less 2 hours of elimination
- The range is 0.03% to 0.052%, around a midpoint of 0.039%
Quick reference
Any amount of alcohol impairs driving, and the only reliable number is zero. This estimate cannot tell you whether you are safe to drive or to do anything else.
| Example | Estimated BAC, midpoint |
|---|---|
| Male, 80 kg, 1 beer, 1 hour | 0.007% |
| Male, 80 kg, 3 beers, 2 hours | 0.039% |
| Female, 60 kg, 2 glasses of wine, 1 hour | 0.061% |
| Female, 60 kg, 1 glass of wine, 2 hours | 0.005% |
| Male, 176 lb, 2 beers, 1 hour | 0.032% |
| Male, 200 lb, 4 beers, 3 hours | 0.035% |
| Female, 130 lb, 1 spirit measure, 1 hour | 0.022% |
| Male, 176 lb, 1 beer, 8 hours | 0% |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
6 sources
p. 2: "males have 58 percent of their body weight as water". Computing a BAC Estimate, NHTSA Office of Program Development and Evaluation (October 1994); the only copy found online is hosted by the Brewers Association, https://www.craftbeer.com/attachments/0000/1170/Computing_a_BAC_Estimate.pdf
Used for
- Share of a man’s body weight that is water, through which alcohol distributes. NHTSA notes this is not Widmark’s R.
p. 2: "females have 49 percent of their weight as water". Computing a BAC Estimate, NHTSA Office of Program Development and Evaluation (October 1994); the only copy found online is hosted by the Brewers Association, https://www.craftbeer.com/attachments/0000/1170/Computing_a_BAC_Estimate.pdf
Used for
- Share of a woman’s body weight that is water, through which alcohol distributes.
p. 3: "On average, blood is composed of 80.6 percent water." Computing a BAC Estimate, NHTSA Office of Program Development and Evaluation (October 1994); the only copy found online is hosted by the Brewers Association, https://www.craftbeer.com/attachments/0000/1170/Computing_a_BAC_Estimate.pdf
Used for
- Share of blood that is water: the alcohol concentration in body water times this gives the concentration in blood.
p. 4: "average metabolism rate for moderate drinkers produces a .017 per hour decline". Computing a BAC Estimate, NHTSA Office of Program Development and Evaluation (October 1994); the only copy found online is hosted by the Brewers Association, https://www.craftbeer.com/attachments/0000/1170/Computing_a_BAC_Estimate.pdf
Used for
- Average hourly fall in BAC for moderate drinkers; heavy drinkers average about .02. Individuals vary.
p. 3: "Since alcohol has a specific gravity of .79". Computing a BAC Estimate, NHTSA Office of Program Development and Evaluation (October 1994); the only copy found online is hosted by the Brewers Association, https://www.craftbeer.com/attachments/0000/1170/Computing_a_BAC_Estimate.pdf
Used for
- Specific gravity of ethanol, used to convert drink volume to alcohol mass.
NIAAA, What Is A Standard Drink? (US definition), https://www.niaaa.nih.gov/alcohols-effects-health/what-standard-drink: "about 14 grams" of pure alcohol.
Used for
- Alcohol in one US standard drink. The definition differs by country.
Common questions about the Blood Alcohol Calculator
- How accurate is an estimate like this?
- Not accurate enough to decide anything by. The share of the body that is water varies substantially between people and within the same person on different days, absorption depends on what and when you last ate, and the elimination rate is an average that individuals depart from in both directions. Published comparisons against measured blood alcohol show errors comfortably larger than the gap between common legal limits.
- Does coffee or food speed up elimination?
- No. Elimination is done by the liver at a rate that caffeine does not change; coffee makes a person more alert without making them less impaired, which is arguably worse. Food eaten before or during drinking slows absorption, so it lowers the peak, but once alcohol is in the blood nothing accelerates its removal except time.
- How long until alcohol clears?
- This page does not print a countdown, deliberately. Producing a time at which you would be at zero implies a precision the estimate does not have, and it reads as permission. The elimination term is visible in the arithmetic, and the honest statement is that the only reliable figure is zero.
- Why do two people drinking the same amount get different readings?
- Body mass is the largest factor: the same alcohol spread through a larger body reaches a lower concentration. The share of the body that is water differs between people with body composition, and alcohol spreads through that water. On top of that, drinking rate, food and individual differences in absorption and elimination all move the result.
- What is a standard drink?
- In the US, 14 grams of alcohol, which is about 355 mL of 5% beer, 150 mL of 12% wine or 44 mL of 40% spirits. The definition differs by country: the UK unit is 8 grams, Australia uses 10, and several European countries use figures in between. This page reports US standard drinks and computes from the actual volumes and strengths you enter.