Blood alcohol content (BAC) calculator

The Widmark formula

Published by Swedish chemist Erik Widmark in 1932, it estimates blood alcohol concentration from how much you drank and your body mass. Alcohol spreads through body water, so the factor r is 0.7 for men and 0.6 for women, who have on average less body water.

\[ \begin{gathered} \text{Pure alcohol (g)} = \dfrac{\text{volume (ml)} \times \text{ABV (\%)}}{100} \times 0.789 \\ \text{Peak (g/L)} = \dfrac{\text{pure alcohol (g)}}{r \times \text{weight in kg}} \quad \to \quad \text{BAC \%} = \dfrac{\text{g/L}}{10} \\ \text{BAC after } t \text{ hours} = \text{peak} - 0.015\% \times t \quad \text{(never below 0)} \end{gathered} \]

0.789 g/ml is the density of ethanol. The average elimination rate is 0.015% BAC per hour (0.010 to 0.020 depending on the person), and coffee, cold showers or water do not speed it up.

Legal BAC limits in the US

You can be charged with impaired driving below 0.08% if your driving is affected. Laws change, check current rules where you live.

What counts as one standard drink

In the US, a standard drink holds 14 grams of pure alcohol: 12 oz of 5% beer, 5 oz of 12% wine or 1.5 oz of 40% spirits. Craft beers and restaurant pours are often bigger. The estimate ignores food in your stomach (which slows absorption), medication, fatigue and health, so it can understate your BAC. After heavy drinking, you may still be over the limit the next morning.

Blood alcohol calculator: a worked Widmark example

An example makes the formula clear. A 176 lb man (80 kg) drinks 3 standard drinks, or 42 g of pure alcohol. First, compute the divider: 80 × 0.7 gives 56. Then divide: 42 ÷ 56, which is about 0.75 g/L, or 0.075% BAC. Finally, subtract elimination: after 2 hours, about 0.045% remains. So the level sits near the legal limit just after the last drink.

In contrast, a 132 lb woman (60 kg) who drinks the same 42 g gets 42 ÷ (60 × 0.6), or about 1.17 g/L, which is 0.117% BAC. In fact, body weight and body water change the result a lot.

Blood alcohol calculator: what changes your BAC

Several factors truly change your BAC. Here are the main ones. However, none lets you drive safely or "hold it" better.

• Weight and sex, which set the water available.
• Food: eating slows how fast alcohol enters the blood.
• Pace: fast drinking pushes the peak higher.
• Fatigue, medication and age, which change your reaction.

So two people can have very different levels after the same number of drinks. To see your reference weight, open the BMI calculator.

Blood alcohol calculator: driving, risk and smart habits

Risk starts well before the legal limit. At 0.02% BAC, reaction and vision already slip a little. Then at 0.08%, crash risk rises sharply. So the only safe rule is not to drive after drinking.

Plan a designated driver, a rideshare or transit before you go out. Also, coffee and a cold shower do not speed up elimination. Only time lowers the level. To track daily habits, try the water intake calculator. Finally, check your calorie needs if you watch your diet.