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Daily Dose of Utilities

Blood Alcohol Content (BAC) Calculator

The Widmark formula estimates BAC as the alcohol drunk divided by body weight times a body-water factor, minus about 0.015% per hour. A 180 lb man who has 4 standard drinks over 2 hours is at roughly 0.071%. This is an estimate only: never use it to decide whether to drive.

A rough estimate only. Real blood alcohol depends on food, medication, health, drink strength and more. Never use this to decide whether it's safe to drive. If you've been drinking, don't drive.
Sex (used by the formula)
Weight unit
lb
h

One standard drink = 12 oz of 5% beer, 5 oz of 12% wine or 1.5 oz of 40% spirits. Strong beers and large pours count as more than one.

Estimated BAC

0.071%

Back to zero in about

4.7 hours

Compared with the US 0.08% driving limit

Under, but impairment starts well below 0.08%

Show the math

  1. 1

    Alcohol consumed

    4 standard drinks × 14 g = 56 g of alcohol

    A US standard drink is 14 g of pure alcohol: about 12 oz of 5% beer, 5 oz of 12% wine or 1.5 oz of 40% spirits.

    = 56 g

Read the steps as text
  1. Alcohol consumed. 4 standard drinks × 14 g = 56 g of alcohol A US standard drink is 14 g of pure alcohol: about 12 oz of 5% beer, 5 oz of 12% wine or 1.5 oz of 40% spirits.
  2. Spread through body water (Widmark). 56 g ÷ (81,647 g × 0.68) × 100 = 0.101% r is the share of body weight alcohol can spread into, about 0.68 on average for men. Women usually reach a higher BAC from the same drinks.
  3. Subtract what's been broken down. 0.101 − 0.015 × 2 h = 0.071% The liver clears about 0.015% BAC per hour on average. Nothing speeds this up: not coffee, water or a cold shower.
  4. Time until it's gone. 0.071 ÷ 0.015 = 4.7 hours

How BAC is estimated

The Widmark formula, developed in the 1930s, is still the basis of most BAC estimates. It divides the grams of alcohol you've drunk by your body weight in grams multiplied by r, the share of your body that alcohol spreads into (about 0.68 for men and 0.55 for women on average), and multiplies by 100. Then it subtracts about 0.015% for every hour since you started drinking, the average rate the liver clears alcohol.

A US standard drink holds 14 g of pure alcohol: about 12 oz of 5% beer, 5 oz of 12% wine or 1.5 oz of 40% spirits. Many craft beers, large wine pours and mixed drinks contain more than one standard drink.

Why this is only an estimate

Real blood alcohol varies widely between people and occasions. Food in your stomach slows absorption, and medication, health, hydration, genetics and drinking speed all play a part. Your BAC can also still be rising for an hour or more after your last drink.

Impairment begins well below the US legal driving limit of 0.08% (0.05% in Utah), and commercial drivers have a 0.04% limit. Only time lowers BAC: coffee, water, food and cold showers don't speed it up. If you've been drinking, don't drive. Plan a ride before you start.

Frequently asked questions

How long does it take to sober up?
BAC falls by about 0.015% an hour, so from 0.08% it takes roughly 5 to 6 hours to reach zero. Nothing speeds this up.
Why do women reach a higher BAC from the same drinks?
On average women have less body water for alcohol to spread into, and often weigh less, so the same amount of alcohol is more concentrated.
Can I use this to decide if I'm safe to drive?
No. It's a rough estimate that can be well off for any individual, and impairment starts below the legal limit. If you've been drinking, don't drive.
What counts as one standard drink?
14 g of pure alcohol: 12 oz of regular beer (5%), 5 oz of wine (12%) or a 1.5 oz shot of 80-proof spirits (40%).

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